C. Davidson Frontispiece See page 107
eclipse instruments at sobral
SPACE TIME
AND
GRAVITATION
AN OUTLINE OF THE GENERAL
RELATIVITY THEORY
BY
A. S. EDDINGTON, M.A., M.Sc., F.R.S.
PLUMIAN PROFESSOR OF ASTRONOMY AND EXPERIMENTAL
PHILOSOPHY, CAMBRIDGE
CAMBRIDGE
AT THE UNIVERSITY PRESS
1920
Perhaps to move
His laughter at their quaint opinions wide
Hereafter, when they come to model heaven
And calculate the stars: how they will wield
The mighty frame: how build, unbuild, contrive
To save appearances.
Paradise Lost.
PREFACE
By his theory of relativity Albert Einstein has provoked a revolution of
thought in physical science.
The achievement consists essentially in this:|Einstein has succeeded in
separating far more completely than hitherto the share of the observer and
the share of external nature in the things we see happen. The perception
of an object by an observer depends on his own situation and circumstances;
for example, distance will make it appear smaller and dimmer.
We make allowance for this almost unconsciously in interpreting what we
see. But it now appears that the allowance made for the motion of the observer
has hitherto been too crude|a fact overlooked because in practice
all observers share nearly the same motion, that of the earth. Physical
space and time are found to be closely bound up with this motion of the
observer; and only an amorphous combination of the two is left inherent
in the external world. When space and time are relegated to their
proper source|the observer|the world of nature which remains appears
strangely unfamiliar; but it is in reality simplied, and the underlying
unity of the principal phenomena is now clearly revealed. The deductions
from this new outlook have, with one doubtful exception, been conrmed
when tested by experiment.
It is my aim to give an account of this work without introducing anything
very technical in the way of mathematics, physics, or philosophy.
The new view of space and time, so opposed to our habits of thought,
must in any case demand unusual mental exercise. The results appear
strange; and the incongruity is not without a humorous side. For the rst
nine chapters the task is one of interpreting a clear-cut theory, accepted
in all its essentials by a large and growing school of physicists|although
perhaps not everyone would accept the author's views of its meaning.
Chapters x and xi deal with very recent advances, with regard to which
opinion is more 
uid. As for the last chapter, containing the author's
speculations on the meaning of nature, since it touches on the rudiments
of a philosophical system, it is perhaps too sanguine to hope that it can
viii PREFACE
ever be other than controversial.
A non-mathematical presentation has necessary limitations; and the
reader who wishes to learn how certain exact results follow from Einstein's,
or even Newton's, law of gravitation is bound to seek the reasons in a
mathematical treatise. But this limitation of range is perhaps less serious
than the limitation of intrinsic truth. There is a relativity of truth, as
there is a relativity of space.|
\For is and is-not though with Rule and Line
And up-and-down without, I could dene."
Alas! It is not so simple. We abstract from the phenomena that which is
peculiar to the position and motion of the observer; but can we abstract
that which is peculiar to the limited imagination of the human brain?
We think we can, but only in the symbolism of mathematics. As the
language of a poet rings with a truth that eludes the clumsy explanations
of his commentators, so the geometry of relativity in its perfect harmony
expresses a truth of form and type in nature, which my bowdlerised version
misses.
But the mind is not content to leave scientic Truth in a dry husk of
mathematical symbols, and demands that it shall be alloyed with familiar
images. The mathematician, who handles x so lightly, may fairly be
asked to state, not indeed the inscrutable meaning of x in nature, but the
meaning which x conveys to him.
Although primarily designed for readers without technical knowledge of
the subject, it is hoped that the book may also appeal to those who have
gone into the subject more deeply. A few notes have been added in the
Appendix mainly to bridge the gap between this and more mathematical
treatises, and to indicate the points of contact between the argument in
the text and the parallel analytical investigation.
It is impossible adequately to express my debt to contemporary literature
and discussion. The writings of Einstein, Minkowski, Hilbert,
Lorentz, Weyl, Robb, and others, have provided the groundwork; in the
give and take of debate with friends and correspondents, the extensive
ramications have gradually appeared.
A. S. E.
1 May, 1920.
CONTENTS
Eclipse Instruments at Sobral . . . . . . . . . . . . . . . Frontispiece
prologue page
What is Geometry? . . . . . . . . . . . . . . . . . . . . . . . 1
chapter i
The FitzGerald Contraction . . . . . . . . . . . . . . . . . 15
chapter ii
Relativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
chapter iii
The World of Four Dimensions . . . . . . . . . . . . . . . . 41
chapter iv
Fields of Force . . . . . . . . . . . . . . . . . . . . . . . . . . 57
chapter v
Kinds of Space . . . . . . . . . . . . . . . . . . . . . . . . . . 69
chapter vi
The New Law of Gravitation and the Old Law . . . . . . 85
chapter vii
Weighing Light . . . . . . . . . . . . . . . . . . . . . . . . . . 101
chapter viii
Other Tests of the Theory . . . . . . . . . . . . . . . . . . 113
chapter ix
Momentum and Energy . . . . . . . . . . . . . . . . . . . . . 125
chapter x page
Towards Infinity . . . . . . . . . . . . . . . . . . . . . . . . . 141
chapter xi
Electricity and Gravitation . . . . . . . . . . . . . . . . . . 153
chapter xii
On the Nature of Things . . . . . . . . . . . . . . . . . . . . 165
appendix
Mathematical Notes . . . . . . . . . . . . . . . . . . . . . . . 183
Historical Note . . . . . . . . . . . . . . . . . . . . . . . . . 191
PROLOGUE
WHAT IS GEOMETRY?
A conversation between|
An experimental Physicist.
A pure Mathematician.
A Relativist, who advocates the newer conceptions of time and space
in physics.
Rel. There is a well-known proposition of Euclid which states that \Any
two sides of a triangle are together greater than the third side." Can either
of you tell me whether nowadays there is good reason to believe that this
proposition is true?
Math. For my part, I am quite unable to say whether the proposition is
true or not. I can deduce it by trustworthy reasoning from certain other
propositions or axioms, which are supposed to be still more elementary. If
these axioms are true, the proposition is true; if the axioms are not true,
the proposition is not true universally. Whether the axioms are true or
not I cannot say, and it is outside my province to consider.
Phys. But is it not claimed that the truth of these axioms is self-evident?
Math. They are by no means self-evident to me; and I think the claim
has been generally abandoned.
Phys. Yet since on these axioms you have been able to found a logical
and self-consistent system of geometry, is not this indirect evidence that
they are true?
Math. No. Euclid's geometry is not the only self-consistent system of
geometry. By choosing a dierent set of axioms I can, for example, arrive
at Lobatchewsky's geometry, in which many of the propositions of Euclid
are not in general true. From my point of view there is nothing to choose
between these dierent geometries.
Rel. How is it then that Euclid's geometry is so much the most important
system?
2 PROLOGUE
Math. I am scarcely prepared to admit that it is the most important.
But for reasons which I do not profess to understand, my friend the Physicist
is more interested in Euclidean geometry than in any other, and is
continually setting us problems in it. Consequently we have tended to
give an undue share of attention to the Euclidean system. There have,
however, been great geometers like Riemann who have done something to
restore a proper perspective.
Rel. (to Physicist). Why are you specially interested in Euclidean
geometry? Do you believe it to be the true geometry?
Phys. Yes. Our experimental work proves it true.
Rel. How, for example, do you prove that any two sides of a triangle
are together greater than the third side?
Phys. I can, of course, only prove it by taking a very large number of
typical cases, and I am limited by the inevitable inaccuracies of experiment.
My proofs are not so general or so perfect as those of the pure
mathematician. But it is a recognised principle in physical science that it
is permissible to generalise from a reasonably wide range of experiment;
and this kind of proof satises me.
Rel. It will satisfy me also. I need only trouble you with a special case.
Here is a triangle ABC; how will you prove that AB + BC is greater
than AC?
Phys. I shall take a scale and measure the three sides.
Rel. But we seem to be talking about dierent things. I was speaking
of a proposition of geometry|properties of space, not of matter. Your
experimental proof only shows how a material scale behaves when you
turn it into dierent positions.
Phys. I might arrange to make the measures with an optical device.
Rel. That is worse and worse. Now you are speaking of properties of
light.
Phys. I really cannot tell you anything about it, if you will not let
me make measurements of any kind. Measurement is my only means of
nding out about nature. I am not a metaphysicist.
Rel. Let us then agree that by length and distance you always mean
a quantity arrived at by measurements with material or optical appliances.
You have studied experimentally the laws obeyed by these mea-
sured lengths, and have found the geometry to which they conform. We
will call this geometry \Natural Geometry"; and it evidently has much
greater importance for you than any other of the systems which the brain
of the mathematician has invented. But we must remember that its subject
matter involves the behaviour of material scales|the properties of
WHAT IS GEOMETRY? 3
matter. Its laws are just as much laws of physics as, for example, the laws
of electromagnetism.
Phys. Do you mean to compare space to a kind of magnetic eld? I
scarcely understand.
Rel. You say that you cannot explore the world without some kind of
apparatus. If you explore with a scale, you nd out the natural geometry;
if you explore with a magnetic needle, you nd out the magnetic eld.
What we may call the eld of extension, or space-eld, is just as much
a physical quality as the magnetic eld. You can think of them both
existing together in the aether, if you like. The laws of both must be
determined by experiment. Of course, certain approximate laws of the
space-eld (Euclidean geometry) have been familiar to us from childhood;
but we must get rid of the idea that there is anything inevitable about
these laws, and that it would be impossible to nd in other parts of the
universe space-elds where these laws do not apply. As to how far space
really resembles a magnetic eld, I do not wish to dogmatise; my point is
that they present themselves to experimental investigation in very much
the same way.
Let us proceed to examine the laws of natural geometry. I have a tapemeasure,
and here is the triangle. AB = 391
2 in., BC = 1
8 in., CA = 397
8 in.
Why, your proposition does not hold!
Phys. You know very well what is wrong. You gave the tape-measure a
big stretch when you measured AB.
Rel. Why shouldn't I?
Phys. Of course, a length must be measured with a rigid scale.
Rel. That is an important addition to our denition of length. But
what is a rigid scale?
Phys. A scale which always keeps the same length.
Rel. But we have just dened length as the quantity arrived at by
measures with a rigid scale; so you will want another rigid scale to test
whether the rst one changes length; and a third to test the second; and
so ad innitum. You remind me of the incident of the clock and time-gun
in Egypt. The man in charge of the time-gun red it by the clock; and the
man in charge of the clock set it right by the time-gun. No, you must not
dene length by means of a rigid scale, and dene a rigid scale by means
of length.
Phys. I admit I am hazy about strict denitions. There is not time for
everything; and there are so many interesting things to nd out in physics,
which take up my attention. Are you so sure that you are prepared with
a logical denition of all the terms you use?
4 PROLOGUE
Rel. Heaven forbid! I am not naturally inclined to be rigorous about
these things. Although I appreciate the value of the work of those who
are digging at the foundations of science, my own interests are mainly in
the upper structure. But sometimes, if we wish to add another storey, it
is necessary to deepen the foundations. I have a denite object in trying
to arrive at the exact meaning of length. A strange theory is 
oating
round, to which you may feel initial objections; and you probably would
not wish to let your views go by default. And after all, when you claim
to determine lengths to eight signicant gures, you must have a pretty
denite standard of right and wrong measurements.
Phys. It is dicult to dene what we mean by rigid; but in practice
we can tell if a scale is likely to change length appreciably in dierent
circumstances.
Rel. No. Do not bring in the idea of change of length in describing the
apparatus for dening length. Obviously the adopted standard of length
cannot change length, whatever it is made of. If a metre is dened as the
length of a certain bar, that bar can never be anything but a metre long;
and if we assert that this bar changes length, it is clear that we must have
changed our minds as to the denition of length. You recognised that my
tape-measure was a defective standard|that it was not rigid. That was
not because it changed length, because, if it was the standard of length,
it could not change length. It was lacking in some other quality.
You know an approximately rigid scale when you see one. What you
are comparing it with is not some non-measurable ideal of length, but
some attainable, or at least approachable, ideal of material constitution.
Ordinary scales have defects|
exure, expansion with temperature, etc.|
which can be reduced by suitable precautions; and the limit, to which you
approach as you reduce them, is your rigid scale. You can dene these
defects without appealing to any extraneous denition of length; for example,
if you have two rods of the same material whose extremities are
just in contact with one another, and when one of them is heated the
extremities no longer can be adjusted to coincide, then the material has a
temperature-coecient of expansion. Thus you can compare experimentally
the temperature-coecients of dierent metals and arrange them in
diminishing sequence. In this sort of way you can specify the nature of
your ideal rigid rod, before you introduce the term length.
Phys. No doubt that is the way it should be dened.
Rel. We must recognise then that all our knowledge of space rests on the
behaviour of material measuring-scales free from certain denable defects
of constitution.
WHAT IS GEOMETRY? 5
Phys. I am not sure that I agree. Surely there is a sense in which the
statement AB = 2CD is true or false, even if we had no conception of a
material measuring-rod. For instance, there is, so to speak, twice as much
paper between A and B, as between C and D.
Rel. Provided the paper is uniform. But then, what does uniformity of
the paper mean? That the amount in given length is constant. We come
back at once to the need of dening length.
If you say instead that the amount of \space" between A and B is twice
that between C and D, the same thing applies. You imagine the intervals
lled with uniform space; but the uniformity simply means that the same
amount of space corresponds to each inch of your rigid measuring-rod.
You have arbitrarily used your rod to divide space into so-called equal
lumps. It all comes back to the rigid rod.
I think you were right at rst when you said that you could not nd out
anything without measurement; and measurement involves some specied
material appliance.
Now you admit that your measures cannot go beyond a certain close
approximation, and that you have not tried all possible conditions. Supposing
that one corner of your triangle was in a very intense gravitational
eld|far stronger than any we have had experience of|I have
good ground for believing that under those conditions you might nd the
sum of two sides of a triangle, as measured with a rigid rod, appreciably
less than the third side. In that case would you be prepared to give up
Euclidean geometry?
Phys. I think it would be risky to assume that the strong force of
gravitation made no dierence to the experiment.
Rel. On my supposition it makes an important dierence.
Phys. I mean that we might have to make corrections to the measures,
because the action of the strong force might possibly distort the measuringrod.
Rel. In a rigid rod we have eliminated any special response to strain.
Phys. But this is rather dierent. The extension of the rod is determined
by the positions taken up by the molecules under the forces to which they
are subjected; and there might be a response to the gravitational force
which all kinds of matter would share. This could scarcely be regarded as
a defect; and our so-called rigid rod would not be free from it any more
than any other kind of matter.
Rel. True; but what do you expect to obtain by correcting the measures?
You correct measures, when they are untrue to standard. Thus you correct
the readings of a hydrogen-thermometer to obtain the readings of a perfect
6 PROLOGUE
gas-thermometer, because the hydrogen molecules have nite size, and
exert special attractions on one another, and you prefer to take as standard
an ideal gas with innitely small molecules. But in the present case, what
is the standard you are aiming at when you propose to correct measures
made with the rigid rod?
Phys. I see the diculty. I have no knowledge of space apart from
my measures, and I have no better standard than the rigid rod. So it is
dicult to see what the corrected measures would mean. And yet it would
seem to me more natural to suppose that the failure of the proposition
was due to the measures going wrong rather than to an alteration in the
character of space.
Rel. Is not that because you are still a bit of a metaphysicist? You
keep some notion of a space which is superior to measurement, and are
ready to throw over the measures rather than let this space be distorted.
Even if there were reason for believing in such a space, what possible
reason could there be for assuming it to be Euclidean? Your sole reason
for believing space to be Euclidean is that hitherto your measures have
made it appear so; if now measures of certain parts of space prefer non-
Euclidean geometry, all reason for assuming Euclidean space disappears.
Mathematically and conceptually Euclidean and non-Euclidean space are
on the same footing; our preference for Euclidean space was based on
measures, and must stand or fall by measures.
Phys. Let me put it this way. I believe that I am trying to measure
something called length, which has an absolute meaning in nature, and is
of importance in connection with the laws of nature. This length obeys
Euclidean geometry. I believe my measures with a rigid rod determine
it accurately when no disturbance like gravitation is present; but in a
gravitational eld it is not unreasonable to expect that the uncorrected
measures may not give it exactly.
Rel. You have three hypotheses there:|(1) there is an absolute thing
in nature corresponding to length, (2) the geometry of these absolute
lengths is Euclidean, and (3) practical measures determine this length
accurately when there is no gravitational force. I see no necessity for these
hypotheses, and propose to do without them. Hypotheses non ngo. The
second hypothesis seems to me particularly objectionable. You assume
that this absolute thing in nature obeys the laws of Euclidean geometry.
Surely it is contrary to scientic principles to lay down arbitrary laws for
nature to obey; we must nd out her laws by experiment. In this case the
only experimental evidence is that measured lengths (which by your own
admission are not necessarily the same as this absolute thing) sometimes
WHAT IS GEOMETRY? 7
obey Euclidean geometry and sometimes do not. Again it would seem
reasonable to doubt your third hypothesis beyond, say, the sixth decimal
place; and that would play havoc with your more delicate measures. But
where I fundamentally dier from you is the rst hypothesis. Is there some
absolute quantity in nature that we try to determine when we measure
length? When we try to determine the number of molecules in a given piece
of matter, we have to use indirect methods, and dierent methods may give
systematically dierent results; but no one doubts that there is a denite
number of molecules, so that there is some meaning in saying that certain
methods are theoretically good and others inaccurate. Counting appears
to be an absolute operation. But it seems to me that other physical
measures are on a dierent footing. Any physical quantity, such as length,
mass, force, etc., which is not a pure number, can only be dened as
the result arrived at by conducting a physical experiment according to
specied rules.
So I cannot conceive of any \length" in nature independent of a denition
of the way of measuring length. And, if there is, we may disregard
it in physics, because it is beyond the range of experiment. Of course,
it is always possible that we may come across some quantity, not given
directly by experiment, which plays a fundamental part in theory. If so,
it will turn up in due course in our theoretical formulae. But it is no good
assuming such a quantity, and laying down a priori laws for it to obey, on
the o-chance of its proving useful.
Phys. Then you will not let me blame the measuring-rod when the
proposition fails?
Rel. By all means put the responsibility on the measuring-rod. Natural
geometry is the theory of the behaviour of material scales. Any proposition
in natural geometry is an assertion as to the behaviour of rigid scales,
which must accordingly take the blame or credit. But do not say that the
rigid scale is wrong, because that implies a standard of right which does
not exist.
Phys. The space which you are speaking of must be a sort of abstraction
of the extensional relations of matter.
Rel. Exactly so. And when I ask you to believe that space can be non-
Euclidean, or, in popular phrase, warped, I am not asking you for any
violent eort of the imagination; I only mean that the extensional relations
of matter obey somewhat modied laws. Whenever we investigate
the properties of space experimentally, it is these extensional relations
that we are nding. Therefore it seems logical to conclude that space as
known to us must be the abstraction of these material relations, and not
8 PROLOGUE
something more transcendental. The reformed methods of teaching geometry
in schools would be utterly condemned, and it would be misleading
to set schoolboys to verify propositions of geometry by measurement, if
the space they are supposed to be studying had not this meaning.
I suspect that you are doubtful whether this abstraction of extensional
relations quite fulls your general idea of space; and, as a necessity of
thought, you require something beyond. I do not think I need disturb
that impression, provided you realise that it is not the properties of this
more transcendental thing we are speaking of when we describe geometry
as Euclidean or non-Euclidean.
Math. The view has been widely held that space is neither physical nor
metaphysical, but conventional. Here is a passage from Poincare's Science
and Hypothesis, which describes this alternative idea of space:
\If Lobatchewsky's geometry is true, the parallax of a very distant star
will be nite. If Riemann's is true, it will be negative. These are the
results which seem within the reach of experiment, and it is hoped that
astronomical observations may enable us to decide between the two geometries.
But what we call a straight line in astronomy is simply the path
of a ray of light. If, therefore, we were to discover negative parallaxes,
or to prove that all parallaxes are higher than a certain limit, we should
have a choice between two conclusions: we could give up Euclidean geometry,
or modify the laws of optics, and suppose that light is not rigorously
propagated in a straight line. It is needless to add that everyone would
look upon this solution as the more advantageous. Euclidean geometry,
therefore, has nothing to fear from fresh experiments."
Rel. Poincare's brilliant exposition is a great help in understanding the
problem now confronting us. He brings out the interdependence between
geometrical laws and physical laws, which we have to bear in mind continually.
We can add on to one set of laws that which we subtract from
the other set. I admit that space is conventional|for that matter, the
meaning of every word in the language is conventional. Moreover, we
have actually arrived at the parting of the ways imagined by Poincare,
though the crucial experiment is not precisely the one he mentions. But
I deliberately adopt the alternative, which, he takes for granted, everyone
would consider less advantageous. I call the space thus chosen physical
space, and its geometry natural geometry, thus admitting that other conventional
meanings of space and geometry are possible. If it were only
a question of the meaning of space|a rather vague term|these other
possibilities might have some advantages. But the meaning assigned to
length and distance has to go along with the meaning assigned to space.
WHAT IS GEOMETRY? 9
Now these are quantities which the physicist has been accustomed to measure
with great accuracy; and they enter fundamentally into the whole of
our experimental knowledge of the world. We have a knowledge of the
so-called extent of the stellar universe, which, whatever it may amount
to in terms of ultimate reality, is not a mere description of location in a
conventional and arbitrary mathematical space. Are we to be robbed of
the terms in which we are accustomed to describe that knowledge?
The law of Boyle states that the pressure of a gas is proportional to
its density. It is found by experiment that this law is only approximately
true. A certain mathematical simplicity would be gained by conventionally
redening pressure in such a way that Boyle's law would be rigorously
obeyed. But it would be high-handed to appropriate the word pressure in
this way, unless it had been ascertained that the physicist had no further
use for it in its original meaning.
Phys. I have one other objection. Apart from measures, we have a
general perception of space, and the space we perceive is at least approximately
Euclidean.
Rel. Our perceptions are crude measures. It is true that our perception
of space is very largely a matter of optical measures with the eyes. If in
a strong gravitational eld optical and mechanical measures diverged, we
should have to make up our minds which was the preferable standard,
and afterwards abide by it. So far as we can ascertain, however, they
agree in all circumstances, and no such diculty arises. So, if physical
measures give us a non-Euclidean space, the space of perception will be
non-Euclidean. If you were transplanted into an extremely intense gravitational
eld, you would directly perceive the non-Euclidean properties of
space.
Phys. Non-Euclidean space seems contrary to reason.
Math. It is not contrary to reason, but contrary to common experience,
which is a very dierent thing, since experience is very limited.
Phys. I cannot imagine myself perceiving non-Euclidean space!
Math. Look at the re
ection of the room in a polished doorknob, and
imagine yourself one of the actors in what you see going on there.
Rel. I have another point to raise. The distance between two points
is to be the length measured with a rigid scale. Let us mark the two
points by particles of matter, because we must somehow identify them
by reference to material objects. For simplicity we shall suppose that the
two particles have no relative motion, so that the distance|whatever it
is|remains constant. Now you will probably agree that there is no such
thing as absolute motion; consequently there is no standard condition of
10 PROLOGUE
the scale which we can call \at rest." We may measure with the scale
moving in any way we choose, and if results for dierent motions disagree,
there is no criterion for selecting the true one. Further, if the particles are
sliding past the scale, it makes all the dierence what instants we choose
for making the two readings.
Phys. You can avoid that by dening distance as the measurement made
with a scale which has the same velocity as the two points. Then they
will always be in contact with two particular divisions of the scale.
Rel. A very sound denition; but unfortunately it does not agree with
the meaning of distance in general use. When the relativist wishes to refer
to this length, he calls it the proper-length; in non-relativity physics it does
not seem to have been used at all. You see it is not convenient to send
your apparatus hurling through the laboratory|after a pair of  particles,
for example. And you could scarcely measure the length of a wave of light
by this convention. So the physicist refers his lengths to apparatus at
rest on the earth; and the mathematician starts with the words \Choose
unaccelerated rectangular axes Ox, Oy, Oz, : : : " and assumes that the
measuring-scales are at rest relatively to these axes. So when the term
length is used some arbitrary standard motion of the measuring apparatus
must always be implied.
Phys. Then if you have xed your standard motion of the measuringrod,
there will be no ambiguity if you take the readings of both particles
at the same moment.
Rel. What is the same moment at dierent places? The conception
of simultaneity in dierent places is a dicult one. Is there a particular
instant in the progress of time on another world, Arcturus, which is the
same as the present instant on the Earth?
Phys. I think so, if there is any connecting link. We can observe an
event, say a change of brightness, on Arcturus, and, allowing for the time
taken by light to travel the distance, determine the corresponding instant
on the earth.
Rel. But then you must know the speed of the earth through the aether.
It may have shortened the light-time by going some way to meet the light
coming from Arcturus.
Phys. Is not that a small matter?
Rel. At a very modest reckoning the motion of the earth in the interval
might alter the light-time by several days. Actually, however, any speed of
the earth through the aether up to the velocity of light is admissible, with-
The proper-length of a light-wave is actually innite.
WHAT IS GEOMETRY? 11
out aecting anything observable. At least, nothing has been discovered
which contradicts this. So the error may be months or years.
Phys. What you have shown is that we have not sucient knowledge
to determine in practice which are simultaneous events on the Earth and
Arcturus. It does not follow that there is no denite simultaneity.
Rel. That is true, but it is at least possible that the reason why we are
unable to determine simultaneity in practice (or, what comes to pretty
much the same thing, our motion through the aether) in spite of many
brilliant attempts, is that there is no such thing as absolute simultaneity
of distant events. It is better therefore not to base our physics on this
notion of absolute simultaneity, which may turn out not to exist, and is
in any case out of reach at present.
But what all this comes to is that time as well as space is implied
in all our measures. The fundamental measurement is not the interval
between two points of space, but between two points of space associated
with instants of time.
Our natural geometry is incomplete at present. We must supplement it
by bringing in time as well as space. We shall need a perfect clock as well
as a rigid scale for our measures. It may be dicult to choose an ideal
standard clock; but whatever denition we decide on must be a physical
denition. We must not dodge it by saying that a perfect clock is one
which keeps perfect time. Perhaps the best theoretical clock would be a
pulse of light travelling in vacuum to and fro between mirrors at the ends
of a rigid scale. The instants of arrival at one end would dene equal
intervals of time.
Phys. I think your unit of time would change according to the motion
of your \clock" through the aether.
Rel. Then you are comparing it with some notion of absolute time. I
have no notion of time except as the result of measurement with some kind
of clock. (Our immediate perception of the 
ight of time is presumably
associated with molecular processes in the brain which play the part of a
material clock.) If you know a better clock, let us adopt it; but, having
once xed on our ideal clock there can be no appeal from its judgments.
You must remember too that if you wish to measure a second at one place,
you must keep your clock xed at what you consider to be one place; so
its motion is dened. The necessity of dening the motion of the clock
emphasises that one cannot consider time apart from space; there is one
geometry comprising both.
Phys. Is it right to call this study geometry? Geometry deals with space
alone.
12 PROLOGUE
Math. I have no objection. It is only necessary to consider time as a
fourth dimension. Your complete natural geometry will be a geometry of
four dimensions.
Phys. Have we then found the long-sought fourth dimension?
Math. It depends what kind of a fourth dimension you were seeking.
Probably not in the sense you intend. For me it only means adding a fourth
variable, t, to my three space-variables x, y, z. It is no concern of mine
what these variables really represent. You give me a few fundamental laws
that they satisfy, and I proceed to deduce other consequences that may be
of interest to you. The four variables may for all I know be the pressure,
density, temperature and entropy of a gas; that is of no importance to
me. But you would not say that a gas had four dimensions because four
mathematical variables were used to describe it. Your use of the term
\dimensions" is probably more restricted than mine.
Phys. I know that it is often a help to represent pressure and volume
as height and width on paper; and so geometry may have applications to
the theory of gases. But is it not going rather far to say that geometry
can deal directly with these things and is not necessarily concerned with
lengths in space?
Math. No. Geometry is nowadays largely analytical, so that in form as
well as in eect, it deals with variables of an unknown nature. It is true
that I can often see results more easily by taking my x and y as lengths
on a sheet of paper. Perhaps it would be helpful in seeing other results if I
took them as pressure and density in a steam-engine; but a steam-engine
is not so handy as a pencil. It is literally true that I do not want to know
the signicance of the variables x, y, z, t that I am discussing. That is
lucky for the Relativist, because although he has dened carefully how
they are to be measured, he has certainly not conveyed to me any notion
of how I am to picture them, if my picture of absolute space is an illusion.
Phys. Yours is a strange subject. You told us at the beginning that you
are not concerned as to whether your propositions are true, and now you
tell us you do not even care to know what you are talking about.
Math. That is an excellent description of Pure Mathematics, which has
already been given by an eminent mathematician.
\Pure mathematics consists entirely of such asseverations as that, if such and
such a proposition is true of anything, then such and such a proposition is true of
that thing. It is essential not to discuss whether the rst proposition is really true,
and not to mention what the anything is of which it is supposed to be true. . . . Thus
mathematics may be dened as the subject in which we never know what we are talking
about, nor whether what we are saying is true."
Bertrand Russell.
WHAT IS GEOMETRY? 13
Rel. I think there is a real sense in which time is a fourth dimension|as
distinct from a fourth variable. The term dimension seems to be associated
with relations of order. I believe that the order of events in nature is one
indissoluble four-dimensional order. We may split it arbitrarily into space
and time, just as we can split the order of space into length, breadth and
thickness. But space without time is as incomplete as a surface without
thickness.
Math. Do you argue that the real world behind the phenomena is fourdimensional?
Rel. I think that in the real world there must be a set of entities related
to one another in a four-dimensional order, and that these are the basis
of the perceptual world so far as it is yet explored by physics. But it is
possible to pick out a four-dimensional set of entities from a basal world of
ve dimensions, or even of three dimensions. The straight lines in threedimensional
space form a four-dimensional set of entities, i.e. they have a
four-fold order. So one cannot predict the ultimate number of dimensions
in the world|if indeed the expression dimensions is applicable.
Phys. What would a philosopher think of these conceptions? Or is he
solely concerned with a metaphysical space and time which is not within
reach of measurement.
Rel. In so far as he is a psychologist our results must concern him. Perception
is a kind of crude physical measurement; and perceptual space and
time is the same as the measured space and time, which is the subjectmatter
of natural geometry. In other respects he may not be so immediately
concerned. Physicists and philosophers have long agreed that motion
through absolute space can have no meaning; but in physics the question
is whether motion through aether has any meaning. I consider that it has
no meaning; but that answer, though it brings philosophy and physics into
closer relation, has no bearing on the philosophic question of absolute motion.
I think, however, we are entitled to expect a benevolent interest from
philosophers, in that we are giving to their ideas a perhaps unexpected
practical application.
Let me now try to sum up my conclusions from this conversation. We
have been trying to give a precise meaning to the term space, so that we
may be able to determine exactly the properties of the space we live in.
There is no means of determining the properties of our space by a priori
reasoning, because there are many possible kinds of space to choose from,
no one of which can be considered more likely than any other. For more
than 2000 years we have believed in a Euclidean space, because certain
experiments favoured it; but there is now reason to believe that these same
14 PROLOGUE
experiments when pushed to greater accuracy decide in favour of a slightly
dierent space (in the neighbourhood of massive bodies). The relativist
sees no reason to change the rules of the game because the result does not
agree with previous anticipations. Accordingly when he speaks of space,
he means the space revealed by measurement, whatever its geometry. He
points out that this is the space with which physics is concerned; and,
moreover, it is the space of everyday perception. If his right to appropriate
the term space in this way is challenged, he would urge that this
is the sense in which the term has always been used in physics hitherto;
it is only recently that conservative physicists, frightened by the revolutionary
consequences of modern experiments, have begun to play with the
idea of a pre-existing space whose properties cannot be ascertained by
experiment|a metaphysical space, to which they arbitrarily assign Euclidean
properties, although it is obvious that its geometry can never be
ascertained by experiment. But the relativist, in dening space as mea-
sured space, clearly recognises that all measurement involves the use of
material apparatus; the resulting geometry is specically a study of the
extensional relations of matter. He declines to consider anything more
transcendental.
My second point is that since natural geometry is the study of extensional
relations of natural objects, and since it is found that their spaceorder
cannot be discussed without reference to their time-order as well, it
has become necessary to extend our geometry to four dimensions in order
to include time.
CHAPTER I
THE FITZGERALD CONTRACTION
In order to reach the Truth, it is necessary, once in one's life, to put every
thing in doubt|so far as possible. Descartes.
Will it take longer to swim to a point 100 yards up-stream and back, or
to a point 100 yards across-stream and back?
In the rst case there is a long toil up against the current, and then a
quick return helped by the current, which is all too short to compensate.
In the second case the current also hinders, because part of the eort
is devoted to overcoming the drift down-stream. But no swimmer will
hesitate to say that the hindrance is the greater in the rst case.
Let us take a numerical example. Suppose the swimmer's speed is
50 yards a minute in still water, and the current is 30 yards a minute.
Thus the speed against the current is 20, and with the current 80 yards
a minute. The up journey then takes 5 minutes and the down journey 11
4
minutes. Total time, 61
4 minutes.
Fig. 1.
O B
E
Going across-stream the swimmer must aim at
a point E above the point B where he wishes to
arrive, so that OE represents his distance travelled
in still water, and EB the amount he has
drifted down. These must be in the ratio 50 to 30,
and we then know from the right-angled triangle
OBE that OB will correspond to 40. Since OB
is 100 yards, OE is 125 yards, and the time taken
is 21
2 minutes. Another 21
2 minutes will be needed
for the return journey. Total time, 5 minutes.
In still water the time would have been 4 minutes.
The up-and-down swim is thus longer than the transverse swim in the
16 THE FITZGERALD CONTRACTION [ch.
ratio 61
4 : 5 minutes. Or we may write the ratio
1
p􀀀1 􀀀 ( 30
50 )2
 which shows how the result depends on the ratio of the speed of the current
to the speed of the swimmer, viz. 30
50 .
A very famous experiment on these lines was tried in America in the
year 1887. The swimmer was a wave of light, which we know swims
through the aether with a speed of 186; 330 miles a second. The aether
was 
owing through the laboratory like a river past its banks. The lightwave
was divided, by partial re
ection at a thinly silvered surface, into two
parts, one of which was set to perform the up-and-down stream journey
and the other the across-stream journey. When the two waves reached
their proper turning-points they were sent back to the starting-point by
mirrors. To judge the result of the race, there was an optical device for
studying interference fringes; because the recomposition of the two waves
after the journey would reveal if one had been delayed more than the
other, so that, for example, the crest of one instead of tting on to the
crest of the other coincided with its trough.
To the surprise of Michelson and Morley, who conducted the experiment,
the result was a dead-heat. It is true that the direction of the current of
aether was not known|they hoped to nd it out by the experiment. That,
however, was got over by trying a number of dierent orientations. Also
it was possible that there might actually be no current at a particular
moment. But the earth has a velocity of 181
2 miles a second, continually
changing direction as it goes round the sun; so that at some time during
the year the motion of a terrestrial laboratory through the aether must
be at least 181
2 miles a second. The experiment should have detected the
delay by a much smaller current; in a repetition of it by Morley and Miller
in 1905, a current of 2 miles a second would have been sucient.
If we have two competitors, one of whom is known to be slower than the
other, and yet they both arrive at the winning-post at the same time, it is
clear that they cannot have travelled equal courses. To test this, the whole
apparatus was rotated through a right angle, so that what had been the
up-and-down course became the transverse course, and vice versa. Our
two competitors interchanged courses, but still the result was a dead-heat.
The surprising character of this result can be appreciated by contrasting
it with a similar experiment on sound-waves. Sound consists of waves in
air or other material, as light consists of waves in aether. It would be
possible to make a precisely similar experiment on sound, with a current
i] THE FITZGERALD CONTRACTION 17
of air past the apparatus instead of a current of aether. In that case the
greater delay of the wave along the direction of the current would certainly
show itself experimentally. Why does light seem to behave dierently?
The straightforward interpretation of this remarkable result is that each
course undergoes an automatic contraction when it is swung from the
transverse to the longitudinal position, so that whichever arm of the apparatus
is placed up-stream it straightway becomes the shorter. The course
is marked out in the rigid material apparatus, and we have to suppose that
the length of any part of the apparatus changes as it is turned in dierent
directions with respect to the aether-current. It is found that the kind of
material|metal, stone or wood|makes no dierence to the experiment.
The contraction must be the same for all kinds of matter; the expected
delay depends only on the ratio of the speed of the aether current to the
speed of light, and the contraction which compensates it must be equally
denite.
This explanation was proposed by FitzGerald, and at rst sight it seems
a strange and arbitrary hypothesis. But it has been rendered very plausible
by subsequent theoretical researches of Larmor and Lorentz. Under
ordinary circumstances the form and size of a solid body is maintained by
the forces of cohesion between its particles. What is the nature of cohesion?
We guess that it is made up of electric forces between the molecules.
But the aether is the medium in which electric force has its seat; hence it
will not be a matter of indierence to these forces how the electric medium
is 
owing with respect to the molecules. When the 
ow changes there will
be a readjustment of cohesive forces, and we must expect the body to take
a new shape and size.
The theory of Larmor and Lorentz enables us to trace in detail the
readjustment. Taking the accepted formulae of electromagnetic theory,
they showed that the new form of equilibrium would be contracted in
just such a way and by just such an amount as FitzGerald's explanation
requires.
The contraction in most cases is extremely minute. We have seen that
when the ratio of the speed of the current to that of the swimmer is 3
5 , a
contraction in the ratio p􀀀1􀀀( 3
5 )2
is needed to compensate for the delay.
The earth's orbital velocity is 1
10000 of the velocity of light, so that it will
give a contraction of p􀀀1􀀀( 1
10000 )2
, or 1 part in 200; 000; 000. This would
mean that the earth's diameter in the direction of its motion is shortened
by 21
2 inches.
Appendix, Note 1.
18 THE FITZGERALD CONTRACTION [ch.
The Michelson-Morley experiment has thus failed to detect our motion
through the aether, because the eect looked for|the delay of one of the
light waves|is exactly compensated by an automatic contraction of the
matter forming the apparatus. Other ingenious experiments have been
tried, electrical and optical experiments of a more technical nature. They
likewise have failed, because there is always an automatic compensation
somewhere. We now believe there is something in the nature of things
which inevitably makes these compensations, so that it will never be possible
to determine our motion through the aether. Whether we are at rest
in it, or whether we are rushing through it with a speed not much less
than that of light, will make no dierence to anything that can possibly
be observed.
This may seem a rash generalization from the few experiments actually
performed; more particularly, since we can only experiment with the small
range of velocity caused by the earth's orbital motion. With a larger range
residual dierences might be disclosed. But there is another reason for
believing that the compensation is not merely approximate but exact. The
compensation has been traced theoretically to its source in the well-known
laws of electromagnetic force; and here it is mathematically exact. Thus
the generalization is justied, at least in so far as the observed phenomena
depend on electromagnetic causes, and in so far as the universally accepted
laws of electromagnetism are accurate.
The generalization here laid down is called the restricted Principle of
Relativity:|It is impossible by any experiment to detect uniform motion
relative to the aether.
There are other natural forces which have not as yet been recognised
as coming within the electromagnetic scheme|gravitation, for example|
and for these other tests are required. Indeed we were scarcely justied in
stating above that the diameter of the earth would contract 21
2 inches, because
the gure of the earth is determined mainly by gravitation, whereas
the Michelson-Morley experiment relates to bodies held together by cohesion.
There is fair evidence of a rather technical kind that the compensation
exists also for phenomena in which gravitation is concerned; and we
shall assume that the principle covers all the forces of nature.
Suppose for a moment it were not so, and that it were possible to determine
a kind of absolute motion of the earth by experiments or observations
involving gravitation. Would this throw light on our motion through the
aether? I think not. It would show that there is some standard of rest
with respect to which the law of gravitation takes a symmetrical and
simple form; presumably this standard corresponds to some gravitational
i] THE FITZGERALD CONTRACTION 19
medium, and the motion determined would be motion with respect to that
medium. Similarly if the motion were revealed by vital or psychical phenomena,
it would be motion relative to some vital or psychical medium.
The aether, dened as the seat of electric forces, must be revealed, if at
all, by electric phenomena.
It is well to remember that there is reasonable justication for adopting
the principle of relativity even if the evidence is insucient to prove it. In
Newtonian dynamics the phenomena are independent of uniform motion
of the system; no explanation is asked for, because it is dicult to see
any reason why there should be an eect. If in other phenomena the
principle fails, then we must seek for an explanation of its failure|and no
doubt a plausible explanation can be devised; but so long as experiment
gives no indication of a failure, it is idle to anticipate such a complication.
Clearly physics cannot concern itself with all the possible complexities
which may exist in nature, but have not hitherto betrayed themselves in
any experiment.
The principle of relativity has implications of a most revolutionary kind.
Let us consider what is perhaps an exaggerated case|or perhaps the actual
case, for we cannot tell. Let the reader suppose that he is travelling
through the aether at 161; 000 miles a second vertically upwards; if he
likes to make the positive assertion that this is his velocity, no one will be
able to nd any evidence to contradict him. For this speed the FitzGerald
contraction is just 1
2 , so that every object contracts to half its original
length when turned into the vertical position.
As you lie in bed, you are, say, 6 feet long. Now stand upright; you are
3 feet. You are incredulous? Well, let us prove it! Take a yard-measure;
when turned vertically it must undergo the FitzGerald contraction, and
become only half a yard. If you measure yourself with it, you will nd
you are just two|half-yards. \But I can see that the yard-measure does
not change length when I turn it." What you perceive is an image of
the rod on the retina of your eye; you imagine that the image occupies
the same space in both positions; but your retina has contracted in the
vertical direction without your knowing it, so that your visual estimates of
vertical length are double what they should be. And so on with every test
you can devise. Because everything is altered in the same way, nothing
appears to be altered at all.
It is possible to devise electrical and optical tests; in that case the
argument is more complicated, because we must consider the eect of the
rapid current of aether on the electric forces and on waves of light. But
the nal conclusion is always the same; the tests will reveal nothing. Here
20 THE FITZGERALD CONTRACTION [ch.
is one illustration. To avoid distortion of the retina, lie on your back on
the 
oor, and watch in a suitably inclined mirror someone turn the rod
from the horizontal to the vertical position. You will, of course, see no
change of length, and it is not possible to blame the retina this time. But
is the appearance in the mirror a faithful reproduction of what is actually
occurring? In a plane mirror at rest the appearance is correct; the rays
of light come o the mirror at the same angle as they fall on to it, like
billiard balls rebounding from an elastic cushion. But if the cushion is in
rapid motion the angle of the billiard-ball will be altered; and similarly the
rapid motion of the mirror through the aether alters the law of re
ection.
Precise calculation shows that the moving mirror will distort the image,
so as to conceal exactly the changes of length which occur.
The mathematician does not need to go through all the possible tests
in detail; he knows that the complete compensation is inherent in the
fundamental laws of nature, and so must occur in every case. So if any
suggestion is made of a device for detecting these eects, he starts at once
to look for the fallacy which must surely be there. Our motion through
the aether may be very much less than the value here adopted, and the
changes of length may be very small; but the essential point is that they
escape notice, not because they are small (if they are small), but because
from their very nature they are undetectable.
There is a remarkable reciprocity about the eects of motion on length,
which can best be illustrated by another example. Suppose that by development
in the powers of aviation, a man 
ies past us at the rate of 161; 000
miles a second. We shall suppose that he is in a comfortable travelling
conveyance in which he can move about, and act normally and that his
length is in the direction of the 
ight. If we could catch an instantaneous
glimpse as he passed, we should see a gure about three feet high, but
with the breadth and girth of a normal human being. And the strange
thing is that he would be sublimely unconscious of his own undignied
appearance. If he looks in a mirror in his conveyance, he sees his usual
proportions; this is because of the contraction of his retina, or the distortion
by the moving mirror, as already explained. But when he looks down
on us, he sees a strange race of men who have apparently gone through
some 
attening-out process; one man looks barely 10 inches across the
shoulders, another standing at right angles is almost \length and breadth,
without thickness." As they turn about they change appearance like the
gures seen in the old-fashioned convex-mirrors. If the reader has watched
a cricket-match through a pair of prismatic binoculars, he will have seen
this eect exactly.
i] THE FITZGERALD CONTRACTION 21
It is the reciprocity of these appearances{that each party should think
the other has contracted|that is so dicult to realise. Here is a paradox
beyond even the imagination of Dean Swift. Gulliver regarded the Lilliputians
as a race of dwarfs; and the Lilliputians regarded Gulliver as a
giant. That is natural. If the Lilliputians had appeared dwarfs to Gulliver,
and Gulliver had appeared a dwarf to the Lilliputians|but no! that is
too absurd for ction, and is an idea only to be found in the sober pages
of science.
This reciprocity is easily seen to be a necessary consequence of the Principle
of Relativity. The aviator must detect a FitzGerald contraction of
objects moving rapidly relatively to him, just as we detect the contraction
of objects moving relatively to us, and as an observer at rest in the aether
detects the contraction of objects moving relatively to the aether. Any
other result would indicate an observable eect due to his own motion
through the aether.
Which is right? Are we or the aviator? Or are both the victims of
illusion? It is not illusion in the ordinary sense, because the impressions
of both would be conrmed by every physical test or scientic calculation
suggested. No one knows which is right. No one will ever know, because
we can never nd out which, if either, is truly at rest in the aether.
It is not only in space but in time that these strange variations occur.
If we observed the aviator carefully we should infer that he was unusually
slow in his movements; and events in the conveyance moving with him
would be similarly retarded|as though time had forgotten to go on. His
cigar lasts twice as long as one of ours. I said \infer" deliberately; we
should see a still more extravagant slowing down of time; but that is easily
explained, because the aviator is rapidly increasing his distance from us
and the light-impressions take longer and longer to reach us. The more
moderate retardation referred to remains after we have allowed for the
time of transmission of light.
But here again reciprocity comes in, because in the aviator's opinion it
is we who are travelling at 161; 000 miles a second past him; and when he
has made all allowances, he nds that it is we who are sluggish. Our cigar
lasts twice as long as his.
Let us examine more closely how the two views are to be reconciled.
Suppose we both light similar cigars at the instant he passes us. At the
end of 30 minutes our cigar is nished. This signal, borne on the waves
of light, hurries out at the rate of 186; 000 miles a second to overtake
the aviator travelling at 161; 000 miles a second, who has had 30 minutes
start. It will take nearly 194 minutes to overtake him, giving a total time
22 THE FITZGERALD CONTRACTION [ch.
of 224 minutes after lighting the cigar. His watch like everything else
about him (including his cigar) is going at half-speed; so it records only
112 minutes elapsed when our signal arrives. The aviator knows, of course,
that this is not the true time when our cigar was nished, and that he
must correct for the time of transmission of the light-signal. He sets himself
this problem|that man has travelled away from me at 161; 000 miles a
second for an unknown time x minutes; he has then sent a signal which
travels the same distance back at 186; 000 miles a second; the total time
is 112 minutes; problem, nd x. Answer, x = 60 minutes. He therefore
judges that our cigar lasted 60 minutes, or twice as long as his own. His
cigar lasted 30 minutes by his watch (because the same retardation aects
both watch and cigar); and that was in our opinion twice as long as ours,
because his watch was going at half-speed.
Here is the full time-table.
Stationary
watch
Stationary Observer Aviator Aviator's
watch
0 min. Lights cigar Lights cigar 0 min.
30 '' Finishes cigar . . . 15 ''
60 '' Inferred time aviator's
cigar nished Finishes cigar 30 ''
112 '' Receives signal aviator's
cigar nished . . . 56 ''
120 '' . . . Inferred time stationary
cigar nished 60 ''
224 '' . . . Receives signal stationary
cigar nished 112 ''
This is analysed from our point of view, not the aviator's; because it
makes out that he was wrong in his inference and we were right. But no
one can tell which was really right.
The argument will repay a careful examination, and it will be recognised
that the chief cause of the paradox is that we assume that we are at rest in
the aether, whereas the aviator assumes that he is at rest. Consequently
whereas in our opinion the light-signal is overtaking him at merely the
dierence between 186; 000 and 161; 000 miles a second, he considers that
it is coming to him through the relatively stationary aether at the normal
speed of light. It must be remembered that each observer is furnished
with complete experimental evidence in support of his own assumption.
If we suggest to the aviator that owing to his high velocity the relative
speed of the wave overtaking him can only be 25; 000 miles a second, he
will reply \I have determined the velocity of the wave relatively to me by
timing it as it passes two points in my conveyance; and it turns out to be
i] THE FITZGERALD CONTRACTION 23
186; 000 miles a second. So I know my correction for light-time is right."
His clocks and scales are all behaving in an extraordinary way from our
point of view, so it is not surprising that he should arrive at a measure of
the velocity of the overtaking wave which diers from ours; but there is
no way of convincing him that our reckoning is preferable.
Although not a very practical problem, it is of interest to inquire what
happens when the aviator's speed is still further increased and approximates
to the velocity of light. Lengths in the direction of 
ight become
smaller and smaller, until for the speed of light they shrink to zero. The
aviator and the objects accompanying him shrink to two dimensions. We
are saved the diculty of imagining how the processes of life can go on
in two dimensions, because nothing goes on. Time is arrested altogether.
This is the description according to the terrestrial observer. The aviator
himself detects nothing unusual; he does not perceive that he has stopped
moving. He is merely waiting for the next instant to come before making
the next movement; and the mere fact that time is arrested means that
he does not perceive that the next instant is a long time coming.
It is a favourite device for bringing home the vast distances of the stars
to imagine a voyage through space with the velocity of light. The youthful
adventurer steps on to his magic carpet loaded with provisions for a century.
He reaches his journey's end, say Arcturus, a decrepit centenarian.
This is wrong. It is quite true that the journey would last something like a
hundred years by terrestrial chronology; but the adventurer would arrive
at his destination no more aged than when he started, and he would not
have had time to think of eating. So long as he travels with the speed
of light he has immortality and eternal youth. If in some way his motion
were reversed so that he returned to the earth again, he would nd that
centuries had elapsed here, whilst he himself did not feel a day older|for
him the voyage had lasted only an instanty.
Our reason for discussing at length the eects of these improbably high
We need not stop to prove this directly. If the aviator could detect anything in his
measurements inconsistent with the hypothesis that he was at rest in the aether (e.g.
a dierence of velocity of overtaking waves of light and waves meeting him) it would
contradict the restricted principle of relativity.
ySince the earth is moving relatively to our adventurer with the velocity of light, we
might be tempted to argue that from this point of view the terrestrial observer would
have perpetual youth whilst the voyager grew older. Evidently, if they met again,
they could disprove one or other of the two arguments. But in order to meet again
the velocity of one of them must be reversed by supernatural means or by an intense
gravitational force so that the conditions are not symmetrical and reciprocity does not
apply. The argument given in the text appears to be the correct one.
24 THE FITZGERALD CONTRACTION [ch.
velocities is simply in order that we may speak of the results in terms of
common experience; otherwise it would be necessary to use the terms of
rened technical measurement. The relativist is sometimes suspected of an
inordinate fondness for paradox; but that is rather a misunderstanding of
his argument. The paradoxes exist when the new experimental discoveries
are woven into the scheme of physics hitherto current, and the relativist
is ready enough to point this out. But the conclusion he draws is that a
revised scheme of physics is needed in which the new experimental results
will nd a natural place without paradox.
To sum up|on any planet moving with a great velocity through the
aether, extraordinary changes of length of objects are continually occurring
as they move about, and there is a slowing down of all natural processes as
though time were retarded. These things cannot be perceived by anyone
on the planet; but similar eects would be detected by any observer having
a great velocity relative to the planet (who makes all allowances for the
eect of the motion on the observations, but takes it for granted that he
himself is at rest in the aether). There is complete reciprocity so that each
of two observers in relative motion will nd the same strange phenomena
occurring to the other; and there is nothing to help us to decide which is
right.
I think that no one can contemplate these results without feeling that
the whole strangeness must arise from something perverse and inappropriate
in our ordinary point of view. Changes go on on a planet, all nicely
balanced by adjustments of natural forces, in such a way that no one on the
planet can possibly detect what is taking place. Can we seriously imagine
that there is anything in the reality behind the phenomena, which re
ects
these changes? Is it not more probable that we ourselves introduce the
complexity, because our method of description is not well-adapted to give
a simple and natural statement of what is really occurring?
The search for a more appropriate apparatus of description leads us
to the standpoint of relativity described in the next chapter. I draw a
distinction between the principle and the standpoint of relativity. The
principle of relativity is a statement of experimental fact, which may be
right or wrong; the rst part of it|the restricted principle|has already
been enunciated. Its consequences can be deduced by mathematical rea-
The last clause is perhaps unnecessary. The correction applied for light transmission
will naturally be based on the observer's own experimental determination of
the velocity of light. According to experiment the velocity of light relatively to him
is apparently the same in all directions, and he will apply the corrections accordingly.
This is equivalent to assuming that he is at rest in the aether; but he need not, and
probably would not, make the assumption explicitly.
i] THE FITZGERALD CONTRACTION 25
soning, as in the case of any other scientic generalization. It postulates
no particular mechanism of nature, and no particular view as to the mean-
ing of time and space, though it may suggest theories on the subject. The
only question is whether it is experimentally true or not.
The standpoint of relativity is of a dierent character. It asserts rst
that certain unproved hypotheses as to time and space have insensibly
crept into current physical theories, and that these are the source of the
diculties described above. Now the most dangerous hypotheses are those
which are tacit and unconscious. So the standpoint of relativity proposes
tentatively to do without these hypotheses (not making any others in
their place); and it discovers that they are quite unnecessary and are
not supported by any known fact. This in itself appears to be sucient
justication for the standpoint. Even if at some future time facts should
be discovered which conrm the rejected hypotheses, the relativist is not
wrong in reserving them until they are required.
It is not our policy to take shelter in impregnable positions; and we
shall not hesitate to draw reasonable conclusions as well as absolutely
proved conclusions from the knowledge available. But to those who think
that the relativity theory is a passing phase of scientic thought, which
may be reversed in the light of future experimental discoveries, we would
point out that, though like other theories it may be developed and corrected,
there is a certain minimum statement possible which represents
irreversible progress. Certain hypotheses enter into all physical descriptions
and theories hitherto current, dating back in some cases for 2000
years, in other cases for 200 years. It can now be proved that these hypotheses
have nothing to do with any phenomena yet observed, and do
not aord explanations of any known fact. This is surely a discovery of
the greatest importance|quite apart from any question as to whether the
hypotheses are actually wrong.
I am not satised with the view so often expressed that the sole aim of
scientic theory is \economy of thought." I cannot reject the hope that
theory is by slow stages leading us nearer to the truth of things. But
unless science is to degenerate into idle guessing, the test of value of any
theory must be whether it expresses with as little redundancy as possible
the facts which it is intended to cover. Accidental truth of a conclusion is
no compensation for erroneous deduction.
The relativity standpoint is then a discarding of certain hypotheses,
which are uncalled for by any known facts, and stand in the way of an
understanding of the simplicity of nature.
26 THE FITZGERALD CONTRACTION
CHAPTER II
RELATIVITY
The views of time and space, which I have to set forth, have their foundation
in experimental physics. Therein is their strength. Their tendency is
revolutionary. From henceforth space in itself and time in itself sink to mere
shadows, and only a kind of union of the two preserves an independent existence.
H. Minkowski
(1908).
There are two parties to every observation|the observed and the observer.
What we see depends not only on the object looked at, but on our own
circumstances|position, motion, or more personal idiosyncracies. Sometimes
by instinctive habit, sometimes by design, we attempt to eliminate
our own share in the observation, and so form a general picture of the
world outside us, which shall be common to all observers. A small speck
on the horizon of the sea is interpreted as a giant steamer. From the
window of our railway carriage we see a cow glide past at fty miles an
hour, and remark that the creature is enjoying a rest. We see the starry
heavens revolve round the earth, but decide that it is really the earth that
is revolving, and so picture the state of the universe in a way which would
be acceptable to an astronomer on any other planet.
The rst step in throwing our knowledge into a common stock must be
the elimination of the various individual standpoints and the reduction to
some specied standard observer. The picture of the world so obtained
is none the less relative. We have not eliminated the observer's share; we
have only xed it denitely.
To obtain a conception of the world from the point of view of no one in
particular is a much more dicult task. The position of the observer can
be eliminated; we are able to grasp the conception of a chair as an object in
nature|looked at all round, and not from any particular angle or distance.
We can think of it without mentally assigning ourselves some position
28 RELATIVITY [ch.
with respect to it. This is a remarkable faculty, which has evidently been
greatly assisted by the perception of solid relief with our two eyes. But the
motion of the observer is not eliminated so simply. We had thought that
it was accomplished; but the discovery in the last chapter that observers
with dierent motions use dierent space- and time-reckoning shows that
the matter is more complicated than was supposed. It may well require a
complete change in our apparatus of description, because all the familiar
terms of physics refer primarily to the relations of the world to an observer
in some specied circumstances.
Whether we are able to go still further and obtain a knowledge of the
world, which not merely does not particularise the observer, but does not
postulate an observer at all; whether if such knowledge could be obtained,
it would convey any intelligible meaning; and whether it could be of any
conceivable interest to anybody if it could be understood|these questions
need not detain us now. The answers are not necessarily negative, but they
lie outside the normal scope of physics.
The circumstances of an observer which aect his observations are his
position, motion and gauge of magnitude. More personal idiosyncracies
disappear if, instead of relying on his crude senses, he employs scientic
measuring apparatus. But scientic apparatus has position, motion and
size, so that these are still involved in the results of any observation. There
is no essential distinction between scientic measures and the measures of
the senses. In either case our acquaintance with the external world comes
to us through material channels; the observer's body can be regarded
as part of his laboratory equipment, and, so far as we know, it obeys
the same laws. We therefore group together perceptions and scientic
measures, and in speaking of \a particular observer" we include all his
measuring appliances.
Position, motion, magnitude-scale|these factors have a profound in-

uence on the aspect of the world to us. Can we form a picture of the
world which shall be a synthesis of what is seen by observers in all sorts of
positions, having all sorts of velocities, and all sorts of sizes? As already
stated we have accomplished the synthesis of positions. We have two eyes,
which have dinned into our minds from babyhood that the world has to be
looked at from more than one position. Our brains have so far responded
as to give us the idea of solid relief, which enables us to appreciate the
three-dimensional world in a vivid way that would be scarcely possible
if we were only acquainted with strictly two-dimensional pictures. We
not merely deduce the three-dimensional world; we see it. But we have
no such aid in synthesising dierent motions. Perhaps if we had been
ii] RELATIVITY 29
endowed with two eyes moving with dierent velocities our brains would
have developed the necessary faculty; we should have perceived a kind of
relief in a fourth dimension so as to combine into one picture the aspect
of things seen with dierent motions. Finally, if we had had two eyes of
dierent sizes, we might have evolved a faculty for combining the points
of view of the mammoth and the microbe.
It will be seen that we are not fully equipped by our senses for forming
an impersonal picture of the world. And it is because the deciency is
manifest that we do not hesitate to advocate a conception of the world
which transcends the images familiar to the senses. Such a world can perhaps
be grasped, but not pictured by the brain. It would be unreasonable
to limit our thought of nature to what can be comprised in sense-pictures.
As Lodge has said, our senses were developed by the struggle for existence,
not for the purpose of philosophising on the world.
Let us compare two well-known books, which might be described as elementary
treatises on relativity, Alice in Wonderland and Gulliver's Trav-
els. Alice was continually changing size, sometimes growing, sometimes
on the point of vanishing altogether. Gulliver remained the same size, but
on one occasion he encountered a race of men of minute size with everything
in proportion, and on another voyage a land where everything was
gigantic. It does not require much re
ection to see that both authors are
describing the same phenomenon|a relative change of scale of observer
and observed. Lewis Carroll took what is probably the ordinary scienti
c view, that the observer had changed, rather than that a simultaneous
change had occurred to all her surroundings. But it would never have appeared
like that to Alice; she could not have \stepped outside and looked
at herself," picturing herself as a giant lling the room. She would have
said that the room had unaccountably shrunk. Dean Swift took the truer
view of the human mind when he made Gulliver attribute his own changes
to the things around him; it never occurred to Gulliver that his own size
had altered; and, if he had thought of the explanation, he could scarcely
have accustomed himself to that way of thinking. But both points of view
are legitimate. The size of a thing can only be imagined as relative to
something else; and there is no means of assigning the change to one end
of the relation rather than the other.
We have seen in the theory of the Michelson-Morley experiment that,
according to current physical views, our standard of size|the rigid measuring-
rod|must change according to the circumstances of its motion; and
the aviator's adventures illustrated a similar change in the standard of duration
of time. Certain rather puzzling irregularities have been discovered
30 RELATIVITY [ch.
in the apparent motions of the Sun, Mercury, Venus and the Moon; but
there is a strong family resemblance between these, which leads us to
believe that the real phenomenon is a failure of the time-keeping of our
standard clock, the Earth. Instances could be multiplied where a change
of the observer or his standards produces or conceals changes in the world
around him.
The object of the relativity theory, however, is not to attempt the hopeless
task of apportioning responsibility between the observer and the external
world, but to emphasise that in our ordinary description and in our
scientic description of natural phenomena the two factors are indissolubly
united. All the familiar terms of physics|length, duration of time,
motion, force, mass, energy, and so on|refer primarily to this relative
knowledge of the world; and it remains to be seen whether any of them
can be retained in a description of the world which is not relative to a
particular observer.
Our rst task is a description of the world independent of the motion of
the observer. The question of the elimination of his gauge of magnitude
belongs to a later development of the theory discussed in Chapter xi. Let
us draw a square ABCD on a sheet of paper, making the sides equal, to
the best of our knowledge. We have seen that an aviator 
ying at 161; 000
miles a second in the direction AB, would judge that the sides AB, DC
had contracted to half their length, so that for him the gure would be
an oblong. If it were turned through a right angle AB and DC would
expand and the other two sides contract|in his judgment. For us, the
lengths of AB and AC are equal; for him, one length is twice the other.
Clearly length cannot be a property inherent in our drawing; it needs the
specication of some observer.
We have seen further that duration of time also requires that an observer
should be specied. The stationary observer and the aviator disagreed as
to whose cigar lasted the longer time.
Thus length and duration are not things inherent in the external world;
they are relations of things in the external world to some specied observer.
If we grasp this all the mystery disappears from the phenomena described
in Chapter i. When the rod in the Michelson-Morley experiment is turned
through a right angle it contracts; that naturally gives the impression that
something has happened to the rod itself. Nothing whatever has happened
to the rod|the object in the external world. Its length has altered, but
length is not an intrinsic property of the rod, since it is quite indeterminate
until some observer is specied. Turning the rod through a right angle
has altered the relation to the observer (implied in the discussion of the
ii] RELATIVITY 31
experiment); but the rod itself, or the relation of a molecule at one end
to a molecule at the other, is unchanged. Measurement of length and
duration is a comparison with partitions of space and time drawn by the
observer concerned, with the help of apparatus which shares his motion.
Nature is not concerned with these partitions; it has, as we shall see later,
a geometry of its own which is of a dierent type.
Current physics has hitherto assumed that all observers are not to be
regarded as on the same footing, and that there is some absolute observer
whose judgments of length and duration are to be treated with respect,
because nature pays attention to his space-time partitions. He is supposed
to be at rest in the aether, and the aether materialises his space-partitions
so that they have a real signicance in the external world. This is sheer
hypothesis, and we shall nd it is unsupported by any facts. Evidently our
proper course is to pursue our investigations, and call in this hypothetical
observer only if we nd there is something which he can help to explain.
We have been leading up from the older physics to the new outlook of
relativity, and the reader may feel some doubt as to whether the strange
phenomena of contraction and time-retardation, that were described in the
last chapter, are to be taken seriously, or are part of a reductio ad absurdum
argument. The answer is that we believe that the phenomena do occur
as described; only the description (like that of all observed phenomena)
concerns the relations of the external world to some observer, and not the
external world itself. The startling character of the phenomena arises from
the natural but fallacious inference that they involve intrinsic changes in
the objects themselves.
We have been considering chie
y the observer's end of the observation;
we must now turn to the other end|the thing observed. Although length
and duration have no exact counterparts in the external world, it is clear
that there is a certain ordering of things and events outside us which we
must now nd more appropriate terms to describe. The order of events
is a four-fold order; we can arrange them as right-and-left, backwardsand-
forwards, up-and-down, sooner-and-later. An individual may at rst
consider these as four independent orders, but he will soon attempt to
combine some of them. It is recognised at once that there is no essential
distinction between right-and-left and backwards-and-forwards. The
observer has merely to turn through a right angle and the two are interchanged.
If he turns through a smaller angle, he has rst to combine
them, and then to redivide them in a dierent way. Clearly it would be a
nuisance to continually combine and redivide; so we get accustomed to the
thought of leaving them combined in a two-fold or two-dimensional order.
32 RELATIVITY [ch.
The amalgamation of up-and-down is less simple. There are obvious reasons
for considering this dimension of the world as fundamentally distinct
from the other two. Yet it would have been a great stumbling-block to
science if the mind had refused to combine space into a three-dimensional
whole. The combination has not concealed the real distinction of horizontal
and vertical, but has enabled us to understand more clearly its
nature|for what phenomena it is relevant, and for what irrelevant. We
can understand how an observer in another country redivides the combination
into a dierent vertical and horizontal. We must now go further
and amalgamate the fourth order, sooner-and-later. This is still harder
for the mind. It does not imply that there is no distinction between space
and time; but it gives a fresh unbiassed start by which to determine what
the nature of the distinction is.
The idea of putting together space and time, so that time is regarded as a
fourth dimension, is not new. But until recently it was regarded as merely
a picturesque way of looking at things without any deep signicance. We
can put together time and temperature in a thermometer chart, or pressure
and volume on an indicator-diagram. It is quite non-committal. But
our theory is going to lead much further than that. We can lay two
dimensional surfaces|sheets of paper|on one another till we build up a
three-dimensional block; but there is a dierence between a block which
is a pile of sheets and a solid block of paper. The solid block is the true
analogy for the four-dimensional combination of space-time; it does not
separate naturally into a particular set of three-dimensional spaces piled
in time-order. It can be redivided into such a pile; but it can be redivided
in any direction we please.
Just as the observer by changing his orientation makes a new division of
the two-dimensional plane into right-and-left, backwards-and-forwards|
just as the observer by changing his longitude makes a new division of
three-dimensional space into vertical and horizontal|so the observer by
changing his motion makes a new division of the four-dimensional order
into time and space.
This will be justied in detail later; it indicates that observers with different
motions will have dierent time and space-reckoning|a conclusion
we have already reached from another point of view.
Although dierent observers separate the four orders dierently, they
all agree that the order of events is four-fold; and it appears that this
undivided four-fold order is the same for all observers. We therefore believe
that it is inherent in the external world; it is in fact the synthesis, which
we have been seeking, of the appearances seen by observers having all
ii] RELATIVITY 33
sorts of positions and all sorts of (uniform) motions. It is therefore to be
regarded as a conception of the real world not relative to any particularly
circumstanced observer.
The term \real world" is used in the ordinary sense of physics, without
any intention of prejudging philosophical questions as to reality. It has the
same degree of reality as was formerly attributed to the three-dimensional
world of scientic theory or everyday conception, which by the advance
of knowledge it replaces. As I have already indicated, it is merely the
accident that we are not furnished with a pair of eyes in rapid relative
motion, which has allowed our brains to neglect to develop a faculty for
visualising this four-dimensional world as directly as we visualise its threedimensional
section.
It is now easy to see that length and duration must be the components
of a single entity in the four-dimensional world of space-time. Just as
we resolve a structure into plan and elevation, so we resolve extension in
the four-dimensional world into length and duration. The structure has
a size and shape independent of our choice of vertical. Similarly with
things in space-time. Whereas length and duration are relative, the single
\extension" of which they are components has an absolute signicance in
nature, independent of the particular decomposition into space and time
separately adopted by the observer.
Consider two events; for example, the stroke of one o'clock and the
stroke of two o'clock by Big Ben. These occupy two points in spacetime,
and there is a denite separation between them. An observer at
Westminster considers that they occur at the same place, and that they
are separated by an hour in time; thus he resolves their four-dimensional
separation into zero distance in space and one hour distance in time. An
observer on the sun considers that they do not occur at the same place;
they are separated by about 70; 000 miles, that being the distance travelled
by the earth in its orbital motion with respect to the sun. It is clear that
he is not resolving in quite the same directions as the terrestrial observer,
since he nds the space-component to be 70; 000 miles instead of zero. But
if he alters one component he must necessarily alter the other; so he will
make the time-component dier slightly from an hour. By analogy with
resolution into components in three-dimensions, we should expect him to
make it less than an hour|having, as it were, borrowed from time to make
space; but as a matter of fact he makes it longer. This is because spacetime
has a dierent geometry, which will be described later. Our present
point is that there is but one separation of two events in four dimensions,
which can be resolved in any number of ways into the components length
34 RELATIVITY [ch.
and duration.
We see further how motion must be purely relative. Take two events A
and B in the history of one particle. We can choose any direction as the
time-direction; let us choose it along AB. Then A and B are separated
only in time and not in space, so the particle is at rest. If we choose a
slightly inclined time-direction, the separation AB will have a component
in space; the two events then do not occur at the same place, that is
to say, the particle has moved. The negation of absolute motion is thus
associated with the possibility of choosing the time-direction in any way
we please. What determines the separation of space and time for any
particular observer can now be seen. Let the observer place himself so
that he is, to the best of his knowledge, at rest. If he is a normal human
being, he will seat himself in an arm-chair; if he is an astronomer, he will
place himself on the sun or at the centre of the stellar universe. Then
all the events happening directly to him will in his opinion occur at the
same place. Their separation will have no space-component, and they will
accordingly be ranged solely in the time-direction. This chain of events,
marking his track through the four-dimensional world, will be his timedirection.
Each observer bases his separation of space and time on his own
track through the world.
Since any separation of space and time is admissible, it is possible for
the astronomer to base his space and time on the track of a solar observer
instead of that of a terrestrial observer; but it must be remembered that
in practice the space and time of the solar observer have to be inferred
indirectly from those of the terrestrial observer; and, if the corrections are
made according to the crude methods hitherto employed, they may be
inferred wrongly (if extreme accuracy is needed).
The most formidable objection to this relativist view of the world is the
aether diculty. We have seen that uniform motion through the aether
cannot be detected by experiment, and therefore it is entirely in accordance
with experiment that such motion should have no counterpart in
the four-dimensional world. Nevertheless, it would almost seem that such
motion must logically exist, if the aether exists; and, even at the expense
of formal simplicity, it ought to be exhibited in any theory which pretends
to give a complete account of what is going on in nature. If a substantial
aether analogous to a material ocean exists, it must rigidify, as it were, a
denite space; and whether the observer or whether nature pays any attention
to that space or not, a fundamental separation of space and time
must be there. Some would cut the knot by denying the aether altogether.
We do not consider that desirable, or, so far as we can see, possible; but
ii] RELATIVITY 35
we do deny that the aether need have such properties as to separate space
and time in the way supposed. It seems an abuse of language to speak of a
division existing, when nothing has ever been found to pay any attention
to the division.
Mathematicians of the nineteenth century devoted much time to theories
of elastic solid and other material aethers. Waves of light were supposed to
be actual oscillations of this substance; it was thought to have the familiar
properties of rigidity and density; it was sometimes even assigned a place
in the table of the elements. The real death-blow to this materialistic
conception of the aether was given when attempts were made to explain
matter as some state in the aether. For if matter is vortex-motion or
beknottedness in aether, the aether cannot be matter{some state in itself.
If any property of matter comes to be regarded as a thing to be explained
by a theory of its structure, clearly that property need not be attributed
to the aether. If physics evolves a theory of matter which explains some
property, it stulties itself when it postulates that the same property exists
unexplained in the primitive basis of matter.
Moreover the aether has ceased to take any very active part in physical
theory and has, as it were, gone into reserve. A modern writer on
electromagnetic theory will generally start with the postulate of an aether
pervading all space; he will then explain that at any point in it there is
an electromagnetic vector whose intensity can be measured; henceforth
his sole dealings are with this vector, and probably nothing more will be
heard of the aether itself. In a vague way it is supposed that this vector
represents some condition of the aether, and we need not dispute that
without some such background the vector would scarcely be intelligible|
but the aether is now only a background and not an active participant in
the theory.
There is accordingly no reason to transfer to this vague background of
aether the properties of a material ocean. Its properties must be determined
by experiment, not by analogy. In particular there is no reason to
suppose that it can partition out space in a denite way, as a material
ocean would do. We have seen in the Prologue that natural geometry
depends on laws of matter; therefore it need not apply to the aether. Permanent
identity of particles is a property of matter, which Lord Kelvin
sought to explain in his vortex-ring hypothesis. This abandoned hypothesis
at least teaches us that permanence should not be regarded as axiomatic,
but may be the result of elaborate constitution. There need not
be anything corresponding to permanent identity in the constituent portions
of the aether; we cannot lay our nger at one spot and say \this
36 RELATIVITY [ch.
piece of aether was a few seconds ago over there." Without any continuity
of identity of the aether motion through the aether becomes meaningless;
and it seems likely that this is the true reason why no experiment ever
reveals it.
This modern theory of the relativity of all uniform motion is essentially
a return to the original Newtonian view, temporarily disturbed by the
introduction of aether problems; for in Newton's dynamics uniform motion
of the whole system has not|and no one would expect it to have|any
eect. But there are considerable diculties in the limitation to uniform
motion. Newton himself seems to have appreciated the diculty; but
the experimental evidence appeared to him to be against any extension
of the principle. Accordingly Newton's laws of mechanics are not of the
general type in which it is unnecessary to particularise the observer; they
hold only for observers with a special kind of motion which is described
as \unaccelerated." The only denition of this epithet that can be given
is that an \unaccelerated" observer is one for whom Newton's laws of
motion hold. On this theory, the phenomena are not indierent to an
acceleration or non-uniform motion of the whole system. Yet an absolute
non-uniform motion through space is just as impossible to imagine as
an absolute uniform motion. The partial relativity of phenomena makes
the diculty all the greater. If we deny a fundamental medium with
continuous identity of its parts, motion uniform or non-uniform should
have no signicance; if we admit such a medium, motion uniform or nonuniform
should be detectable; but it is much more dicult to devise a
plan of the world according to which uniform motion has no signicance
and non-uniform motion is signicant.
It is through experiment that we have been led back to the principle
of relativity for uniform motion. In seeking some kind of extension of
this principle to accelerated motion, we are led by the feeling that, having
got so far, it is dicult and arbitrary to stop at this point. We now
try to conceive a system of nature for which all kinds of motion of the
observer are indierent. It will be a completion of our synthesis of what
is perceived by observers having all kinds of motions with respect to one
another, removing the restriction to uniform motion. The experimental
tests must follow after the consequences of this generalisation have been
deduced.
The task of formulating such a theory long appeared impossible. It was
pointed out by Newton that, whereas there is no criterion for detecting
whether a body is at rest or in uniform motion, it is easy to detect whether
it is in rotation. For example the bulge of the earth's equator is a sign
ii] RELATIVITY 37
that the earth is rotating, since a plastic body at rest would be spherical.
This problem of rotation aords a hint as to the cause of the incomplete
relativity of Newtonian mechanics. The laws of motion are formulated
with respect to an unaccelerated observer, and do not apply to a frame of
reference rotating with the earth. Yet mathematicians frequently do use
a rotating frame. Some modication of the laws is then necessary; and
the modication is made by introducing a centrifugal force|not regarded
as a real force like gravitation, but as a mathematical ction employed to
correct for the improper choice of a frame of reference. The bulge of the
earth's equator may be attributed indierently to the earth's rotation or
to the outward pull of the centrifugal force introduced when the earth is
regarded as non-rotating.
Now it is generally assumed that the centrifugal force is something
sui generis, which could always be distinguished experimentally from any
other natural phenomenon. If then on choosing a frame of reference we
nd that a centrifugal force is detected, we can at once infer that the
frame of reference is a \wrong" one; rotating and non-rotating frames
can be distinguished by experiment, and rotation is thus strictly absolute.
But this assumes that the observed eects of centrifugal force cannot be
produced in any other way than by rotation of the observer's frame of reference.
If once it is admitted that centrifugal force may not be completely
distinguishable by experiment from another kind of force|gravitation|
perceived even by Newton's unaccelerated observer, the argument ceases
to apply. We can never determine exactly how much of the observed eld
of force is centrifugal force and how much is gravitation; and we cannot
nd experimentally any denite standard that is to be considered absolutely
non-rotating.
The question then, whether there exists a distinction between \right"
frames of reference and \wrong" frames, turns on whether the use of a
\wrong" frame produces eects experimentally distinguishable from any
natural eects which can be perceived when a \right" frame is used. If
there is no such dierence, all frames may be regarded as on the same
footing and equally right. In that case we can have a complete relativity of
natural phenomena. Since the eect of departing from Newton's standard
frame is the introduction of a eld of force, this generalised relativity
theory must be largely occupied with the nature of elds of force.
The precise meaning of the statement that all frames of reference are
on the same footing is rather dicult to grasp. We believe that there are
absolute things in the world|not only matter, but certain characteristics
in empty space or aether. In the atmosphere a frame of reference which
38 RELATIVITY [ch.
moves with the air is dierentiated from other frames moving in a dierent
manner; this is because, besides discharging the normal functions of
a frame of reference, the air-frame embodies certain of the absolute properties
of the matter existing in the region. Similarly, if in empty space
we choose a frame of reference which more or less follows the lines of the
absolute structure in the region, the frame will usurp some of the absolute
qualities of that structure. What we mean by the equivalence of all frames
is that they are not dierentiated by any qualities formerly supposed to
be intrinsic in the frames themselves|rest, rectangularity, acceleration|
independent of the absolute structure of the world that is referred to them.
Accordingly the objection to attributing absolute properties to Newton's
frame of reference is not that it is impossible for a frame of reference to
acquire absolute properties, but that the Newtonian frame has been laid
down on the basis of relative knowledge without any attempt to follow the
lines of absolute structure.
Force, as known to us observationally, is like the other quantities of
physics, a relation. The force, measured with a spring-balance, for example,
depends on the acceleration of the observer holding the balance;
and the term may, like length and duration, have no exact counterpart in
a description of nature independent of the observer. Newton's view assumes
that there is such a counterpart, an active cause in nature which is
identical with the force perceived by his standard unaccelerated observer.
Although any other observer perceives this force with additions of his own,
it is implied that the original force in nature and the observer's additions
can in some way be separated without ambiguity. There is no experimental
foundation for this separation, and the relativity view is that a eld of
force can, like length and duration, be nothing but a link between nature
and the observer. There is, of course, something at the far end of the
link, just as we found an extension in four dimensions at the far end of
the relations denoted by length and duration. We shall have to study the
nature of this unknown whose relation to us appears as force. Meanwhile
we shall realise that the alteration of perception of force by non-uniform
motion of the observer, as well as the alteration of the perception of length
by his uniform motion, is what might be expected from the nature of these
quantities as relations solely.
We proceed now to a more detailed study of the four-dimensional world,
of the things which occur in it, and of the laws by which they are regulated.
It is necessary to dive into this absolute world to seek the truth about
nature; but the physicist's object is always to obtain knowledge which
can be applied to the relative and familiar aspect of the world. The
ii] RELATIVITY 39
absolute world is of so dierent a nature, that the relative world, with
which we are acquainted, seems almost like a dream. But if indeed we
are dreaming, our concern is with the baseless fabric of our vision. We
do not suggest that physicists ought to translate their results into terms
of four-dimensional space for the empty satisfaction of working in the
realm of reality. It is rather the opposite. They explore the new eld and
bring back their spoils|a few simple generalisations|to apply them to
the practical world of three-dimensions. Some guiding light will be given
to the attempts to build a scheme of things entire. For the rest, physics
will continue undisturbedly to explore the relative world, and to employ
the terms applicable to relative knowledge, but with a fuller appreciation
of its relativity.
40 RELATIVITY
CHAPTER III
THE WORLD OF FOUR DIMENSIONS
Here is a portrait of a man at eight years old, another at fteen, another at
seventeen, another at twenty-three, and so on. All these are evidently sections,
as it were, Three-Dimensional representations of his Four-Dimensional being,
which is a xed and unalterable thing. H. G. Wells, The Time Machine.
The distinction between horizontal and vertical is not an illusion; and the
man who thinks it can be disregarded is likely to come to an untimely end.
Yet we cannot arrive at a comprehensive view of nature unless we combine
horizontal and vertical dimensions into a three-dimensional space. By
doing this we obtain a better idea of what the distinction of horizontal and
vertical really is in those cases where it is relevant, e.g. the phenomena of
motion of a projectile. We recognise also that vertical is not a universally
dierentiated direction in space, as the 
at-earth philosophers might have
imagined.
Similarly by combining the time-ordering and space-ordering of the
events of nature into a single order of four dimensions, we shall not only
obtain greater simplicity for the phenomena in which the separation of
time and space is irrelevant, but we shall understand better the nature of
the dierentiation when it is relevant.
A point in this space-time, that is to say a given instant at a given
place, is called an \event." An event in its customary meaning would be
the physical happening which occurs at and identies a particular place
and time. However, we shall use the word in both senses, because it is
scarcely possible to think of a point in space-time without imagining some
identifying occurrence.
In the ordinary geometry of two or three dimensions, the distance between
two points is something which can be measured, usually with a
rigid scale; it is supposed to be the same for all observers, and there is no
need to specify horizontal and vertical directions or a particular system of
42 THE WORLD OF FOUR DIMENSIONS [ch.
coordinates. In four-dimensional space-time there is likewise a certain extension
or generalised distance between two events, of which the distance
in space and the separation in time are particular components. This extension
in space and time combined is called the \interval" between the
two events; it is the same for all observers, however they resolve it into
space and time separately. We may think of the interval as something
intrinsic in external nature|an absolute relation of the two events, which
postulates no particular observer. Its practical measurement is suggested
by analogy with the distance of two points in space.
Fig. 2.
O X1 X2 x
y
Y1
Y2
M
P1
P2
In two dimensions on a plane, two points P1,
P2 (Fig. 2) can be specied by their rectangular
coordinates (x1; y1) and (x2; y2), when arbitrary
axes have been selected. In the gure,
OX1 = x1, OY1 = y1, etc. We have
P1P2
2 = P1M2 +MP2
2
= X1X2
2 + Y1Y2
2
= (x2 􀀀 x1)2 + (y2 􀀀 y1)2;
so that if s is the distance between P1 and P2
s2 = (x2 􀀀 x1)2 + (y2 􀀀 y1)2:
The extension to three dimensions is, as we should expect,
s2 = (x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2:
Introducing the times of the events t1, t2, we should naturally expect that
the interval in the four-dimensional world would be given by
s2 = (x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2 + (t2 􀀀 t1)2:
An important point arises here. It was, of course, assumed that the
same scale was used for measuring x and y and z. But how are we to use
the same scale for measuring t? We cannot use a scale at all; some kind
of clock is needed. The most natural connection between the measure of
time and length is given by the fact that light travels 300; 000 kilometres
in 1 second. For the four-dimensional world we shall accordingly regard
1 second as the equivalent of 300; 000 kilometres, and measure lengths and
times in seconds or kilometres indiscriminately; in other words we make
the velocity of light the unit of velocity. It is not essential to do this, but
it greatly simplies the discussion.
iii] THE WORLD OF FOUR DIMENSIONS 43
Secondly, the formulae here given for s2 are the characteristic formulae
of Euclidean geometry. So far as three-dimensional space is concerned
the applicability of Euclidean geometry is very closely conrmed by experiment.
But space-time is not Euclidean; it does, however, conform (at
least approximately) to a very simple modication of Euclidean geometry
indicated by the corrected formula
s2 = (x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2 􀀀 (t2 􀀀 t1)2:
There is only a sign altered; but that minus sign is the secret of the
dierences of the manifestations of time and space in nature.
This change of sign is often found puzzling at the start. We could not
dene s by the expression originally proposed (with the positive sign), because
the expression does not dene anything objective. Using the space
and time of one observer, one value is obtained; for another observer, another
value is obtained. But if s is dened by the expression now given, it
is found that the same result is obtained by all observers. The quantity s
is thus something which concerns solely the two events chosen; we give it a
name|the interval between the two events. In ordinary space the distance
between two points is the corresponding property, which concerns only the
two points and not the extraneous coordinate system of location which is
used. Hence interval, as here dened, is the analogue of distance; and the
analogy is strengthened by the evident resemblance of the formula for s in
both cases. Moreover, when the dierence of time vanishes, the interval
reduces to the distance. But the discrepancy of sign introduces certain
important dierences. These dierences are summed up in the statement
that the geometry of space is Euclidean, but the geometry of space-time
is semi-Euclidean or \hyperbolic." The association of a geometry with
any continuum always implies the existence of some uniquely measurable
quantity like interval or distance; in ordinary space, geometry without the
idea of distance would be meaningless.
For the moment the diculty of thinking in terms of an unfamiliar
geometry may be evaded by a dodge. Instead of real time t, consider
imaginary time  ; that is to say, let
t = p􀀀1:
Then (t2 􀀀 t1)2 = 􀀀(2 􀀀 1)2;
so that
s2 = (x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2 + (2 􀀀 1)2:
Appendix, Note 2.
44 THE WORLD OF FOUR DIMENSIONS [ch.
Everything is now symmetrical and there is no distinction between  and
the other variables. The continuum formed of space and imaginary time
is completely isotropic for all measurements; no direction can be picked
out in it as fundamentally distinct from any other.
The observer's separation of this continuum into space and time consists
in slicing it in some direction, viz. that perpendicular to the path along
which he is himself travelling. The section gives three-dimensional space
at some moment, and the perpendicular dimension is (imaginary) time.
Clearly the slice may be taken in any direction; there is no question of a
true separation and a ctitious separation. There is no conspiracy of the
forces of nature to conceal our absolute motion|because, looked at from
this broader point of view, there is nothing to conceal. The observer is at
liberty to orient his rectangular axes of x, y, z and  arbitrarily, just as
in three-dimensions he can orient his axes of x, y, z arbitrarily.
It can be shown that the dierent space and time used by the aviator
in Chapter i correspond to an orientation of the time-axis along his own
course in the four-dimensional world, whereas the ordinary time and space
are given when the time-axis is oriented along the course of a terrestrial observer.
The FitzGerald contraction and the change of time-measurement
are given exactly by the usual formulae for rotation of rectangular axes.
It is not very protable to speculate on the implication of the mysterious
factor p􀀀1, which seems to have the property of turning time into space.
It can scarcely be regarded as more than an analytical device. To follow
out the theory of the four-dimensional world in more detail, it is necessary
to return to real time, and face the diculties of a strange geometry.
Consider a particular observer, S, and represent time according to his
reckoning by distance up the page parallel to OT. One dimension of his
space will be represented by horizontal distance parallel to OX; another
will stand out at right angles from the page; and the reader must imagine
the third as best he can. Fortunately it will be sucient for us to consider
only the one dimension of space OX and deal with the phenomena of
\line-land," i.e. we limit ourselves to motion to and fro in one straight line
in space.
The two lines U0OU, V 0OV , at 45° to the axes, represent the tracks of
points which progress 1 unit horizontally (in space) for 1 unit vertically
(in time); thus they represent points moving with unit velocity. We have
chosen the velocity of light as unit velocity; hence U0OU, V 0OV will be
the tracks of pulses of light in opposite directions along the straight line.
Appendix, Note 3.
iii] THE WORLD OF FOUR DIMENSIONS 45
Fig. 3.
O
X
T
U
U′
V
V ′
P
P′
N
Any event P within the sector UOV is indubitably after the event O,
whatever system of time-reckoning is adopted. For it would be possible
for a material particle to travel from O to P, the necessary velocity being
less than that of light; and no rational observer would venture to state
that the particle had completed its journey before it had begun it. It
would, in fact, be possible for an observer travelling along NP to receive
a light-signal or wireless telegram announcing the event O, just as he
reached N, since ON is the track of such a message; and then after the
time NP he would have direct experience of the event P. To have actual
evidence of the occurrence of one event before experiencing the second is
a clear proof of their absolute order in nature, which should convince not
merely the observer concerned but any other observer with whom he can
communicate.
Similarly events in the sector U0OV 0 are indubitably before the event O.
With regard to an event P0 in the sector UOV 0 or V OU0 we cannot assert
that it is absolutely before or after O. According to the time-reckoning of
our chosen observer S, P0 is after O, because it lies above the line OX;
but there is nothing absolute about this. The track OP0 corresponds to
a velocity greater than that of light, so that we know of no particle or
physical impulse which could follow the track. An observer experiencing
46 THE WORLD OF FOUR DIMENSIONS [ch.
the event P0 could not get news of the event O by any known means until
after P0 had happened. The order of the two events can therefore only
be inferred by estimating the delay of the message and this estimate will
depend on the observer's mode of reckoning space and time.
Space-time is thus divided into three zones with respect to the event O.
U0OV 0 belongs to the indubitable past. UOV is the indubitable future.
UOV 0 and V OU0 are (absolutely) neither past nor future, but simply
\elsewhere." It may be remarked that, as we have no means of identifying
points in space as \the same point," and as the events O and P might quite
well happen to the same particle of matter, the events are not necessarily
to be regarded as in dierent places, though the observer S will judge
them so; but the events O and P0 cannot happen to the same particle,
and no observer could regard them as happening at the same place. The
main interest of this analysis is that it shows that the arbitrariness of
time-direction is not inconsistent with the existence of regions of absolute
past and future.
Although there is an absolute past and future, there is between them
an extended neutral zone; and simultaneity of events at dierent places
has no absolute meaning. For our selected observer all events along OX
are simultaneous with one another; for another observer the line of events
simultaneous with O would lie in a dierent direction. The denial of
absolute simultaneity is a natural complement to the denial of absolute
motion. The latter asserts that we cannot nd out what is the same place
at two dierent times; the former that we cannot nd out what is the
same time at two dierent places. It is curious that the philosophical
denial of absolute motion is readily accepted, whilst the denial of absolute
simultaneity appears to many people revolutionary.
The division into past and future (a feature of time-order which has no
analogy in space-order) is closely associated with our ideas of causation
and free will. In a perfectly determinate scheme the past and future may
be regarded as lying mapped out|as much available to present exploration
as the distant parts of space. Events do not happen; they are just there,
and we come across them. \The formality of taking place" is merely the
indication that the observer has on his voyage of exploration passed into
the absolute future of the event in question; and it has no important
signicance. We can be aware of an eclipse in the year 1999, very much as
we are aware of an unseen companion to Algol. Our knowledge of things
where we are not, and of things when we are not, is essentially the same|
an inference (sometimes a mistaken inference) from brain impressions,
including memory, here and now.
iii] THE WORLD OF FOUR DIMENSIONS 47
So, if events are determinate, there is nothing to prevent a person from
being aware of an event before it happens; and an event may cause other
events previous to it. Thus the eclipse of the Sun in May 1919 caused
observers to embark in March. It may be said that it was not the eclipse,
but the calculations of the eclipse, which caused the embarkation; but I
do not think any such distinction is possible, having regard to the indirect
character of our acquaintance with all events except those at the precise
point of space where we stand. A detached observer contemplating our
world would see some events apparently causing events in their future,
others apparently causing events in their past|the truth being that all
are linked by determinate laws, the so-called causal events being merely
conspicuous foci from which the links radiate.
The recognition of an absolute past and future seems to depend on the
possibility of events which are not governed by a determinate scheme. If,
say, the event O is an ultimatum, and the person describing the path NP
is a ruler of the country aected, then it may be manifest to all observers
that it is his knowledge of the actual occurrence of the event O which has
caused him to create the event P. P must then be in the absolute future
Fig. 4.
O X
X1
T T1
U
U′
V
V ′
L
L′
F
G
H
M K
K′ M′
of O, and, as we have seen, must lie in the sector UOV . But the inference
48 THE WORLD OF FOUR DIMENSIONS [ch.
is only permissible, if the event P could be determined by the event O, and
was not predetermined by causes anterior to both|if it was possible for it
to happen or not, consistently with the laws of nature. Since physics does
not attempt to cover indeterminate events of this kind, the distinction of
absolute past and future is not directly important for physics; but it is of
interest to show that the theory of four-dimensional space-time provides
an absolute past and future, in accordance with common requirements,
although this can usually be ignored in applications to physics.
Consider now all the events which are at an interval of one unit from O,
according to the denition of the interval s
s2 = 􀀀(x2 􀀀 x1)2 􀀀 (y2 􀀀 y1)2 􀀀 (z2 􀀀 z1)2 + (t2 􀀀 t1)2: (1)
We have changed the sign of s2, because usually (though not always) the
original s2 would have come out negative. In Euclidean space points distant
a unit interval lie on a circle; but, owing to the change in geometry
due to the altered sign of (t2 􀀀 t1)2, they now lie on a rectangular hyperbola
with two branches KLM, K0L0M0. Since the interval is an absolute
quantity, all observers will agree that these points are at unit interval
from O.
Now make the following construction:|draw a straight line OFT1 to
meet the hyperbola in F; draw the tangent FG at F, meeting the lightline
U0OU in G; complete the parallelogram OFGH; produce OH to X1.
We now assert that an observer S1 who chooses OT1 for his time-direction
will regard OX1 as his space direction and will consider OF and OH to
be the units of time and space.
The two observers make their partitions of space and time in dierent
ways, as illustrated in Figs. 5 and 6, where in each case the partitions
are at unit distance (in space and time) according to the observers' own
reckoning. The same diagram of events in the world will serve for both
observers; S1 merely removes S's partitions and overlays his own, locating
the events in his space and time accordingly. It will be seen at once
that the lines of unit velocity|progress of one unit of space for one unit
of time|agree, so that the velocity of a pulse of light is unity for both
observers. It can be shown from the properties of the hyperbola that the
locus of points at any interval s from O, given by equation (1), viz.
s2 = (t 􀀀 t0)2 􀀀 (x 􀀀 x0)2;
is the same locus (a hyperbola) for both systems of reckoning x and t.
The two observers will always agree on the measures of intervals, though
iii] THE WORLD OF FOUR DIMENSIONS 49
they will disagree about lengths, durations, and the velocities of everything
except light. This rather complex transformation is mathematically
equivalent to the simple rotation of the axes required when imaginary time
is used.
It must not be supposed that there is any natural distinction corresponding
to the dierence between the square-partitions of observer S
and the diamond-shaped partitions of observer S1. We might say that S1
transplants the space-time world unchanged from Fig. 5 to Fig. 6, and then
distorts it until the diamonds shown become squares; or we might equally
well start with this distorted space-time, partitioned by S1 into squares,
and then S's partitions would be represented by diamonds. It cannot be
said that either observer's space-time is distorted absolutely, but one is
distorted relatively to the other. It is the relation of order which is intrinsic
in nature, and is the same both for the squares and diamonds; shape
is put into nature by the observer when he has chosen his partitions.
Fig. 5. Fig. 6.
O X
T
O
X1
T1
We can now deduce the FitzGerald contraction. Consider a rod of unit
length at rest relatively to the observer S. The two extremities are at
rest in his space, and consequently remain on the same space-partitions;
hence their tracks in four dimensions PP0, QQ0 (Fig. 7) are entirely in
the time-direction. The real rod in nature is the four-dimensional object
shown in section as P0PQQ0. Overlay the same gure with S1's space and
time partitions, shown by the dotted lines. Taking a section at any one
\time," the instantaneous rod is P1Q1, viz. the section of P0PQQ0 by S1's
time-line. Although on paper P1Q1 is actually longer than PQ, it is seen
50 THE WORLD OF FOUR DIMENSIONS [ch.
that it is a little shorter than one of S1's space-partitions; and accordingly
S1 judges that it is less than one unit long|it has contracted on account
of its motion relative to him.
Fig. 7.
P Q
P′ Q′
P1
Q1
R
S
R′
S′
R1 S1
Similarly RR0 􀀀 SS0 is a rod of unit length at rest relatively to S1.
Overlaying S's partitions we see that it occupies R1S1 at a particular
instant for S; and this is less than one of S's partitions. Thus S judges it
to have contracted on account of its motion relative to him.
Fig. 8.
M
L
N
N′
L′
M′
In the same way we can illustrate
the problem of the duration
of the cigar; each observer believed
the other's cigar to last the longer
time. Taking LM (Fig. 8) to represent
the duration of S's cigar (two
units), we see that in S1's reckoning
it reaches over a little more than two
time-partitions. Moreover it has not
kept to one space-partition, i.e. it
has moved. Similarly L0N0 is the duration
of S1's cigar (two time-units
for him); and it lasts a little beyond two unit-partitions in S's timereckoning.
(Note, in comparing the two diagrams, L0;M0;N0 are the same
points as L;M;N.)
iii] THE WORLD OF FOUR DIMENSIONS 51
If in Fig. 4 we had taken the line OT1 very near to OU, our diamonds
would have been very elongated, and the unit-divisions OF;OH very large.
This kind of partition would be made by an observer whose course through
the world is OT1, and who is accordingly travelling with a velocity approaching
that of light relative to S. In the limit, when the velocity
reaches that of light, both space-unit and time-unit become innite, so
that in the natural units for an observer travelling with the speed of light,
all the events in the nite experience of S take place \in no time" and the
size of every object is zero. This applies, however, only to the two dimensions
x and t; the space-partitions parallel to the plane of the paper are
not aected by this motion along x. Consequently for an observer travelling
with the speed of light all ordinary objects become two-dimensional,
preserving their lateral dimensions, but innitely thin longitudinally. The
fact that events take place \in no time" is usually explained by saying
that the inertia of any particle moving with the velocity of light becomes
innite so that all molecular processes in the observer must stop; many
things may happen in S's world in a twinkling of an eye|of S1's eye.
However successful the theory of a four-dimensional world may be, it is
dicult to ignore a voice inside us which whispers \At the back of your
mind, you know that a fourth dimension is all nonsense." I fancy that that
voice must often have had a busy time in the past history of physics. What
nonsense to say that this solid table on which I am writing is a collection of
electrons moving with prodigious speeds in empty spaces, which relatively
to electronic dimensions are as wide as the spaces between the planets in
the solar system! What nonsense to say that the thin air is trying to crush
my body with a load of 14 lbs. to the square inch! What nonsense that
the star-cluster, which I see through the telescope obviously there now, is
a glimpse into a past age 50; 000 years ago! Let us not be beguiled by this
voice. It is discredited.
But the statement that time is a fourth dimension may suggest unnecessary
diculties which a more precise denition avoids. It is in the external
world that the four dimensions are united|not in the relations of the external
world to the individual which constitute his direct acquaintance
with space and time. Just in that process of relation to an individual, the
order falls apart into the distinct manifestations of space and time. An individual
is a four-dimensional object of greatly elongated form; in ordinary
language we say that he has considerable extension in time and insigni-
cant extension in space. Practically he is represented by a line|his track
through the world. When the world is related to such an individual, his
own asymmetry is introduced into the relation; and that order of events
52 THE WORLD OF FOUR DIMENSIONS [ch.
which is parallel with his track, that is to say with himself, appears in his
experience to be dierentiated from all other orders of events.
Probably the best known exposition of the fourth dimension is that
given in E. Abbott's popular book Flatland. It may be of interest to
see how far the four-dimensional world of space-time conforms with his
anticipations. He lays stress on three points.
(1) As a four-dimensional body moves, its section by the three-dimensional
world may vary; thus a rigid body can alter size and shape.
(2) It should be possible for a body to enter a completely closed room,
by travelling into it in the direction of the fourth dimension, just as we can
bring our pencil down on to any point within a square without crossing
its sides.
(3) It should be possible to see the inside of a solid, just as we can see
the inside of a square by viewing it from a point outside its plane.
The rst phenomenon is manifested by the FitzGerald contraction.
If quantity of matter is to be identied with its mass, the second phenomenon
does not happen. It could easily be conceived of as happening,
but it is provided against by a special law of nature|the conservation of
mass. It could happen, but it does not happen.
The third phenomenon does not happen for two reasons. A natural body
extends in time as well as in space, and is therefore four-dimensional; but
for the analogy to hold, the object must have one dimension less than
the world, like the square seen from the third dimension. If the solid
suddenly went out of existence so as to present a plane section towards
time, we should still fail to see the interior of it; because light-tracks in
four-dimensions are restricted to certain lines like UOV; U0OV 0 in Fig. 3,
whereas in three-dimensions light can traverse any straight line. This
could be remedied by interposing some kind of dispersive medium, so that
light of some wave-length could be found travelling with every velocity
and following every track in space-time; then, looking at a solid which
suddenly went out of existence, we should receive at the same moment
light-impressions from every particle in its interior (supposing them selfluminous).
We actually should see the inside of it.
How our poor eyes are to disentangle this overwhelming experience is
quite another question.
The interval is a quantity so fundamental for us that we may consider
its measurement in some detail. Suppose we have a scale AB divided
into kilometres, say, and at each division is placed a clock also registering
kilometres. (It will be remembered that time can be measured in seconds
or kilometres indierently.) When the clocks are correctly set and viewed
iii] THE WORLD OF FOUR DIMENSIONS 53
Fig. 9.
A 1 2 3 4 5 B
from A the sum of the readings of any clock and the division beside it is
the same for all, since the scale-reading gives the correction for the time
taken by light, travelling with unit velocity, to reach A. This is shown
in Fig. 9 where the clock-readings are given as though they were being
viewed from A.
Now lay the scale in line with the two events; note the clock and scalereadings
t1; x1, of the rst event, and the corresponding readings t2; x2, of
the second event. Then by the formula already given
s2 = (t2 􀀀 t1)2 􀀀 (x2 􀀀 x1)2:
But suppose we took a dierent standard of rest, and set the scale moving
uniformly in the direction AB. Then the divisions would have advanced
to meet the second event, and (x2􀀀x1) would be smaller. This is compensated,
because t2 􀀀 t1 also becomes altered. A is now advancing to meet
the light coming from any of the clocks along the rod; the light arrives too
quickly, and in the initial adjustment described above the clock must be
set back a little. The clock-reading of the event is thus smaller. There are
other small corrections arising from the FitzGerald contraction, etc.; and
the net result is that, it does not matter what uniform motion is given to
the scale, the nal result for s is always the same.
In elementary mechanics we are taught that velocities can be compounded
by adding. If B's velocity relative to A (as observed by either of
them) is 100 km. per sec., and C's velocity relative to B is 100 km. per sec.
in the same direction, then C's velocity relative to A should be 200 km.
per sec. This is not quite accurate; the true answer is 199:999978 km. per
sec. The discrepancy is not dicult to explain. The two velocities and
their resultant are not all reckoned with respect to the same partitions of
space and time. When B measures C's velocity relative to him he uses
his own space and time, and it must be corrected to reduce to A's space
and time units, before it can be added on to a velocity measured by A.
54 THE WORLD OF FOUR DIMENSIONS [ch.
If we continue the chain, introducing D whose velocity relative to C,
and measured by C, is 100 km. per sec., and so on ad innitum, we never
obtain an innite velocity with respect to A, but gradually approach the
limiting velocity of 300; 000 km. per sec., the speed of light. This speed
has the remarkable property of being absolute, whereas every other speed
is relative. If a speed of 100 km. per sec. or of 100; 000 km. per sec. is
mentioned, we have to ask|speed relative to what? But if a speed of
300; 000 km. per sec. is mentioned, there is no need to ask the question;
the answer is|relative to any and every piece of matter. A  particle shot
o from radium can move at more than 200; 000 km. per sec.; but the speed
of light relative to an observer travelling with it is still 300; 000 km. per
sec. It reminds us of the mathematicians' transnite number Aleph; you
can subtract any number you like from it, and it still remains the same.
The velocity of light plays a conspicuous part in the relativity theory,
and it is of importance to understand what is the property associated
with it which makes it fundamental. The fact that the velocity of light
is the same for all observers is a consequence rather than a cause of its
pre-eminent character. Our rst introduction of it, for the purpose of coordinating
units of length and time, was merely conventional with a view to
simplifying the algebraic expressions. Subsequently, considerable use has
been made of the fact that nothing is known in physics which travels with
greater speed, so that in practice our determinations of simultaneity depend
on signals transmitted with this speed. If some new kind of ray with
a higher speed were discovered, it would perhaps tend to displace lightsignals
and light-velocity in this part of the work, time-reckoning being
modied to correspond; on the other hand, this would lead to greater complexity
in the formulae, because the FitzGerald contraction which aects
space-measurement depends on light-velocity. But the chief importance
of the velocity of light is that no material body can exceed this velocity.
This gives a general physical distinction between paths which are time-like
and space-like, respectively|those which can be traversed by matter, and
those which cannot. The material structure of the four-dimensional world
is brous, with the threads all running along time-like tracks; it is a tangled
warp without a woof. Hence, even if the discovery of a new ray led us
to modify the reckoning of time and space, it would still be necessary in
the study of material systems to preserve the present absolute distinction
of time-like and space-like intervals, under a new name if necessary.
It may be asked whether it is possible for anything to have a speed
greater than the velocity of light. Certainly matter cannot attain a greater
speed; but there might be other things in nature which could. \Mr
iii] THE WORLD OF FOUR DIMENSIONS 55
Speaker," said Sir Boyle Roche, \not being a bird, I could not be in
two places at the same time." Any entity with a speed greater than light
would have the peculiarity of Sir Boyle Roche's bird. It can scarcely be
said to be a self-contradictory property to be in two places at the same
time any more than for an object to be at two times in the same place.
The perplexities of the quantum theory of energy sometimes seem to suggest
that the possibility ought not to be overlooked; but, on the whole,
the evidence seems to be against the existence of anything moving with a
speed beyond that of light.
The standpoint of relativity and the principle of relativity are quite
independent of any views as to the constitution of matter or light. Hitherto
our only reference to electrical theory has been in connection with Larmor
and Lorentz's explanation of the FitzGerald contraction; but now from the
discussion of the four-dimensional world, we have found a more general
explanation of the change of length. The case for the electrical theory of
matter is actually weakened, because many experimental eects formerly
thought to depend on the peculiar properties of electrical forces are now
found to be perfectly general consequences of the relativity of observational
knowledge.
Whilst the evidence for the electrical theory of matter is not so conclusive,
as at one time appeared, the theory may be accepted without
serious misgivings. To postulate two entities, matter and electric charges,
when one will suce is an arbitrary hypothesis, unjustiable in our present
state of knowledge. The great contribution of the electrical theory to this
subject is a precise explanation of the property of inertia. It was shown
theoretically by J. J. Thomson that if a charged conductor is to be moved
or stopped, additional eort will be necessary simply on account of the
charge. The conductor has to carry its electric eld with it, and force is
needed to set the eld moving. This property is called inertia, and it is
measured by mass. If, keeping the charge constant, the size of the conductor
is diminished, this inertia increases. Since the smallest separable
particles of matter are found by experiment to be very minute and to carry
charges, the suggestion arises that these charges may be responsible for
the whole of the inertia detected in matter. The explanation is sucient;
and there seems no reason to doubt that all inertia is of this electrical
kind.
When the calculations are extended to charges moving with high velocities,
it is found that the electrical inertia is not strictly constant but
depends on the speed; in all cases the variation is summed up in the statement
that the inertia is simply proportional to the total energy of the
56 THE WORLD OF FOUR DIMENSIONS
electromagnetic eld. We can say if we like that the mass of a charged
particle at rest belongs to its electrostatic energy; when the charge is set
in motion, kinetic energy is added, and this kinetic energy also has mass.
Hence it appears that mass (inertia) and energy are essentially the same
thing, or, at the most, two aspects of the same thing. It must be remembered
that on this view the greater part of the mass of matter is due to
concealed energy, which is not as yet releasable.
The question whether electrical energy not bound to electric charges
has mass, is answered in the armative in the case of light. Light has
mass. Presumably also gravitational energy has mass; or, if not, mass will
be created when, as often happens, gravitational energy is converted into
kinetic energy. The mass of the whole (negative) gravitational energy of
the earth is of the order minus a billion tons.
The theoretical increase of the mass of an electron with speed has been
conrmed experimentally, the agreement with calculation being perfect if
the electron undergoes the FitzGerald contraction by its motion. This has
been held to indicate that the electron cannot have any inertia other than
that due to the electromagnetic eld carried with it. But the conclusion
(though probable enough) is not a fair inference; because these results,
obtained by special calculation for electrical inertia, are found to be predicted
by the theory of relativity for any kind of inertia. This will be
shown in Chapter ix. The factor giving the increase of mass with speed
is the same as that which aects length and time. Thus if a rod moves at
such a speed that its length is halved, its mass will be doubled. Its density
will be increased four-fold, since it is both heavier and less in volume.
We have thought it necessary to include this brief summary of the electrical
theory of matter and mass, because, although it is not required by
the relativity theory, it is so universally accepted in physics that we can
scarcely ignore it. Later on we shall reach in a more general way the identi
cation of mass with energy and the variation of mass with speed; but,
since the experimental measurement of inertia involves the study of a body
in non-uniform motion, it is not possible to enter on a satisfactory discussion
of mass until the more general theory of relativity for non-uniform
motion has been developed.
CHAPTER IV
FIELDS OF FORCE
For whenever bodies fall through water and thin air, they must quicken their
descents in proportion to their weights, because the body of water and subtle
nature of air cannot retard everything in equal degree, but more readily give
way overpowered by the heavier; on the other hand empty void cannot oer
resistance to anything in any direction at any time, but must, as its nature
craves, continually give way; and for this reason all things must be moved
and borne along with equal velocities though of unequal weights through the
unresisting void. Lucretius, De Natura Rerum.
The primary conception of force is associated with the muscular sensation
felt when we make an eort to cause or prevent the motion of matter. Similar
eects on the motion of matter can be caused by non-living agency,
and these also are regarded as due to forces. As is well known, the scienti
c measure of a force is the momentum that it communicates to a
body in given time. There is nothing very abstract about a force transmitted
by material contact; modern physics shows that the momentum
is communicated by a process of molecular bombardment. We can visualise
the mechanism, and see the molecules carrying the motion in small
parcels across the boundary into the body that is being acted on. Force
is no mysterious agency; it is merely a convenient summary of this 
ow
of motion, which we can trace continuously if we take the trouble. It is
true that the diculties are only set back a stage, and the exact mode by
which the momentum is redistributed during a molecular collision is not
yet understood; but, so far as it goes, this analysis gives a clear idea of
the transmission of motion by ordinary forces.
But even in elementary mechanics an important natural force appears,
which does not seem to operate in this manner. Gravitation is not resolvable
into a succession of molecular blows. A massive body, such as the
earth, seems to be surrounded by a eld of latent force, ready, if another
58 FIELDS OF FORCE [ch.
body enters the eld, to become active, and transmit motion. One usually
thinks of this in
uence as existing in the space round the earth even
when there is no test-body to be aected, and in a rather vague way it is
suspected to be some state of strain or other condition of an unperceived
medium.
Although gravitation has been recognised for thousands of years, and
its laws were formulated with sucient accuracy for almost all purposes
more than 200 years ago, it cannot be said that much progress has been
made in explaining the nature or mechanism of this in
uence. It is said
that more than 200 theories of gravitation have been put forward; but
the most plausible of these have all had the defect that they lead nowhere
and admit of no experimental test. Many of them would nowadays be
dismissed as too materialistic for our taste|lling space with the hum of
machinery|a procedure curiously popular in the nineteenth century. Few
would survive the recent discovery that gravitation acts not only on the
molecules of matter, but on the undulations of light.
The nature of gravitation has seemed very mysterious, yet it is a remarkable
fact that in a limited region it is possible to create an articial
eld of force which imitates a natural gravitational eld so exactly that,
so far as experiments have yet gone, no one can tell the dierence. Those
who seek for an explanation of gravitation naturally aim to nd a model
which will reproduce its eects; but no one before Einstein seems to have
thought of nding the clue in these articial elds, familiar as they are.
When a lift starts to move upwards the occupants feel a characteristic
sensation, which is actually identical with a sensation of increased
weight. The feeling disappears as soon as the motion becomes uniform;
it is associated only with the change of motion of the lift, that is to say,
the acceleration. Increased weight is not only a matter of sensation; it is
shown by any physical experiments that can be performed. The usual laboratory
determination of the value of gravity by Atwood's machine would,
if carried out inside the accelerated lift, give a higher value. A springbalance
would record higher weights. Projectiles would follow the usual
laws of motion but with a higher value of gravity. In fact, the upward
acceleration of the lift is in its mechanical eects exactly similar to an
additional gravitational eld superimposed on that normally present.
Perhaps the equivalence is most easily seen when we produce in this
manner an articial eld which just neutralises the earth's eld of gravitation.
Jules Verne's book Round the Moon tells the story of three men
in a projectile shot from a cannon into space. The author enlarges on
their amusing experiences when their weight vanished altogether at the
iv] FIELDS OF FORCE 59
neutral point, where the attraction of the earth and moon balance one another.
As a matter of fact they would not have had any feeling of weight
at any time during their journey after they left the earth's atmosphere.
The projectile was responding freely to the pull of gravity, and so were its
occupants. When an occupant let go of a plate, the plate could not \fall"
any more than it was doing already, and so it must remain poised.
It will be seen that the sensation of weight is not felt when we are free to
respond to the force of gravitation; it is only felt when something interferes
to prevent our falling. It is primarily the 
oor or the chair which causes
the sensation of weight by checking the fall. It seems literally true to say
that we never feel the force of the earth's gravitation; what we do feel is
the bombardment of the soles of our boots by the molecules of the ground,
and the consequent impulses spreading upwards through the body. This
point is of some importance, since the idea of the force of gravitation as
something which can be felt, predisposes us to a materialistic view of its
nature.
Another example of an articial eld of force is the centrifugal force of
the earth's rotation. In most books of Physical Constants will be found a
table of the values of \g," the acceleration due to gravity, at dierent latitudes.
But the numbers given do not relate to gravity alone; they are the
resultant of gravity and the centrifugal force of the earth's rotation. These
are so much alike in their eects that for practical purposes physicists have
not thought it worth while to distinguish them.
Similar articial elds are produced when an aeroplane changes its
course or speed; and one of the diculties of navigation is the impossibility
of discriminating between these and the true gravitation of the earth
with which they combine. One usually nds that the practical aviator
requires little persuasion of the relativity of force.
To nd a unifying idea as to the origin of these articial elds of force,
we must return to the four-dimensional world of space-time. The observer
is progressing along a certain track in this world. Now his course need
not necessarily be straight. It must be remembered that straight in the
four-dimensional world means something more than straight in space; it
implies also uniform velocity, since the velocity determines the inclination
of the track to the time-axis.
The observer in the accelerated lift travels upwards in a straight line,
say 1 foot in the rst second, 4 feet in two seconds, 9 feet in three seconds,
and so on. If we plot these points as x and t on a diagram we obtain
a curved track. Presently the speed of the lift becomes uniform and the
track in the diagram becomes straight. So long as the track is curved
60 FIELDS OF FORCE [ch.
(accelerated motion) a eld of force is perceived; it disappears when the
track becomes straight (uniform motion).
Again the observer on the earth is carried round in a circle once a day
by the earth's rotation; allowing for steady progress through time, the
track in four dimensions is a spiral. For an observer at the north pole the
track is straight, and there the centrifugal force is zero.
Clearly the articial eld of force is associated with curvature of track,
and we can lay down the following rule:|
Whenever the observer's track through the four-dimensional world is
curved he perceives an articial eld of force.
The eld of force is not only perceived by the observer in his sensations,
but reveals itself in his physical measures. It should be understood, however,
that the curvature of track must not have been otherwise allowed
for. Naturally if the observer in the lift recognises that his measures are
aected by his own acceleration and applies the appropriate corrections,
the articial force will be removed by the process. It only exists if he is
unaware of, or does not choose to consider, his acceleration.
The centrifugal force is often called \unreal." From the point of view
of an observer who does not rotate with the earth, there is no centrifugal
force; it only arises for the terrestrial observer who is too lazy to make
other allowance for the eects of the earth's rotation. It is commonly
thought that this \unreality" quite dierentiates it from a \real" force like
gravity; but if we try to nd the grounds of this distinction they evade
us. The centrifugal force is made to disappear if we choose a suitable
standard observer not rotating with the earth; the gravitational force was
made to disappear when we chose as standard observer an occupant of
Jules Verne's falling projectile. If the possibility of annulling a eld of
force by choosing a suitable standard observer is a test of unreality, then
gravitation is equally unreal with centrifugal force.
It may be urged that we have not stated the case quite fairly. When
we choose the non-rotating observer the centrifugal force disappears completely
and everywhere. When we choose the occupant of the falling projectile,
gravitation disappears in his immediate neighbourhood; but he
would notice that, although unsupported objects round him experienced
no acceleration relative to him, objects on the other side of the earth
would fall towards him. So far from getting rid of the eld of force, he
has merely removed it from his own surroundings, and piled it up elsewhere.
Thus gravitation is removable locally, but centrifugal force can
be removed everywhere. The fallacy of this argument is that it speaks
as though gravitation and centrifugal force were distinguishable experii
v] FIELDS OF FORCE 61
mentally. It presupposes the distinction that we are challenging. Looking
simply at the resultant of gravitation and centrifugal force, which is all
that can be observed, neither observer can get rid of the resultant force at
all parts of space. Each has to be content with leaving a residuum. The
non-rotating observer claims that he has got rid of all the unreal part,
leaving a remainder (the usual gravitational eld) which he regards as really
existing. We see no justication for this claim, which might equally
well be made by Jules Verne's observer.
It is not denied that the separation of centrifugal and gravitational force
generally adopted has many advantages for mathematical calculation. If
it were not so, it could not have endured so long. But it is a mathematical
separation only, without physical basis; and it often happens that the
separation of a mathematical expression into two terms of distinct nature,
though useful for elementary work, becomes vitiated for more accurate
work by the occurrence of minute cross-terms which have to be taken into
account.
Newtonian mechanics proceeds on the supposition that there is some
super-observer. If he feels a eld of force, then that force really exists.
Lesser beings, such as the occupants of the falling projectile, have other
ideas, but they are the victims of illusion. It is to this super-observer
that the mathematician appeals when he starts a dynamical investigation
with the words \Take unaccelerated rectangular axes, Ox, Oy, Oz : : : ."
Unaccelerated rectangular axes are the measuring-appliances of the superobserver.
It is quite possible that there might be a super-observer, whose views
have a natural right to be regarded as the truest, or at least the simplest.
A society of learned shes would probably agree that phenomena were
best described from the point of view of a sh at rest in the ocean. But
relativity mechanics nds that there is no evidence that the circumstances
of any observer can be such as to make his views pre-eminent. All are
on an equality. Consider an observer A in a projectile falling freely to
the earth, and an observer B in space out of range of any gravitational
attraction. Neither A nor B feel any eld of force in their neighbourhood.
Yet in Newtonian mechanics an articial distinction is drawn between
their circumstances; B is in no eld of force at all, but A is really in a eld
of force, only its eects are neutralised by his acceleration. But what is
this acceleration of A? Primarily it is an acceleration relative to the earth;
but then that can equally well be described as an acceleration of the earth
relative to A, and it is not fair to regard it as something located with
A. Its importance in Newtonian philosophy is that it is an acceleration
62 FIELDS OF FORCE [ch.
relative to what we have called the super-observer. This potentate has
drawn planes and lines partitioning space, as space appears to him. I fear
that the time has come for his abdication.
Suppose the whole system of the stars were falling freely under the
uniform gravitation of some vast external mass, like a drop of rain falling
to the ground. Would this make any dierence to phenomena? None at
all. There would be a gravitational eld; but the consequent acceleration
of the observer and his landmarks would produce a eld of force annulling
it. Who then shall say what is absolute acceleration?
We shall accordingly give up the attempt to separate articial elds of
force and natural gravitational elds; and call the whole measured eld of
force the gravitational eld, generalising the expression. This eld is not
absolute, but always requires that some observer should be specied.
It may avoid some mystication if we state at once that there are certain
intricacies in the gravitational in
uence radiating from heavy matter which
are distinctive. A theory which did not admit this would run counter to
common sense. What our argument has shown is that the characteristic
symptom in a region in the neighbourhood of matter is not the eld of
force; it must be something more intricate. In due course we shall have to
explain the nature of this more complex eect of matter on the condition
of the world.
Our previous rule, that the observer perceives an articial eld of force
when he deviates from a straight track, must now be superseded. We
need rather a rule determining when he perceives a eld of force of any
kind. Indeed the original rule has become meaningless, because a straight
track is no longer an absolute conception. Uniform motion in a straight
line is not the same for an observer rotating with the earth as for a nonrotating
observer who takes into account the sinuosity of the rotation.
We have decided that these two observers are on the same footing and
their judgments merit the same respect. A straight-line in space-time is
accordingly not an absolute conception, but is only dened relative to
some observer.
Now we have seen that so long as the observer and his measuringappliances
are unconstrained (falling freely) the eld of force immediately
round him vanishes. It is only when he is de
ected from his proper track
that he nds himself in the midst of a eld of force. Leaving on one
side the question of the motion of electrically charged bodies, which must
be reserved for more profound treatment, the observer can only leave his
proper track if he is being disturbed by material impacts, e.g. the molecules
of the ground bombarding the soles of his boots. We may say then that a
iv] FIELDS OF FORCE 63
body does not leave its natural track without visible cause; and any eld
of force round an observer is the result of his leaving his natural track
by such cause. There is nothing mysterious about this eld of force; it is
merely the re
ection in the phenomena of the observer's disturbance; just
as the 
ight of the houses and hedgerows past our railway-carriage is the
re
ection of our motion with the train.
Our attention is thus directed to the natural tracks of unconstrained
bodies, which appear to be marked out in some absolute way in the fourdimensional
world. There is no question of an observer here; the body
takes the same course in the world whoever is watching it. Dierent
observers will describe the track as straight, parabolical, or sinuous, but
it is the same absolute locus.
Now we cannot pretend to predict without reference to experiment the
laws determining the nature of these tracks; but we can examine whether
our knowledge of the four-dimensional world is already sucient to specify
denite tracks of this kind, or whether it will be necessary to introduce
new hypothetical factors. It will be found that it is already sucient.
So far we have had to deal with only one quantity which is independent
of the observer and has therefore an absolute signicance in the world,
namely the interval between two events in space and time. Let us choose
two fairly distant events P1 and P2. These can be joined by a variety
of tracks, and the interval-length from P1 to P2 along any track can be
measured. In order to make sure that the interval-length is actually being
measured along the selected track, the method is to take a large number
of intermediate points on the track, measure the interval corresponding
to each subdivision, and take the sum. It is virtually the same process as
measuring the length of a twisty road on a map with a piece of cotton.
The interval-length along a particular track is thus something which can
be measured absolutely, since all observers agree as to the measurement
of the interval for each subdivision. It follows that all observers will agree
as to which track (if any) is the shortest track between the two points,
judged in terms of interval-length.
This gives a means of dening certain tracks in space-time as having
an absolute signicance, and we proceed tentatively to identify them with
the natural tracks taken by freely moving particles.
In one respect we have been caught napping. Dr A. A. Robb has pointed
out the curious fact that it is not the shortest track, but the longest
track, which is unique. There are any number of tracks from P1 to P2 of
It is here assumed that P2 is in the future of P1 so that it is possible for a particle
to travel from P1 to P2. If P1 and P2 are situated like O and P0 in Fig. 3, the interval64
FIELDS OF FORCE [ch.
zero interval-length; there is just one which has maximum length. This
is because of the peculiar geometry which the minus sign of (t2 􀀀 t1)2
introduces. For instance, it will be seen from equation (1), p. 48, that
when
(x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2 = (t2 􀀀 t1)2;
that is to say when the resultant distance travelled in space is equal to
the distance travelled in time, then s is zero. This happens when the
velocity is unity|the velocity of light. To get from P1 to P2 by a path
of no interval-length, we must simply keep on travelling with the velocity
of light, cruising round if necessary, until the moment comes to turn up
at P2. On the other hand there is evidently an upper limit to the intervallength
of the track, because each portion of s is always less than the
corresponding portion of (t2 􀀀 t1), and s can never exceed t2 􀀀 t1.
There is a physical interpretation of interval-length along the path of
a particle which helps to give a more tangible idea of its meaning. It is
the time as perceived by an observer, or measured by a clock, carried on
the particle. This is called the proper-time; and, of course, it will not in
general agree with the time-reckoning of the independent onlooker who is
supposed to be watching the whole proceedings. To prove this, we notice
from equation (1) that if x2 = x1; y2 = y1 and z2 = z1, then s = t2􀀀t1. The
condition x2 = x1, etc. means that the particle must remain stationary
relative to the observer who is measuring x, y, z, t. To secure this we
mount our observer on the particle and then the interval-length s will be
t2 􀀀 t1, which is the time elapsed according to his clock.
We can use proper-time as generally equivalent to interval-length; but
it must be admitted that the term is not very logical unless the track in
question is a natural track. For any other track, the drawback to dening
the interval-length as the time measured by a clock which follows the
track, is that no clock could follow the track without violating the laws of
nature. We may force it into the track by continually hitting it; but that
treatment may not be good for its time-keeping qualities. The original
denition by equation (1) is the more general denition.
We are now able to state formally our proposed law of motion|Every
particle moves so as to take the track of greatest interval-length between
two events, except in so far as it is disturbed by impacts of other particles
or electrical forces.
This cannot be construed into a truism like Newton's rst law of motion.
The reservation is not an undened agency like force, whose meaning can
length is imaginary, and the shortest track is unique.
iv] FIELDS OF FORCE 65
be extended to cover any breakdown of the law. We reserve only direct
material impacts and electromagnetic causes, the latter being outside our
present eld of discussion.
Consider, for example, two events in space-time, viz. the position of the
earth at the present moment, and its position a hundred years ago. Call
these events P2 and P1. In the interim the earth (being undisturbed by
impacts) has moved so as to take the longest track from P1 to P2|or, if
we prefer, so as to take the longest possible proper-time over the journey.
In the weird geometry of the part of space-time through which it passes (a
geometry which is no doubt associated in some way with our perception of
the existence of a massive body, the sun) this longest track is a spiral|a
circle in space, drawn out into a spiral by continuous displacement in time.
Any other course would have had shorter interval-length.
In this way the study of elds of force is reduced to a study of geometry.
To a certain extent this is a retrograde step; we adopt Kepler's description
of the sun's gravitational eld instead of Newton's. The eld of force is
completely described if the tracks through space and time of particles
projected in every possible way are prescribed. But we go back in order
to go forward in a new direction. To express this unmanageable mass of
detail in a unied way, a world-geometry is found in which the tracks of
greatest length are the actual tracks of the particles. It only remains to
express the laws of this geometry in a concise form. The change from a
mechanical to a geometrical theory of elds of force is not so fundamental
a change as might be supposed. If we are now reducing mechanics to a
branch of natural geometry, we have to remember that natural geometry
is equally a branch of mechanics, since it is concerned with the behaviour
of material measuring-appliances.
Reference has been made to weird geometry. There is no help for it, if
the longest track can be a spiral like that known to be described by the
earth. Non-Euclidean geometry is necessary. In Euclidean geometry the
shortest track is always a straight line; and the slight modication of Euclidean
geometry described in Chapter iii is found to give a straight line as
the longest track. The status of non-Euclidean geometry has already been
thrashed out in the Prologue; and there seems to be no reason whatever
for preferring Euclid's geometry unless observations decide in its favour.
Equation (1), p. 48, is the expression of the Euclidean (or semi-Euclidean)
geometry we have hitherto adopted; we shall have to modify it, if we adopt
non-Euclidean geometry.
But the point arises that the geometry arrived at in Chapter iii was not
arbitrary. It was the synthesis of measures made with clocks and scales,
66 FIELDS OF FORCE [ch.
by observers with all kinds of uniform motion relative to one another;
we cannot modify it arbitrarily to t the behaviour of moving particles
like the earth. Now, if the worst came to the worst, and we could not
reconcile a geometry based on measures with clocks and scales and a
geometry based on the natural tracks of moving particles|if we had to
select one or the other and keep to it|I think we ought to prefer to use
the geometry based on the tracks of moving particles. The free motion of
a particle is an example of the simplest possible kind of phenomenon; it is
unanalysable; whereas, what the readings of any kind of clock record, what
the extension of a material rod denotes, may evidently be complicated
phenomena involving the secrets of molecular constitution. Each geometry
would be right in its own sphere; but the geometry of moving particles
would be the more fundamental study. But it turns out that there is
probably no need to make the choice; clocks, scales, moving particles,
light-pulses, give the same geometry. This might perhaps be expected
since a clock must comprise moving particles of some kind.
A formula, such as equation (1), based on experiment can only be veri-
ed to a certain degree of approximation. Within certain limits it will be
possible to introduce modications. Now it turns out that the free motion
of a particle is a much more sensitive way of exploring space-time, than
any practicable measures with scales and clocks. If then we employ our
accurate knowledge of the motion of particles to correct the formula, we
shall nd that the changes introduced are so small that they are inappreciable
in any practical measures with scales and clocks. There is only one
case where a possible detection of the modication is indicated; this refers
to the behaviour of a clock on the surface of the sun, but the experiment
is one of great diculty and no conclusive answer has been given. We
conclude then that the geometry of space and time based on the motions
of particles is accordant with the geometry based on the cruder observations
with clocks and scales; but if subsequent experiment should reveal
a discrepancy, we shall adhere to the moving particle on account of its
greater simplicity.
The proposed modication can be regarded from another point of view.
Equation (1) is the synthesis of the experiences of all observers in uniform
motion. But uniform motion means that their four-dimensional tracks are
straight lines. We must suppose that the observers were moving in their
natural tracks; for, if not, they experienced elds of force, and presumably
allowed for these in their calculations, so that reduction was made to the
natural tracks. If then equation (1) shows that the natural tracks are
straight lines, we are merely getting out of the equation that which we
iv] FIELDS OF FORCE 67
originally put into it.
The formula needs generalising in another way. Suppose there is a region
of space-time where, for some observer, the natural tracks are all straight
lines and equation (1) holds rigorously. For another (accelerated) observer
the tracks will be curved, and the equation will not hold. At the best it
is of a form which can only hold good for specially selected observers.
Although it has become necessary to throw our formula into the meltingpot,
that does not create any diculty in measuring the interval. Without
going into technical details, it may be pointed out that the innovations
arise solely from the introduction of gravitational elds of force into our
scheme. When there is no force, the tracks of all particles are straight lines
as our previous geometry requires. In any small region we can choose an
observer (falling freely) for whom the force vanishes, and accordingly the
original formula holds good. Thus it is only necessary to modify our rule
for determining the interval by two provisos (1) that the interval measured
must be small, (2) that the scales and clocks used for measuring it must
be falling freely. The second proviso is natural, because, if we do not leave
our apparatus to fall freely, we must allow for the strain that it undergoes.
The rst is not a serious disadvantage, because a larger interval can be
split up into a number of small intervals and the parts measured separately.
In mathematical problems the same device is met with under the name
of integration. To emphasise that the formula is strictly true only for
innitesimal intervals, it is written with a new notation
ds2 = 􀀀dx2 􀀀 dy2 􀀀 dz2 + dt2 (2)
where dx stands for the small dierence x2 􀀀 x1, etc.
The condition that the measuring appliances must not be subjected to
a eld of force is illustrated by Ehrenfest's paradox. Consider a wheel
revolving rapidly. Each portion of the circumference is moving in the direction
of its length, and might be expected to undergo the FitzGerald
contraction due to its velocity; each portion of a radius is moving transversely
and would therefore have no longitudinal contraction. It looks as
though the rim of the wheel should contract and the spokes remain the
same length, when the wheel is set revolving. The conclusion is absurd,
for a revolving wheel has no tendency to buckle|which would be the only
way of reconciling these conditions. The point which the argument has
overlooked is that the results here appealed to apply to unconstrained
bodies, which have no acceleration relative to the natural tracks in space.
Each portion of the rim of the wheel has a radial acceleration, and this
aects its extensional properties. When accelerations as well as velocities
68 FIELDS OF FORCE
occur a more far-reaching theory is needed to determine the changes of
length.
To sum up|the interval between two (near) events is something quantitative
which has an absolute signicance in nature. The track between two
(distant) events which has the longest interval-length must therefore have
an absolute signicance. Such tracks are called geodesics. Geodesics can
be traced practically, because they are the tracks of particles undisturbed
by material impacts. By the practical tracing of these geodesics we have
the best means of studying the character of the natural geometry of the
world. An auxiliary method is by scales and clocks, which, it is believed,
when unconstrained, measure a small interval according to formula (2).
The identity of the two methods of exploring the geometry of the world
is connected with a principle which must now be enunciated denitely.
We have said that no experiments have been able to detect a dierence
between a gravitational eld and an articial eld of force such as the
centrifugal force. This is not quite the same thing as saying that it has
been proved that there is no dierence. It is well to be explicit when a
positive generalisation is made from negative experimental evidence. The
generalisation which it is proposed to adopt is known as the Principle of
Equivalence.
A gravitational eld of force is precisely equivalent to an articial eld
of force, so that in any small region it is impossible by any conceivable
experiment to distinguish between them.
In other words, force is purely relative.
CHAPTER V
KINDS OF SPACE
The danger of asserting dogmatically that an axiom based on the experience
of a limited region holds universally will now be to some extent apparent to
the reader. It may lead us to entirely overlook, or when suggested at once
reject, a possible explanation of phenomena. The hypothesis that space is not
homaloidal [
at], and again that its geometrical character may change with
the time, may or may not be destined to play a great part in the physics
of the future; yet we cannot refuse to consider them as possible explanations
of physical phenomena, because they may be opposed to the popular dogmatic
belief in the universality of certain geometrical axioms|a belief which has risen
from centuries of indiscriminating worship of the genius of Euclid.
W. K. Clifford (and K. Pearson), Common Sense of the Exact Sciences.
On any surface it requires two independent numbers or \coordinates" to
specify the position of a point. For this reason a surface, whether 
at
or curved, is called a two-dimensional space. Points in three-dimensional
space require three, and in four-dimensional space-time four numbers or
coordinates.
To locate a point on a surface by two numbers, we divide the surface into
meshes by any two systems of lines which cross one another. Attaching
consecutive numbers to the lines, or better to the channels between them,
one number from each system will identify a particular mesh; and if the
subdivision is suciently ne any point can be specied in this way with
all the accuracy needed. This method is used, for example, in the Post
Oce Directory of London for giving the location of streets on the map.
The point (4; 2) will be a point in the mesh where channel No. 4 of the
rst system crosses channel No. 2 of the second. If this indication is not
suciently accurate, we must divide channel No. 4 into ten parts numbered
4:0, 4:1 etc. The subdivision must be continued until the meshes are so
small that all points in one mesh can be considered identical within the
limits of experimental detection.
70 KINDS OF SPACE [ch.
The diagrams, Figs. 10, 11, 12, illustrate three of the many kinds of
mesh-systems commonly used on a 
at surface.
If we speak of the properties of the triangle formed by the points (1; 2),
(3; 0), (4; 4), we shall be at once asked, What mesh-system are you using?
No one can form a picture of the triangle until that information has been
given. But if we speak of the properties of a triangle whose sides are
of lengths 2, 3, 4 inches, anyone with a graduated scale can draw the
triangle, and follow our discussion of its properties. The distance between
two points can be stated without referring to any mesh-system. For this
reason, if we use a mesh-system, it is important to nd formulae connecting
the absolute distance with the particular system that is being used.
In the more complicated kinds of mesh-systems it makes a great simpli-
cation if we content ourselves with the formulae for very short distances.
The mathematician then nds no diculty in extending the results to long
distances by the process called integration. We write ds for the distance
between two points close together, x1 and x2 for the two numbers speci-
Fig. 10 Fig. 11 Fig. 12
fying the location of one of them, dx1 and dx2 for the small dierences of
these numbers in passing from the rst point to the second. But in using
one of the particular mesh-systems illustrated in the diagrams, we usually
replace x1, x2 by particular symbols sanctioned by custom, viz. (x1; x2)
becomes (x; y), (r; ), (; ) for Figs. 10, 11, 12, respectively.
The formulae, found by geometry, are:
For rectangular coordinates (x; y), Fig. 10,
ds2 = dx2 + dy2:
For polar coordinates (r; ), Fig. 11,
ds2 = dr2 + r2 d2:
v] KINDS OF SPACE 71
For oblique coordinates (; ), Fig. 12,
ds2 = d2 􀀀 2 dd + d2;
where  is the cosine of the angle between the lines of partition.
As an example of a mesh-system on a curved surface, we may take the
lines of latitude and longitude on a sphere.
For latitude and longitude (; )
ds2 = d2 + cos2  d2:
These expressions form a test, and in fact the only possible test, of the
kind of coordinates we are using. It may perhaps seem inconceivable that
an observer should for an instant be in doubt whether he was using the
mesh-system of Fig. 10 or Fig. 11. He sees at a glance that Fig. 11 is
not what he would call a rectangular mesh-system. But in that glance,
he makes measures with his eye, that is to say he determines ds for pairs
of points, and he notices how these values are related to the number of
intervening channels. In fact he is testing which formula for ds will t. For
centuries man was in doubt whether the earth was 
at or round|whether
he was using plane rectangular coordinates or some kind of spherical coordinates.
In some cases an observer adopts his mesh-system blindly and
long afterwards discovers by accurate measures that ds does not t the
formula he assumed|that his mesh-system is not exactly of the nature
he supposed it was. In other cases he deliberately sets himself to plan
out a mesh-system of a particular variety, say rectangular coordinates; he
constructs right angles and rules parallel lines; but these constructions are
all measurements of the way the x-channels and y-channels ought to go,
and the rules of construction reduce to a formula connecting his measures
ds with x and y.
The use of special symbols for the coordinates, varying according to the
kind of mesh-system used, thus anticipates a knowledge which is really
derived from the form of the formulae. In order not to give away the
secret prematurely, it will be better to use the symbols x1, x2 in all cases.
The four kinds of coordinates already considered then give respectively
the relations,
ds2 = dx1
2 + dx2
2 (rectangular);
ds2 = dx1
2 + x1
2 dx2
2 (polar);
ds2 = dx1
2 􀀀 2 dx1dx2 + dx2
2 (oblique);
ds2 = dx1
2 + cos2 x1 dx2
2 (latitude and longitude):
72 KINDS OF SPACE [ch.
If we have any mesh-system and want to know its nature, we must make a
number of measures of the length ds between adjacent points (x1; x2) and
(x1 + dx1; x2 + dx2) and test which formula ts. If, for example, we then
nd that ds2 is always equal to dx1
2 + x1
2 dx2
2, we know that our meshsystem
is like that in Fig. 11, x1 and x2 being the numbers usually denoted
by the polar coordinates r, . The statement that polar coordinates are
being used is unnecessary, because it adds nothing to our knowledge which
is not already contained in the formula. It is merely a matter of giving a
name; but, of course, the name calls to our minds a number of familiar
properties which otherwise might not occur to us.
For instance, it is characteristic of the polar coordinate system that
there is only one point for which x1 (or r) is equal to 0, whereas in the
other systems x1 = 0 gives a line of points. This is at once apparent from
the formula; for if we have two points for which x1 = 0 and x1 + dx1 = 0,
respectively, then
dx1
2 + x1
2 dx2
2 = 0:
The distance ds between the two points vanishes, and accordingly they
must be the same point.
The examples given can all be summed up in one general expression
ds2 = g11 dx1
2 + 2g12 dx1dx2 + g22 dx2
2;
where g11, g12, g22 may be constants or functions of x1 and x2. For instance,
in the fourth example their values are 1, 0, cos2 x1. It is found that all
possible mesh-systems lead to values of ds2 which can be included in an
expression of this general form; so that mesh-systems are distinguished
by three functions of position g11, g12, g22 which can be determined by
making physical measurements. These three quantities are sometimes
called potentials.
We now come to a point of far-reaching importance. The formula for
ds2 teaches us not only the character of the mesh-system, but the nature
of our two-dimensional space, which is independent of any mesh-system.
If ds2 satises any one of the rst three formulae, then the space is like
a 
at surface; if it satises the last formula, then the space is a surface
curved like a sphere. Try how you will, you cannot draw a mesh-system
on a 
at (Euclidean) surface which agrees with the fourth formula.
If a being limited to a two-dimensional world nds that his measures
agree with the rst formula, he can make them agree with the second or
third formulae by drawing the meshes dierently. But to obtain the fourth
formula he must be translated to a dierent world altogether.
v] KINDS OF SPACE 73
We thus see that there are dierent kinds of two-dimensional space,
betrayed by dierent metrical properties. They are naturally visualised
as dierent surfaces in Euclidean space of three dimensions. This picture
is helpful in some ways, but perhaps misleading in others. The metrical
relations on a plane sheet of paper are not altered when the paper
is rolled into a cylinder|the measures being, of course, conned to the
two-dimensional world represented by the paper, and not allowed to take
a short cut through space. The formulae apply equally well to a plane
surface or a cylindrical surface; and in so far as our picture draws a distinction
between a plane and a cylinder, it is misleading. But they do
not apply to a sphere, because a plane sheet of paper cannot be wrapped
round a sphere. A genuinely two-dimensional being could not be cognisant
of the dierence between a cylinder and a plane; but a sphere would appear
as a dierent kind of space, and he would recognise the dierence by
measurement.
Of course there are many kinds of mesh-systems, and many kinds of
two-dimensional spaces, besides those illustrated in the four examples.
Clearly it is not going to be a simple matter to discriminate the dierent
kinds of spaces by the values of the g's. There is no characteristic, visible
to cursory inspection, which suggests why the rst three formulae should
all belong to the same kind of space, and the fourth to a dierent one.
Mathematical investigation has discovered what is the common link between
the rst three formulae. The g11, g12, g22 satisfy in all three cases
a certain dierential equationy; and whenever this dierential equation is
satised, the same kind of space occurs.
No doubt it seems a very clumsy way of approaching these intrinsic
dierences of kinds of space|to introduce potentials which specically
refer to a particular mesh-system, although the mesh-system can have
nothing to do with the matter. It is worrying not to be able to express
the dierences of space in a purer form without mixing them up with
irrelevant dierences of potential. But we have neither the vocabulary
nor the imagination for a description of absolute properties as such. All
physical knowledge is relative to space and time partitions; and to gain an
understanding of the absolute it is necessary to approach it through the
relative. The absolute may be dened as a relative which is always the
same no matter what it is relative toz. Although we think of it as self-
One should perhaps rather say a roll, to avoid any question of joining the two
edges.
yAppendix, Note 4.
zCf. p. 28, where a distinction was drawn between knowledge which does not par74
KINDS OF SPACE [ch.
existing, we cannot give it a place in our knowledge without setting up
some dummy to relate it to. And similarly the absolute dierences of space
always appear as related to some mesh-system, although the mesh-system
is only a dummy and has nothing to do with the problem.
The results for two dimensions can be generalised, and applied to fourdimensional
space-time. Distance must be replaced by interval, which it
will be remembered, is an absolute quantity, and therefore independent of
the mesh-system used. Partitioning space-time by any system of meshes,
a mesh being given by the crossing of four channels, we must specify a
point in space-time by four coordinate numbers, x1, x2, x3, x4. By analogy
the general formula will be
ds2 = g11 dx1
2 + g22 dx2
2 + g33 dx3
2 + g44 dx4
2
+ 2g12 dx1dx2 + 2g13 dx1dx3 + 2g14 dx1dx4
+ 2g23 dx2dx3 + 2g24 dx2dx4 + 2g34 dx3dx4: (3)
The only dierence is that there are now ten g's, or potentials, instead
of three, to summarise the metrical properties of the mesh-system. It is
convenient in specifying special values of the potentials to arrange them
in the standard form
g11 g12 g13 g14
g22 g23 g24
g33 g34
g44
The space-time already discussed at length in Chapter iii corresponded to
the formula (2), p. 67,
ds2 = 􀀀dx2 􀀀 dy2 􀀀 dz2 + dt2:
Here (x; y; z; t) are the conventional symbols for (x1; x2; x3; x4) when this
special mesh-system is used, viz. rectangular coordinates and time. Comparing
with (3) the potentials have the special values
􀀀1 0 0 0
􀀀1 0 0
􀀀1 0
+1
These are called the \Galilean values." If the potentials have these values
everywhere, space-time may be called \
at," because the geometry is that
ticularise the observer and knowledge which does not postulate an observer at all.
v] KINDS OF SPACE 75
of a plane surface drawn in Euclidean space of ve dimensions. Recollecting
what we found for two dimensions, we shall realise that a quite
dierent set of values of the potentials may also belong to 
at space-time,
because the meshes may be drawn in dierent ways. We must clearly
understand that
(1) The only way of discovering what kind of space-time is being dealt
with is from the values of the potentials, which are determined practically
by measurements of intervals,
(2) Dierent values of the potentials do not necessarily indicate dierent
kinds of space-time,
(3) There is some complicated mathematical property common to all
values of the potentials which belong to the same space-time, which is
not shared by those which belong to a dierent kind of space-time. This
property is expressed by a set of dierential equations.
It can now be deduced that the space-time in which we live is not quite

at. If it were, a mesh-system could be drawn for which the g's have the
Galilean values, and the geometry with respect to these partitions of space
and time would be that discussed in Chapter iii. For that geometry the
geodesics, giving the natural tracks of particles, are straight lines.
Thus in 
at space-time the law of motion is that (with suitably chosen
coordinates) every particle moves uniformly in a straight line except when
it is disturbed by the impacts of other particles. Clearly this is not true of
our world; for example, the planets do not move in straight lines although
they do not suer any impacts. It is true that if we conne attention to
a small region like the interior of Jules Verne's projectile, all the tracks
become straight lines for an appropriate observer, or, as we generally say,
he detects no eld of force. It needs a large region to bring out the
dierences of geometry. That is not surprising, because we cannot expect
to tell whether a surface is 
at or curved unless we consider a reasonably
large portion of it.
According to Newtonian ideas, at a great distance from all matter beyond
the reach of any gravitation, particles would all move uniformly in
straight lines. Thus at a great distance from all matter space-time tends
to become perfectly 
at. This can only be checked by experiment to a
certain degree of accuracy, and there is some doubt as to whether it is
rigorously true. We shall leave this afterthought to Chapter x, meanwhile
assuming with Newton that space-time far enough away from everything
is 
at, although near matter it is curved. It is this puckering near matter
which accounts for its gravitational eects.
76 KINDS OF SPACE [ch.
Just as we picture dierent kinds of two-dimensional space as dierently
curved surfaces in our ordinary space of three-dimensions, so we are
now picturing dierent kinds of four-dimensional space-time as dierently
curved surfaces in a Euclidean space of ve dimensions. This is a picture
only. The fth dimension is neither space nor time nor anything
that can be perceived; so far as we know, it is nonsense. I should not
describe it as a mathematical ction, because it is of no great advantage
in a mathematical treatment. It is even liable to mislead because it draws
distinctions, like the distinction between a plane and a roll, which have no
meaning. It is, like the notion of a eld of force acting in space and time,
merely introduced to bolster up Euclidean geometry, when Euclidean geometry
has been found inappropriate. The real dierence between the
various kinds of space-time is that they have dierent kinds of geometry,
involving dierent properties of the g's. It is no explanation to say that
this is because the surfaces are dierently curved in a real Euclidean space
of ve dimensions. We should naturally ask for an explanation why the
space of ve dimensions is Euclidean; and presumably the answer would
be, because it is a plane in a real Euclidean space of six dimensions, and
so on ad innitum.
The value of the picture to us is that it enables us to describe important
properties with common terms like \pucker" and \curvature" instead of
technical terms like \dierential invariant." We have, however, to be on
our guard, because analogies based on three-dimensional space do not
always apply immediately to many-dimensional space. The writer has
keen recollections of a period of much perplexity, when he had not realised
that a four-dimensional space with \no curvature" is not the same as a
\
at" space! Three-dimensional geometry does not prepare us for these
surprises.
Picturing the space-time in the gravitational eld round the earth as
a pucker, we notice that we cannot locate the pucker at a point; it is
\somewhere round" the point. At any special point the pucker can be
pressed out 
at, and the irregularity runs o somewhere else. That is
what the inhabitants of Jules Verne's projectile did; they 
attened out
the pucker inside the projectile so that they could not detect any eld of
force there; but this only made things worse somewhere else, and they
would nd an increased eld of force (relative to them) on the other side
A fth dimension suces for illustrating the property here considered; but for an
exact representation of the geometry of the world, Euclidean space of ten dimensions
is required. We may well ask whether there is merit in Euclidean geometry sucient
to justify going to such extremes.
v] KINDS OF SPACE 77
of the earth.
What determines the existence of the pucker is not the values of the g's
at any point, or, what comes to the same thing, the eld of force there.
It is the way these values link on to those at other points|the gradient
of the g's, and more particularly the gradient of the gradient. Or, as has
already been said, the kind of space-time is xed by dierential equations.
Thus, although a gravitational eld of force is not an absolute thing,
and can be imitated or annulled at any point by an acceleration of the
observer or a change of his mesh-system, nevertheless the presence of a
heavy particle does modify the world around it in an absolute way which
cannot be imitated articially. Gravitational force is relative; but there is
this more complex character of gravitational in
uence which is absolute.
The question must now be put, Can every possible kind of space-time
occur in an empty region in nature? Suppose we give the ten potentials
perfectly arbitrary values at every point; that will specify the geometry of
some mathematically possible space-time. But could that kind of spacetime
actually occur|by any arrangement of the matter round the region?
The answer is that only certain kinds of space-time can occur in an
empty region in nature. The law which determines what kinds can occur
is the law of gravitation.
It is indeed clear that, since we have reduced the theory of elds of force
to a theory of the geometry of the world, if there is any law governing elds
of force (including the gravitational eld), that law must be of the nature
of a restriction on the possible geometries of the world.
The choice of g's in any special problem is thus arrived at by a threefold
sorting out: (1) many sets of values can be dismissed because they
can never occur in nature, (2) others, while possible, do not relate to the
kind of space-time present in the problem considered, (3) of those which
remain, one set of values relates to the particular mesh-system that has
been chosen. We have now to nd the law governing the rst discrimination.
What is the criterion that decides what values of the g's give a kind
of space-time possible in nature?
In solving this problem Einstein had only two clues to guide him.
(1) Since it is a question of whether the kind of space-time is possible,
the criterion must refer to those properties of the g's which distinguish
dierent kinds of space-time, not to those which distinguish dierent kinds
of mesh-system in the same space-time. The formulae must therefore not
be altered in any way, if we change the mesh-system.
(2) We know that 
at space-time can occur in nature (at great distances
from all gravitating matter). Hence the criterion must be satised by any
78 KINDS OF SPACE [ch.
values of the g's belonging to 
at space-time.
It is remarkable that these slender clues are sucient to indicate almost
uniquely a particular law. Afterwards the further test must be applied|
whether the law is conrmed by observation.
The irrelevance of the mesh-system to the laws of nature is sometimes
expressed in a slightly dierent way. There is one type of observation
which, we can scarcely doubt, must be independent of any possible circumstances
of the observer, namely a complete coincidence in space and
time. The track of a particle through four-dimensional space-time is called
its world-line. Now, the world-lines of two particles either intersect or they
do not intersect; the standpoint of the observer is not involved. In so far
as our knowledge of nature is a knowledge of intersections of world-lines,
it is absolute knowledge independent of the observer. If we examine the
nature of our observations, distinguishing what is actually seen from what
is merely inferred, we nd that, at least in all exact measurements, our
knowledge is primarily built up of intersections of world-lines of two or
more entities, that is to say their coincidences. For example, an electrician
states that he has observed a current of 5 milliamperes. This is his
inference: his actual observation was a coincidence of the image of a wire
in his galvanometer with a division of a scale. A meteorologist nds that
the temperature of the air is 75°; his observation was the coincidence of the
top of the mercury-thread with division 75 on the scale of his thermometer.
It would be extremely clumsy to describe the results of the simplest
physical experiment entirely in terms of coincidence. The absolute observation
is, whether or not the coincidence exists, not when or where or
under what circumstances the coincidence exists; unless we are to resort
to relative knowledge, the place, time and other circumstances must in
their turn be described by reference to other coincidences. But it seems
clear that if we could draw all the world-lines so as to show all the intersections
in their proper order, but otherwise arbitrary, this would contain
a complete history of the world, and nothing within reach of observation
would be omitted.
Let us draw such a picture, and imagine it embedded in a jelly. If we
deform the jelly in any way, the intersections will still occur in the same
order along each world-line and no additional intersections will be created.
The deformed jelly will represent a history of the world, just as accurate
as the one originally drawn; there can be no criterion for distinguishing
which is the best representation.
Suppose now we introduce space and time-partitions, which we might
do by drawing rectangular meshes in both jellies. We have now two ways
v] KINDS OF SPACE 79
of locating the world-lines and events in space and time, both on the
same absolute footing. But clearly it makes no dierence in the result of
the location whether we rst deform the jelly and then introduce regular
meshes, or whether we introduce irregular meshes in the undeformed jelly.
And so all mesh-systems are on the same footing.
This account of our observational knowledge of nature shows that there
is no shape inherent in the absolute world, so that when we insert a meshsystem,
it has no shape initially, and a rectangular mesh-system is intrinsically
no dierent from any other mesh-system.
Returning to our two clues, condition (1) makes an extraordinarily clean
sweep of laws that might be suggested; among them Newton's law is swept
away. The mode of rejection can be seen by an example; it will be sucient
to consider two dimensions. If in one mesh-system (x; y)
ds2 = g11dx2 + 2g12dxdy + g22dy2;
and in another system (x0; y0)
ds2 = g011 dx02 + 2g012 dx0dy0 + g022 dy02;
the same law must be satised if the unaccented letters are throughout
replaced by accented letters. Suppose the law g11 = g22 is suggested.
Change the mesh-system by spacing the y-lines twice as far apart, that is
to say take y0 = 1
2y, with x0 = x. Then
ds2 = g11 dx2 + 2g12 dxdy + g22 dy2
= g11 dx02 + 4g12 dx0dy0 + 4g22 dy02;
so that g110 = g11; g220 = 4g22:
And if g11 is equal to g22, g011 cannot be equal to g022.
After a few trials the reader will begin to be surprised that any possible
law could survive the test. It seems so easy to defeat any formula that is
set up by a simple change of mesh-system. Certainly it is unlikely that
anyone would hit on such a law by trial. But there are such laws, composed
of exceedingly complicated mathematical expressions. The theory of these
is called the \theory of tensors," and had already been worked out by the
pure mathematicians Riemann, Christoel, Ricci, Levi-Civita who, it may
be presumed, never dreamt of a physical application for it.
One law of this kind is the condition for 
at space-time, which is generally
written in the simple, but not very illuminating, form
B
 = 0: (4)
80 KINDS OF SPACE [ch.
The quantity on the left is called the Riemann-Christoel tensor, and it
is written out in a less abbreviated form in the Appendix. It must be
explained that the letters , , ,  indicate gaps, which are to be lled up
by any of the numbers 1, 2, 3, 4, chosen at pleasure. (When the expression
is written out at length, the gaps are in the suxes of the x's and g's.)
Filling the gaps in dierent ways, a large number of expressions, B1
111,
B4
123, B1
432, etc., are obtained. The equation (4) states that all of these are
zero. There are 44, or 256, of these expressions altogether, but many of
them are repetitions. Only 20 of the equations are really necessary; the
others merely say the same thing over again.
It is clear that the law (4) is not the law of gravitation for which we
are seeking, because it is much too drastic. If it were a law of nature,
then only 
at space-time could exist in nature, and there would be no
such thing as gravitation. It is not the general condition, but a special
case|when all attracting matter is innitely remote.
But in nding a general condition, it may be a great help to know a special
case. Would it do to select a certain number of the 20 equations to be
satised generally, leaving the rest to be satised only in the special case?
Unfortunately the equations hang together; and, unless we take them all,
it is found that the condition is not independent of the mesh-system. But
there happens to be one way of building up out of the 20 conditions a less
stringent set of conditions independent of the mesh-system. Let
G11 = B1
111 + B2
112 + B3
113 + B4
114;
and, generally
G = B1
1 + B2
2 + B3
3 + B4
4;
then the conditions
G = 0 (5)
will satisfy our requirements for a general law of nature.
This law is independent of the mesh-system, though this can only be
proved by elaborate mathematical analysis. Evidently, when all the B's
vanish, equation (5) is satised; so, when 
at space-time occurs, this law
of nature is not violated. Further it is not so stringent as the condition for

atness, and admits of the occurrence of a limited variety of non-Euclidean
geometries. Rejecting duplicates, it comprises 10 equations; but four of
Appendix, Note 5.
v] KINDS OF SPACE 81
these can be derived from the other six, so that it gives six conditions,
which happens to be the number required for a law of gravitation.
The suggestion is thus reached that
G = 0
may be the general law of gravitation. Whether it is so or not can only be
settled by experiment. In particular, it must in ordinary cases reduce to
something so near the Newtonian law, that the remarkable conrmation
of the latter by observation is accounted for. Further it is necessary to
examine whether there are any exceptional cases in which the dierence
between it and Newton's law can be tested. We shall see that these tests
are satised.
What would have been the position if this suggested law had failed? We
might continue the search for other laws satisfying the two conditions laid
down; but these would certainly be far more complicated mathematically.
I believe too that they would not help much, because practically they
would be indistinguishable from the simpler law here suggested|though
this has not been demonstrated rigorously. The other alternative is that
there is something causing force in nature not comprised in the geometrical
scheme hitherto considered, so that force is not purely relative, and
Newton's super-observer exists.
Perhaps the best survey of the meaning of our theory can be obtained
from the standpoint of a ten-dimensional Euclidean continuum, in which
space-time is conceived as a particular four-dimensional surface. It has
to be remarked that in ten dimensions there are gradations intermediate
between a 
at surface and a fully curved surface, which we shall speak
of as curved in the \rst degree" or \second degreey." The distinction is
something like that of curves in ordinary space, which may be curved like
a circle, or twisted like a helix; but the analogy is not very close. The full
\curvature" of a surface is a single quantity called G, built up out of the
various terms G in somewhat the same way as these are built up out of
B
. The following conclusions can be stated.
Isolate a region of empty space-time; and suppose that everywhere outside the
region the potentials are known. It should then be possible by the law of gravitation to
determine the nature of space-time in the region. Ten dierential equations together
with the boundary-values would suce to determine the ten potentials throughout the
region; but that would determine not only the kind of space-time but the mesh-system,
whereas the partitions of the mesh-system can be continued across the region in any
arbitrary way. The four sets of partitions give a four-fold arbitrariness; and to admit
of this, the number of equations required is reduced to six.
yThis is not a recognised nomenclature.
82 KINDS OF SPACE [ch.
If B
 = 0 (20 conditions)
space-time is 
at. This is the state of the world at an innite distance
from all matter and all forms of energy.
If G = 0 (6 conditions)
space-time is curved in the rst degree. This is the state of the world in
an empty region|not containing matter, light or electromagnetic elds,
but in the neighbourhood of these forms of energy.
If G = 0 (1 condition)
space-time is curved in the second degree. This is the state of the world in
a region not containing matter or electrons (bound energy), but containing
light or electromagnetic elds (free energy).
If G is not zero
space-time is fully curved. This is the state of the world in a region
containing continuous matter.
According to current physical theory continuous matter does not exist,
so that strictly speaking the last case never arises. Matter is built
of electrons or other nuclei. The regions lying between the electrons are
not fully curved, whilst the regions inside the electrons must be cut out
of space-time altogether. We cannot imagine ourselves exploring the inside
of an electron with moving particles, light-waves, or material clocks
and measuring-rods; hence, without further denition, any geometry of
the interior, or any statement about space and time in the interior, is
meaningless. But in common life, and frequently in physics, we are not
concerned with this microscopic structure of matter. We need to know,
not the actual values of the g's at a point, but their average values through
a region, small from the ordinary standpoint but large compared with the
molecular structure of matter. In this macroscopic treatment molecular
matter is replaced by continuous matter, and uncurved space-time studded
with holes is replaced by an equivalent fully curved space-time without
holes.
It is natural that our senses should have developed faculties for perceiving
some of these intrinsic distinctions of the possible states of the world
v] KINDS OF SPACE 83
around us. I prefer to think of matter and energy, not as agents causing
the degrees of curvature of the world, but as parts of our perceptions of
the existence of the curvature.
It will be seen that the law of gravitation can be summed up in the
statement that in an empty region space-time can be curved only in the
rst degree.
84 KINDS OF SPACE
CHAPTER VI
THE NEW LAW OF GRAVITATION
AND THE OLD LAW
I don't know what I may seem to the world, but, as to myself, I seem to have
been only as a boy playing on the sea-shore, and diverting myself in now and
then nding a smoother pebble or a prettier shell than ordinary, whilst the
great ocean of truth lay all undiscovered before me. Sir Isaac Newton.
Was there any reason to feel dissatised with Newton's law of gravitation?
Observationally it had been subjected to the most stringent tests, and
had come to be regarded as the perfect model of an exact law of nature.
The cases, where a possible failure could be alleged, were almost insignificant.
There are certain unexplained irregularities in the moon's motion;
but astronomers generally looked|and must still look|in other directions
for the cause of these discrepancies. One failure only had led to a serious
questioning of the law; this was the discordance of motion of the perihelion
of Mercury. How small was this discrepancy may be judged from the
fact that, to meet it, it was proposed to amend square of the distance to
the 2:00000016 power of the distance. Further it seemed possible, though
unlikely, that the matter causing the zodiacal light might be of sucient
mass to be responsible for this eect.
The most serious objection against the Newtonian law as an exact law
was that it had become ambiguous. The law refers to the product of the
masses of the two bodies; but the mass depends on the velocity|a fact
unknown in Newton's day. Are we to take the variable mass, or the mass
reduced to rest? Perhaps a learned judge, interpreting Newton's statement
like a last will and testament, could give a decision; but that is scarcely
the way to settle an important point in scientic theory.
Further distance, also referred to in the law, is something relative to an
observer. Are we to take the observer travelling with the sun or with the
86 THE NEW LAW OF GRAVITATION [ch.
other body concerned, or at rest in the aether or in some gravitational
medium?
Finally is the force of gravitation propagated instantaneously, or with
the velocity of light, or some other velocity? Until comparatively recently
it was thought that conclusive proof had been given that the speed of gravitation
must be far higher than that of light. The argument was something
like this. If the Sun attracts Jupiter towards its present position S, and
Jupiter attracts the Sun towards its present position J, the two forces are
in the same line and balance. But if the Sun attracts Jupiter towards its
previous position S0, and Jupiter attracts the Sun towards its previous
position J0, when the force of attraction started out to cross the gulf, then
the two forces give a couple. This couple will tend to increase the angular
Fig. 13.
S
S′
J
J′
momentum of the system, and, acting cumulatively, will soon cause an
appreciable change of period, disagreeing with observation if the speed is
at all comparable with that of light. The argument is fallacious, because
the eect of propagation will not necessarily be that S is attracted in the
direction towards J0. Indeed it is found that if S and J are two electric
charges, S will be attracted very approximately towards J (not J0) in spite
of the electric in
uence being propagated with the velocity of light. In
vi] AND THE OLD LAW 87
the theory given in this book, gravitation is propagated with the speed of
light, and there is no discordance with observation.
It is often urged that Newton's law of gravitation is much simpler than
Einstein's new law. That depends on the point of view; and from the
point of view of the four-dimensional world Newton's law is far more complicated.
Moreover, it will be seen that if the ambiguities are to be cleared
up, the statement of Newton's law must be greatly expanded.
Some attempts have been made to expand Newton's law on the basis
of the restricted principle of relativity (p. 18) alone. This was insucient
to determine a denite amendment. Using the principle of equivalence,
or relativity of force, we have arrived at a denite law proposed in the
last chapter. Probably the question has arisen in the reader's mind, why
should it be called the law of gravitation? It may be plausible as a law
of nature; but what has the degree of curvature of space-time to do with
attractive forces, whether real or apparent?
A race of 
at-sh once lived in an ocean in which there were only two
dimensions. It was noticed that in general shes swam in straight lines,
unless there was something obviously interfering with their free courses.
This seemed a very natural behaviour. But there was a certain region
where all the sh seemed to be bewitched; some passed through the region
but changed the direction of their swim, others swam round and round
indenitely. One sh invented a theory of vortices, and said that there
were whirlpools in that region which carried everything round in curves.
By-and-by a far better theory was proposed; it was said that the shes were
all attracted towards a particularly large sh|a sun-sh|which was lying
asleep in the middle of the region; and that was what caused the deviation
of their paths. The theory might not have sounded particularly plausible
at rst; but it was conrmed with marvellous exactitude by all kinds of
experimental tests. All sh were found to possess this attractive power
in proportion to their sizes; the law of attraction was extremely simple,
and yet it was found to explain all the motions with an accuracy never
approached before in any scientic investigations. Some sh grumbled that
they did not see how there could be such an in
uence at a distance; but
it was generally agreed that the in
uence was communicated through the
ocean and might be better understood when more was known about the
nature of water. Accordingly, nearly every sh who wanted to explain the
attraction started by proposing some kind of mechanism for transmitting
it through the water.
But there was one sh who thought of quite another plan. He was
Appendix, Note 6.
88 THE NEW LAW OF GRAVITATION [ch.
impressed by the fact that whether the sh were big or little they always
took the same course, although it would naturally take a bigger force to
de
ect the bigger sh. He therefore concentrated attention on the courses
rather than on the forces. And then he arrived at a striking explanation of
the whole thing. There was a mound in the world round about where the
sun-sh lay. Flat-sh could not appreciate it directly because they were
two-dimensional; but whenever a sh went swimming over the slopes of
the mound, although he did his best to swim straight on, he got turned
round a bit. (If a traveller goes over the left slope of a mountain, he must
consciously keep bearing away to the left if he wishes to keep to his original
direction relative to the points of the compass.) This was the secret of the
mysterious attraction, or bending of the paths, which was experienced in
the region.
The parable is not perfect, because it refers to a hummock in space
alone, whereas we have to deal with hummocks in space-time. But it
illustrates how a curvature of the world we live in may give an illusion
of attractive force, and indeed can only be discovered through some such
eect. How this works out in detail must now be considered.
In the form G = 0, Einstein's law expresses conditions to be satised
in a gravitational eld produced by any arbitrary distribution of attracting
matter. An analogous form of Newton's law was given by Laplace in his
celebrated expression r2V = 0. A more illuminating form of the law is
obtained if, instead of putting the question what kinds of space-time can
exist under the most general conditions in an empty region, we ask what
kind of space-time exists in the region round a single attracting particle?
We separate out the eect of a single particle, just as Newton did. We
can further simplify matters by introducing some denite mesh-system,
which, of course, must be of a type which is not inconsistent with the kind
of space-time found.
We need only consider space of two dimensions|sucient for the socalled
plane orbit of a planet|time being added as the third dimension.
The remaining dimension of space can always be added, if desired, by
conditions of symmetry. The result of long algebraic calculations is that,
round a particle
ds2 = 􀀀
1


dr2 􀀀 r2 d2 + 
 dt2 (6)
where 
 = 1 􀀀
2m
r
.
Appendix, Note 7.
vi] AND THE OLD LAW 89
The quantity m is the gravitational mass of the particle|but we are
not supposed to know that at present. r and  are polar coordinates, the
mesh-system being as in Fig. 11; or rather they are the nearest thing to
polar coordinates that can be found in space which is not truly 
at.
The fact is that this expression for ds2 is found in the rst place simply
as a particular solution of Einstein's equations of the gravitational eld;
it is a variety of hummock (apparently the simplest variety) which is not
curved beyond the rst degree. There could be such a state of the world
under suitable circumstances. To nd out what those circumstances are,
we have to trace some of the consequences, nd out how any particle moves
when ds2 is of this form, and then examine whether we know of any case
in which these consequences are found observationally. It is only after
having ascertained that this form of ds2 does correspond to the leading
observed eects attributable to a particle of mass m at the origin that we
have the right to identify this particular solution with the one we hoped
to nd.
It will be a sucient illustration of this procedure, if we indicate how the
position of the matter causing this particular solution is located. Wherever
the formula (6) holds good there can be no matter, because the law which
applies to empty space is satised. But if we try to approach the origin
(r = 0), a curious thing happens. Suppose we take a measuring-rod, and,
laying it radially, start marking o equal lengths with it along a radius,
gradually approaching the origin. Keeping the time t constant, and d
being zero for radial measurements, the formula (6) reduces to
ds2 = 􀀀
1


dr2
or dr2 = 􀀀
 ds2:
We start with r large. By-and-by we approach the point where r = 2m.
But here, from its denition, 
 is equal to 0. So that, however large the
measured interval ds may be, dr = 0. We can go on shifting the measuringrod
through its own length time after time, but dr is zero; that is to say,
we do not reduce r. There is a magic circle which no measurement can
bring us inside. It is not unnatural that we should picture something
obstructing our closer approach, and say that a particle of matter is lling
up the interior.
The fact is that so long as we keep to space-time curved only in the rst
degree, we can never round o the summit of the hummock. It must end in
an innite chimney. In place of the chimney, however, we round it o with
a small region of greater curvature. This region cannot be empty because
90 THE NEW LAW OF GRAVITATION [ch.
the law applying to empty space does not hold. We describe it therefore as
containing matter|a procedure which practically amounts to a denition
of matter. Those familiar with hydrodynamics may be reminded of the
problem of the irrotational rotation of a 
uid; the conditions cannot be
satised at the origin, and it is necessary to cut out a region which is lled
by a vortex-lament.
A word must also be said as to the coordinates r and t used in (6). They
correspond to our ordinary notion of radial distance and time|as well as
any variables in a non-Euclidean world can correspond to words which, as
ordinarily used, presuppose a Euclidean world. We shall thus call r and t,
distance and time. But to give names to coordinates does not give more
information|and in this case gives considerably less information|than
is already contained in the formula for ds2. If any question arises as to
the exact signicance of r and t it must always be settled by reference to
equation (6).
The want of 
atness in the gravitational eld is indicated by the deviation
of the coecient 
 from unity. If the mass m = 0, 
 = 1, and
space-time is perfectly 
at. Even in the most intense gravitational elds
known, the deviation is extremely small. For the sun, the quantity m,
called the gravitational mass, is only 1:47 kilometres, for the earth it is
5 millimetres. In any practical problem the ratio 2m=r must be exceedingly
small. Yet it is on the small corresponding dierence in 
 that the
whole of the phenomena of gravitation depend.
The coecient 
 appears twice in the formula, and so modies the

atness of space-time in two ways. But as a rule these two ways are by no
means equally important. Its appearance as a coecient of dt2 produces
much the most striking eects. Suppose that it is wished to measure
the interval between two events in the history of a planet. If the events
are, say 1 second apart in time, dt = 1 second = 300; 000 kilometres.
Thus dt2 = 90; 000; 000; 000 sq. km. Now no planet moves more than
50 kilometres in a second, so that the change dr associated with the lapse
of 1 second in the history of the planet will not be more than 50 km. Thus
dr2 is not more than 2500 sq. km. Evidently the small term 2m=r has a
much greater chance of making an impression where it is multiplied by dt2
than where it is multiplied by dr2.
Accordingly as a rst approximation, we ignore the coecient of dr2,
and consider only the meaning of
ds2 = 􀀀dr2 􀀀 r2 d2 + (1 􀀀 2m=r) dt2: (7)
Appendix, Note 8.
vi] AND THE OLD LAW 91
We shall now show that particles situated in this kind of space-time will
appear to be under the in
uence of an attractive force directed towards
the origin.
Let us consider the problem of mapping a small portion of this kind of
world on a plane.
It is rst necessary to dene carefully the distinction which is here drawn
between a \picture" and a \map." If we are given the latitudes and
longitudes of a number of places on the earth, we can make a picture by
taking latitude and longitude as vertical and horizontal distances, so that
the lines of latitude and longitude form a mesh-system of squares; but
that does not give a true map. In an ordinary map of Europe the lines of
longitude run obliquely and the lines of latitude are curved. Why is this?
Because the map aims at showing as accurately as possible all distances
in their true proportions. Distance is the important thing which it is
desired to represent correctly. In four dimensions interval is the analogue
of distance, and a map of the four-dimensional world will aim at showing
all the intervals in their correct proportions. Our natural picture of spacetime
takes r and t as horizontal and vertical distances, e.g. when we plot
the graph of the motion of a particle; but in a true map, representing the
intervals in their proper proportions, the r and t lines run obliquely or in
curves across the map.
The instructions for drawing latitude and longitude lines (; ) on a
map, are summed up in the formula for ds, p. 71,
ds2 = d2 + cos2  d2;
and similarly the instructions for drawing the r and t lines are given by
the formula (7).
The map is shown in Fig. 14. It is not dicult to see why the t-lines
converge to the left of the diagram. The factor 1􀀀2m=r decreases towards
the left where r is small; and consequently any change of t corresponds
to a shorter interval, and must be represented in the map by a shorter
distance on the left. It is less easy to see why the r-lines take the courses
shown; by analogy with latitude and longitude we might expect them to
be curved the other way. But we discussed in Chapter iii how the slope
of the time-direction is connected with the slope of the space-direction;
and it will be seen that the map gives approximately diamond-shaped
This is usually the object, though maps are sometimes made for a dierent purpose,
e.g. Mercator's Chart.
92 THE NEW LAW OF GRAVITATION [ch.
Fig. 14.
P
Q
partitions of the kind represented in Fig. 6.
Like all maps of curved surfaces, the diagram is only accurate in the
limit when the area covered is very small.
It is important to understand clearly the meaning of this map. When
we speak in the ordinary way of distance from the sun and the time at a
point in the solar system, we mean the two variables r and t. These are
not the result of any precise measures with scales and clocks made at a
point, but are mathematical variables most appropriate for describing the
whole solar system. They represent a compromise, because it is necessary
to deal with a region too large for accurate representation on a plane map.
We should naturally picture them as rectangular coordinates partitioning
space-time into square meshes, as in Fig. 15; but such a picture is not
a true map, because it does not represent in their true proportions the
intervals between the various points in the picture. It is not possible to
draw any map of the whole curved region without distortion; but a small
The substitution x = r + 1
2 t2m=r2, y = t(1 􀀀 m=r), gives ds2 = 􀀀dx2 + dy2, if
squares of m are negligible. The map is drawn with x and y as rectangular coordinates.
vi] AND THE OLD LAW 93
Fig. 15.
P
Q
enough portion can be represented without distortion if the partitions of
equal r and t are drawn as in Fig. 14. To get back from the true map
to the customary picture of r and t as perpendicular space and time, we
must strain Fig. 14 until all the meshes become squares as in Fig. 15.
Now in the map the geometry is Euclidean and the tracks of all material
particles will be straight lines. Take such a straight track PQ, which will
necessarily be nearly vertical, unless the velocity is very large. Strain the
gure so as to obtain the customary representation of r and t (in Fig. 15),
and the track PQ will become curved|curved towards the left, where the
sun lies. In each successive vertical interval (time), a successively greater
progress is made to the left horizontally (space). Thus the velocity towards
the sun increases. We say that the particle is attracted to the sun.
The mathematical reader should nd no diculty in proving from the
diagram that for a particle with small velocity the acceleration towards
the sun is approximately m=r2, agreeing with the Newtonian law.
Tracks for very high speeds may be aected rather dierently. The track
corresponding to a wave of light is represented by a straight line at 45° to
the horizontal in Fig. 14. It would require very careful drawing to trace
what happens to it when the strain is made transforming to Fig. 15; but
94 THE NEW LAW OF GRAVITATION [ch.
actually, whilst becoming more nearly vertical, it receives a curvature in
the opposite direction. The eect of the gravitation of the sun on a lightwave,
or very fast particle, proceeding radially is actually a repulsion!
The track of a transverse light-wave, coming out from the plane of the
paper, will be aected like that of a particle of zero velocity in distorting
from Fig. 14 to Fig. 15. Hence the sun's in
uence on a transverse lightwave
is always an attraction. The acceleration is simply m=r2 as for a
particle at rest.
The result that the expression found for the geometry of the gravitational
eld of a particle leads to Newton's law of attraction is of great
importance. It shows that the law, G = 0, proposed on theoretical
grounds, agrees with observation at least approximately. It is no drawback
that the Newtonian law applies only when the speed is small; all
planetary speeds are small compared with the velocity of light, and the
considerations mentioned at the beginning of this chapter suggest that
some modication may be needed for speeds comparable with that of
light.
Another important point to notice is that the attraction of gravitation
is simply a geometrical deformation of the straight tracks. It makes no
dierence what body or in
uence is pursuing the track, the deformation is
a general discrepancy between the \mental picture" and the \true map"
of the portion of space-time considered. Hence light is subject to the
same disturbance of path as matter. This is involved in the Principle of
Equivalence; otherwise we could distinguish between the acceleration of
a lift and a true increase of gravitation by optical experiments; in that
case the observer for whom light-rays appear to take straight tracks might
be described as absolutely unaccelerated and there could be no relativity
theory. Physicists in general have been prepared to admit the likelihood of
an in
uence of gravitation on light similar to that exerted on matter; and
the problem whether or not light has \weight" has often been considered.
The appearance of 
 as the coecient of dt2 is responsible for the main
features of Newtonian gravitation; the appearance of 1=
 as the coecient
of dr2 is responsible for the principal deviations of the new law from the
old. This classication seems to be correct; but the Newtonian law is
ambiguous and it is dicult to say exactly what are to be regarded as
discrepancies from it. Leaving aside now the time-term as suciently
vi] AND THE OLD LAW 95
discussed, we consider the space-terms alone
ds2 =
1


dr2 + r2 d2:
The expression shows that space considered alone is non-Euclidean in
the neighbourhood of an attracting particle. This is something entirely
outside the scope of the old law of gravitation. Time can only be explored
by something moving, whether a free particle or the parts of a clock,
so that the non-Euclidean character of space-time can be covered up by
introducing a eld of force, suitably modifying the motion, as a convenient
ction. But space can be explored by static methods; and theoretically
its non-Euclidean character could be ascertained by suciently precise
measures with rigid scales.
If we lay our measuring scale transversely and proceed to measure the
circumference of a circle of nominal radius r, we see from the formula that
the measured length ds is equal to r d, so that, when we have gone right
round the circle,  has increased by 2 and the measured circumference is
2r. But when we lay the scale radially the measured length ds is equal to
dr=p
, which is always greater than dr. Thus, in measuring a diameter,
we obtain a result greater than 2r, each portion being greater than the
corresponding change of r.
Thus if we draw a circle, placing a massive particle near the centre
so as to produce a gravitational eld, and measure with a rigid scale
the circumference and the diameter, the ratio of the measured circumference
to the measured diameter will not be the famous number  =
3:141592653589793238462643383279 : : : but a little smaller. Or if we inscribe
a regular hexagon in this circle its sides will not be exactly equal
to the radius of the circle. Placing the particle near, instead of at, the
centre, avoids measuring the diameter through the particle, and so makes
the experiment a practical one. But though practical, it is not practicable
to determine the non-Euclidean character of space in this way. Sucient
renement of measures is not attainable. If the mass of a ton were placed
inside a circle of 5 yards radius, the defect in the value of  would only
appear in the twenty-fourth or twenty-fth place of decimals.
It is of value to put the result in this way, because it shows that the
relativist is not talking metaphysics when he says that space in the gravitational
eld is non-Euclidean. His statement has a plain physical meaning,
We change the sign of ds2, so that ds, when real, means measured space instead
of measured time.
96 THE NEW LAW OF GRAVITATION [ch.
which we may some day learn how to test experimentally. Meanwhile we
can test it by indirect methods.
Suppose that a plane eld is uniformly studded with hurdles. The distance
between any two points will be proportional to the number of hurdles
that must be passed over in getting from one point to the other by
the straight route|in fact the minimum number of hurdles. We can use
counts of hurdles as the equivalent of distance, and map the eld by these
counts. The map can be drawn on a plane sheet of paper without any inconsistency,
since the eld is plane. Let us now dismiss from our minds all
idea of distances in the eld or straight lines in the eld, and assume that
distances on the map merely represent the minimum number of hurdles
between two points; straight lines on the map will represent the corresponding
routes. This has the advantage that if an earthquake occurs,
deforming the eld, the map will still be correct. The path of fewest hurdles
will still cross the same hurdles as before the earthquake; it will be
twisted out of the straight line in the eld; but we should gain nothing by
taking a straighter course, since that would lead through a region where
the hurdles are more crowded. We do not alter the number of hurdles in
any path by deforming it.
This can be illustrated by Figs. 14 and 15. Fig. 14 represents the original
undistorted eld with the hurdles uniformly placed. The straight line
PQ represents the path of fewest hurdles from P to Q, and its length is
proportional to the number of hurdles. Fig. 15 represents the distorted
eld, with PQ distorted into a curve; but PQ is still the path of fewest
hurdles from P to Q, and the number of hurdles in the path is the same
as before. If therefore we map according to hurdle-counts we arrive at
Fig. 14 again, just as though no deformation had taken place.
To make any dierence in the hurdle-counts, the hurdles must be taken
up and replanted. Starting from a given point as centre, let us arrange
them so that they gradually thin out towards the boundaries of the eld.
Now choose a circle with this point as centre;|but rst, what is a circle?
It has to be dened in terms of hurdle-counts; and clearly it must be a
curve such that the minimum number of hurdles between any point on
it and the centre is a constant (the radius). With this denition we can
defy earthquakes. The number of hurdles in the circumference of such a
circle will not bear the same proportion to the number in the radius as
in the eld of uniform hurdles; owing to the crowding near the centre,
the ratio will be less. Thus we have a suitable analogy for a circle whose
circumference is less than  times its diameter.
This analogy enables us to picture the condition of space round a heavy
vi] AND THE OLD LAW 97
particle, where the ratio of the circumference of a circle to the diameter
is less than . Hurdle-counts will no longer be accurately mappable on a
plane sheet of paper, because they do not conform to Euclidean geometry.
Now suppose a heavy particle wishes to cross this eld, passing near but
not through the centre. In Euclidean space, with the hurdles uniformly
distributed, it travels in a straight line, i.e. it goes between any two points
by a path giving the fewest hurdle jumps. We may assume that in the
non-Euclidean eld with rearranged hurdles, the particle still goes by the
path of least eort. In fact, in any small portion we cannot distinguish
between the rearrangement and a distortion; so we may imagine that the
particle takes each portion as it comes according to the rule, and is not
troubled by the rearrangement which is only visible to a general survey of
the whole eld.
Now clearly it will pay not to go straight through the dense portion,
but to keep a little to the outside where the hurdles are sparser|not too
much, or the path will be unduly lengthened. The particle's track will
thus be a little concave to the centre, and an onlooker will say that it
has been attracted to the centre. It is rather curious that we should call
it attraction, when the track has rather been avoiding the central region;
but it is clear that the direction of motion has been bent round in the way
attributable to an attractive force.
This bending of the path is additional to that due to the Newtonian
force of gravitation which depends on the second appearance of 
 in the
formula. As already explained it is in general a far smaller eect and will
appear only as a minute correction to Newton's law. The only case where
the two rise to equal importance is when the track is that of a light-wave,
or of a particle moving with a speed approaching that of light; for then
dr2 rises to the same order of magnitude as dt2.
To sum up, a ray of light passing near a heavy particle will be bent,
rstly, owing to the non-Euclidean character of the combination of time
with space. This bending is equivalent to that due to Newtonian gravitation,
and may be calculated in the ordinary way on the assumption that
light has weight like a material body. Secondly, it will be bent owing to the
non-Euclidean character of space alone, and this curvature is additional
to that predicted by Newton's law. If then we can observe the amount of
curvature of a ray of light, we can make a crucial test of whether Einstein's
or Newton's theory is obeyed.
There must be some absolute track, and if absolute signicance can only be associated
with hurdle-counts and not with distances in the eld, the path of fewest hurdles
is the only track capable of absolute denition.
98 THE NEW LAW OF GRAVITATION [ch.
This separation of the attraction into two parts is useful in a comparison
of the new theory with the old; but from the point of view of relativity it
is articial. Our view is that light is bent just in the same way as the track
of a material particle moving with the same velocity would be bent. Both
causes of bending may be ascribed either to weight or to non-Euclidean
space-time, according to the nomenclature preferred. The only dierence
between the predictions of the old and new theories is that in one case the
weight is calculated according to Newton's law of gravitation, in the other
case according to Einstein's.
There is an alternative way of viewing this eect on light according
to Einstein's theory, which, for many reasons is to be preferred. This
depends on the fact that the velocity of light in the gravitational eld
is not a constant (unity) but becomes smaller as we approach the sun.
This does not mean that an observer determining the velocity of light
experimentally at a spot near the sun would detect the decrease; if he
performed Fizeau's experiment, his result in kilometres per second would
be exactly the same as that of a terrestrial observer. It is the coordinate
velocity that is here referred to, described in terms of the quantities r, ,
t, introduced by the observer who is contemplating the whole solar system
at the same time.
It will be remembered that in discussing the approximate geometry of
space-time in Fig. 3, we found that certain events like P were in the
absolute past or future of O, and others like P0 were neither before nor
after O, but elsewhere. Analytically the distinction is that for the interval
OP, ds2 is positive; for OP0, ds2 is negative. In the rst case the interval
is real or \time-like"; in the second it is imaginary or \space-like." The
two regions are separated by lines (or strictly, cones) in crossing which
ds2 changes from positive to negative; and along the lines themselves ds is
zero. It is clear that these lines must have important absolute signicance
in the geometry of the world. Physically their most important property
is that pulses of light travel along these tracks, and the motion of a lightpulse
is always given by the equation ds = 0.
Using the expression for ds2 in a gravitational eld, we accordingly have
for light
0 = 􀀀
1


dr2 􀀀 r2 d2 + 
 dt2:
For radial motion, d = 0, and therefore
dr
dt2
= 
2:
vi] AND THE OLD LAW 99
For transverse motion, dr = 0, and therefore
r d
dt 2
= 
:
Thus the coordinate velocity of light travelling radially is 
, and of light
travelling transversely is p
, in the coordinates chosen.
The coordinate velocity must depend on the coordinates chosen; and it
is more convenient to use a slightly dierent system in which the velocity
of light is the same in all directions, viz. 
 or 1 􀀀 2m=r. This diminishes
as we approach the sun|an illustration of our previous remark that a
pulse of light proceeding radially is repelled by the sun.
The wave-motion in a ray of light can be compared to a succession of
long straight waves rolling onward in the sea. If the motion of the waves
is slower at one end than the other, the whole wave-front must gradually
slew round, and the direction in which it is rolling must change. In the sea
this happens when one end of the wave reaches shallow water before the
other, because the speed in shallow water is slower. It is well known that
this causes waves proceeding diagonally across a bay to slew round and
come in parallel to the shore; the advanced end is delayed in the shallow
water and waits for the other. In the same way when the light waves pass
near the sun, the end nearest the sun has the smaller velocity and the
wave-front slews round; thus the course of the waves is bent.
Light moves more slowly in a material medium than in vacuum, the
velocity being inversely proportional to the refractive index of the medium.
The phenomenon of refraction is in fact caused by a slewing of the wavefront
in passing into a region of smaller velocity. We can thus imitate the
gravitational eect on light precisely, if we imagine the space round the
sun lled with a refracting medium which gives the appropriate velocity
of light. To give the velocity 1 􀀀 2m=r, the refractive index must be
1=(1 􀀀 2m=r), or, very approximately, 1 + 2m=r. At the surface of the
sun, r = 697; 000 km., m = 1:47 km., hence the necessary refractive
index is 1:00000424. At a height above the sun equal to the radius it
is 1:00000212.
This is obtained by writing r+m instead of r, or diminishing the nominal distance
of the sun by 11
2 kilometres. This change of coordinates simplies the problem, but
can, of course, make no dierence to anything observable. After we have traced the
course of the light ray in the coordinates chosen, we have to connect the results with
experimental measures, using the corresponding formula for ds2. This nal connection
of mathematical and experimental results is, however, comparatively simple, because
it relates to measuring operations performed in a terrestrial observatory where the
dierence of 
 from unity is negligible.
100 THE NEW LAW OF GRAVITATION
Any problem on the paths of rays near the sun can now be solved by the
methods of geometrical optics applied to the equivalent refracting medium.
It is not dicult to show that the total de
ection of a ray of light passing
at a distance r from the centre of the sun is (in circular measure)
4m
r
;
whereas the de
ection of the same ray calculated on the Newtonian theory
would be
2m
r
:
For a ray grazing the surface of the sun the numerical value of this
de
ection is
100:75 (Einstein's theory);
000:87 (Newtonian theory):
CHAPTER VII
WEIGHING LIGHT
Query 1. Do not Bodies act upon Light at a distance, and by their action
bend its Rays, and is not this action (caeteris paribus) strongest at the least
distance? Newton, Opticks.
We come now to the experimental test of the in
uence of gravitation
on light discussed theoretically in the last chapter. It is not the general
purpose of this book to enter into details of experiments; and if we followed
this plan consistently, we should, as hitherto, summarise the results of the
observations in a few lines. But it is this particular test which has turned
public attention towards the relativity theory, and there appears to be
widespread desire for information. We shall therefore tell the story of
the eclipse expeditions in some detail. It will make a break in the long
theoretical arguments, and will illustrate the important applications of
this theory to practical observations.
It must be understood that there were two questions to answer: rstly,
whether light has weight (as suggested by Newton), or is indierent to
gravitation; secondly, if it has weight, is the amount of the de
ection in
accordance with Einstein's or Newton's laws?
It was already known that light possesses mass or inertia like other
forms of electromagnetic energy. This is manifested in the phenomena
of radiation-pressure. Some force is required to stop a beam of light by
holding an obstacle in its path; a searchlight experiences a minute force
of recoil just as if it were a machine-gun ring material projectiles. The
force, which is predicted by orthodox electromagnetic theory, is exceedingly
minute; but delicate experiments have detected it. Probably this
inertia of radiation is of great cosmical importance, playing a great part
in the equilibrium of the more diuse stars. Indeed it is probably the
agent which has carved the material of the universe into stars of roughly
uniform mass. Possibly the tails of comets are a witness to the power of
102 WEIGHING LIGHT [ch.
the momentum of sunlight, which drives outwards the smaller or the more
absorptive particles.
It is legitimate to speak of a pound of light as we speak of a pound of
any other substance. The mass of ordinary quantities of light is however
extremely small, and I have calculated that at the low charge of 3d. a
unit, an Electric Light Company would have to sell light at the rate of
£140,000,000 a pound. All the sunlight falling on the earth amounts to
160 tons daily.
It is perhaps not easy to realise how a wave-motion can have inertia,
and it is still more dicult to understand what is meant by its having
weight. Perhaps this will be better understood if we put the problem in
a concrete form. Imagine a hollow body, with radiant heat or light-waves
traversing the hollow; the mass of the body will be the sum of the masses
of the material and of the radiant energy in the hollow; a greater force
will be required to shift it because of the light-waves contained in it. Now
let us weigh it with scales or a spring-balance. Will it also weigh heavier
on account of the radiation contained, or will the weight be that of the
solid material alone? If the former, then clearly from this aspect light has
weight; and it is not dicult to deduce the eect of this weight on a freely
moving light-beam not enclosed within a hollow.
The eect of weight is that the radiation in the hollow body acquires
each second a downward momentum proportional to its mass. This in the
long run is transmitted to the material enclosing it. For a free light-wave
in space, the added momentum combines with the original momentum,
and the total momentum determines the direction of the ray, which is
accordingly bent. Newton's theory suggests no means for bringing about
the bending, but contents itself with predicting it on general principles.
Einstein's theory provides a means, viz. the variation of velocity of the
waves.
Hitherto mass and weight have always been found associated in strict
proportionality. One very important test had already shown that this proportionality
is not conned to material energy. The substance uranium
contains a great deal of radio-active energy, presumably of an electromagnetic
nature, which it slowly liberates. The mass of this energy must be
an appreciable fraction of the whole mass of the substance. But it was
shown by experiments with the Eotvos torsion-balance that the ratio of
weight to mass for uranium is the same as for all other substances; so the
energy of radio-activity has weight. Still even this experiment deals only
with bound electromagnetic energy, and we are not justied in deducing
the properties of the free energy of light.
vii] WEIGHING LIGHT 103
It is easy to see that a terrestrial experiment has at present no chance of
success. If the mass and weight of light are in the same proportion as for
matter, the ray of light will be bent just like the trajectory of a material
particle. On the earth a ri
e bullet, like everything else, drops 16 feet
in the rst second, 64 feet in two seconds, and so on, below its original
line of 
ight; the ri
e must thus be aimed above the target. Light would
also drop 16 feet in the rst second; but, since it has travelled 186; 000
miles along its course in that time, the bend is inappreciable. In fact any
Fig. 16.
S
E F P′
Q
P
terrestrial course is described so quickly that gravitation has scarcely had
time to accomplish anything.
The experiment is therefore transferred to the neighbourhood of the
sun. There we get a pull of gravitation 27 times more intense than on the
earth; and|what is more important|the greater size of the sun permits
a much longer trajectory throughout which the gravitation is reasonably
powerful. The de
ection in this case may amount to something of the order
of a second of arc, which for the astronomer is a fairly large quantity.
In Fig. 16 the line EFQP shows the track of a ray of light from a
distant star P which reaches the earth E. The main part of the bending
of the ray occurs as it passes the sun S; and the initial course PQ and
the nal course FE are practically straight. Since the light rays enter the
observer's eye or telescope in the direction FE, this will be the direction
in which the star appears. But its true direction from the earth is QP,
the initial course. So the star appears displaced outwards from its true
position by an angle equal to the total de
ection of the light.
It must be noticed that this is only true because a star is so remote that
its true direction with respect to the earth E is indistinguishable from its
direction with respect to the point Q. For a source of light within the solar
system, the apparent displacement of the source is by no means equal to
the de
ection of the light-ray. It is perhaps curious that the attraction
of light by the sun should produce an apparent displacement of the star
away from the sun; but the necessity for this is clear.
Or 32 feet according to Einstein's law. The fall increases with the speed of the
motion.
104 WEIGHING LIGHT [ch.
The bending aects stars seen near the sun, and accordingly the only
chance of making the observation is during a total eclipse when the moon
cuts o the dazzling light. Even then there is a great deal of light from
the sun's corona which stretches far above the disc. It is thus necessary
to have rather bright stars near the sun, which will not be lost in the
glare of the corona. Further the displacements of these stars can only be
measured relatively to other stars, preferably more distant from the sun
and less displaced; we need therefore a reasonable number of outer bright
stars to serve as reference points.
In a superstitious age a natural philosopher wishing to perform an important
experiment would consult an astrologer to ascertain an auspicious
moment for the trial. With better reason, an astronomer to-day consulting
the stars would announce that the most favourable day of the year for
weighing light is May 29. The reason is that the sun in its annual journey
round the ecliptic goes through elds of stars of varying richness, but on
May 29 it is in the midst of a quite exceptional patch of bright stars|part
of the Hyades|by far the best star-eld encountered. Now if this problem
had been put forward at some other period of history, it might have
been necessary to wait some thousands of years for a total eclipse of the
sun to happen on the lucky date. But by strange good fortune an eclipse
did happen on May 29, 1919. Owing to the curious sequence of eclipses
a similar opportunity will recur in 1938; we are in the midst of the most
favourable cycle. It is not suggested that it is impossible to make the test
at other eclipses; but the work will necessarily be more dicult.
Attention was called to this remarkable opportunity by the Astronomer
Royal in March, 1917; and preparations were begun by a Committee of
the Royal Society and Royal Astronomical Society for making the observations.
Two expeditions were sent to dierent places on the line of totality
to minimise the risk of failure by bad weather. Dr A. C. D. Crommelin
and Mr C. Davidson went to Sobral in North Brazil; Mr E. T. Cottingham
and the writer went to the Isle of Principe in the Gulf of Guinea, West
Africa. The instrumental equipment for both expeditions was prepared at
Greenwich Observatory under the care of the Astronomer Royal; and here
Mr Davidson made the arrangements which were the main factor in the
success of both parties.
The circumstances of the two expeditions were somewhat dierent and
it is scarcely possible to treat them together. We shall at rst follow
the fortunes of the Principe observers. They had a telescope of focal
length 11 feet 4 inches. On their photographs 1 second of arc (which
was about the largest displacement to be measured) corresponds to about
vii] WEIGHING LIGHT 105
1
1500 inch|by no means an inappreciable quantity. The aperture of the
object-glass was 13 inches, but as used it was stopped down to 8 inches
to give sharper images. It is necessary, even when the exposure is only a
few seconds, to allow for the diurnal motion of the stars across the sky,
making the telescope move so as to follow them. But since it is dicult to
mount a long and heavy telescope in the necessary manner in a temporary
installation in a remote part of the globe, the usual practice at eclipses is to
keep the telescope rigid and re
ect the stars into it by a coelostat|a plane
mirror kept revolving at the right rate by clock-work. This arrangement
was adopted by both expeditions.
The observers had rather more than a month on the island to make their
preparations. On the day of the eclipse the weather was unfavourable.
When totality began the dark disc of the moon surrounded by the corona
was visible through cloud, much as the moon often appears through cloud
on a night when no stars can be seen. There was nothing for it but to
carry out the arranged programme and hope for the best. One observer
was kept occupied changing the plates in rapid succession, whilst the other
gave the exposures of the required length with a screen held in front of
the object-glass to avoid shaking the telescope in any way.
For in and out, above, about, below
'Tis nothing but a Magic Shadow-show
Played in a Box whose candle is the Sun
Round which we Phantom Figures come and go.
Our shadow-box takes up all our attention. There is a marvellous spectacle
above, and, as the photographs afterwards revealed, a wonderful
prominence-
ame is poised a hundred thousand miles above the surface of
the sun. We have no time to snatch a glance at it. We are conscious only
of the weird half-light of the landscape and the hush of nature, broken
by the calls of the observers, and beat of the metronome ticking out the
302 seconds of totality.
Sixteen photographs were obtained, with exposures ranging from 2 to
20 seconds. The earlier photographs showed no stars, though they portrayed
the remarkable prominence; but apparently the cloud lightened
somewhat towards the end of totality, and a few images appeared on the
later plates. In many cases one or other of the most essential stars was
missing through cloud, and no use could be made of them; but one plate
was found showing fairly good images of ve stars, which were suitable
for a determination. This was measured on the spot a few days after the
106 WEIGHING LIGHT [ch.
eclipse in a micrometric measuring-machine. The problem was to determine
how the apparent positions of the stars, aected by the sun's gravitational
eld, compared with the normal positions on a photograph taken
when the sun was out of the way. Normal photographs for comparison had
been taken with the same telescope in England in January. The eclipse
photograph and a comparison photograph were placed lm to lm in the
measuring-machine so that corresponding images fell close together, and
the small distances were measured in two rectangular directions. From
these the relative displacements of the stars could be ascertained. In
comparing two plates, various allowances have to be made for refraction,
aberration, plate-orientation, etc.; but since these occur equally in determinations
of stellar parallax, for which much greater accuracy is required,
the necessary procedure is well-known to astronomers.
The results from this plate gave a denite displacement, in good accordance
with Einstein's theory and disagreeing with the Newtonian prediction.
Although the material was very meagre compared with what had
been hoped for, the writer (who it must be admitted was not altogether
unbiassed) believed it convincing.
It was not until after the return to England that any further conrmation
was forthcoming. Four plates were brought home undeveloped,
as they were of a brand which would not stand development in the hot
climate. One of these was found to show sucient stars; and on measurement
it also showed the de
ection predicted by Einstein, conrming the
other plate.
The bugbear of possible systematic error aects all investigations of this
kind. How do you know that there is not something in your apparatus responsible
for this apparent de
ection? Your object-glass has been shaken
up by travelling; you have introduced a mirror into your optical system;
perhaps the 50° rise of temperature between the climate at the equator and
England in winter has done some kind of mischief. To meet this criticism,
a dierent eld of stars was photographed at night in Principe and also in
England at the same altitude as the eclipse eld. If the de
ection were really
instrumental, stars on these plates should show relative displacements
of a similar kind to those on the eclipse plates. But on measuring these
check-plates no appreciable displacements were found. That seems to be
satisfactory evidence that the displacement observed during the eclipse is
really due to the sun being in the region, and is not due to dierences in
instrumental conditions between England and Principe. Indeed the only
This was possible because at Principe the eld of stars was re
ected in the coelostat
mirror, whereas in England it was photographed direct.
vii] WEIGHING LIGHT 107
possible loophole is a dierence between the night conditions at Principe
when the check-plates were taken, and the day, or rather eclipse, conditions
when the eclipse photographs were taken. That seems impossible
since the temperature at Principe did not vary more than 1° between day
and night.
The problem appeared to be settled almost beyond doubt; and it was
with some condence that we awaited the return of the other expedition
from Brazil. The Brazil party had had ne weather and had gained far
more extensive material on their plates. They had remained two months
after the eclipse to photograph the same region before dawn, when clear
of the sun, in order that they might have comparison photographs taken
under exactly the same circumstances. One set of photographs was secured
with a telescope similar to that used at Principe. In addition they
used a longer telescope of 4 inches aperture and 19 feet focal length.
The photographs obtained with the former were disappointing. Although
the full number of stars expected (about 12) were shown, and numerous
plates had been obtained, the denition of the images had been spoiled
by some cause, probably distortion of the coelostat-mirror by the heat of
the sunshine falling on it. The observers were pessimistic as to the value
of these photographs; but they were the rst to be measured on return to
England, and the results came as a great surprise after the indications of
the Principe plates. The measures pointed with all too good agreement to
the \half-de
ection," that is to say, the Newtonian value which is one-half
the amount required by Einstein's theory. It seemed dicult to pit the
meagre material of Principe against the wealth of data secured from the
clear sky of Sobral. It is true the Sobral images were condemned, but
whether so far as to invalidate their testimony on this point was not at
rst clear; besides the Principe images were not particularly well-dened,
and were much enfeebled by cloud. Certain compensating advantages of
the latter were better appreciated later. Their strong point was the satisfactory
check against systematic error aorded by the photographs of the
check-eld; there were no check-plates taken at Sobral, and, since it was
obvious that the discordance of the two results depended on systematic error
and not on the wealth of material, this distinctly favoured the Principe
results. Further, at Principe there could be no evil eects from the sun's
rays on the mirror, for the sun had withdrawn all too shyly behind the
veil of cloud. A further advantage was provided by the check-plates at
See Frontispiece. The two telescopes are shown and the backs of the two coelostatmirrors
which re
ect the sky into them. The clock driving the larger mirror is seen on
the pedestal on the left.
108 WEIGHING LIGHT [ch.
Principe, which gave an independent determination of the dierence of
scale of the telescope as used in England and at the eclipse; for the Sobral
plates this scale-dierence was eliminated by the method of reduction,
with the consequence that the results depended on the measurement of a
much smaller relative displacement.
There remained a set of seven plates taken at Sobral with the 4-inch
lens; their measurement had been delayed by the necessity of modifying a
micrometer to hold them, since they were of unusual size. From the rst
no one entertained any doubt that the nal decision must rest with them,
since the images were almost ideal, and they were on a larger scale than
the other photographs. The use of this instrument must have presented
considerable diculties|the unwieldy length of the telescope, the slower
speed of the lens necessitating longer exposures and more accurate driving
of the clock-work, the larger scale rendering the focus more sensitive to
disturbances|but the observers achieved success, and the perfection of
the negatives surpassed anything that could have been hoped for.
These plates were now measured and they gave a nal verdict denitely
conrming Einstein's value of the de
ection, in agreement with the results
obtained at Principe.
It will be remembered that Einstein's theory predicts a de
ection of
100:74 at the edge of the sun, the amount falling o inversely as the
distance from the sun's centre. The simple Newtonian de
ection is half
this, 000:87. The nal results (reduced to the edge of the sun) obtained at
Sobral and Principe with their \probable accidental errors" were
Sobral 100:98  000:12,
Principe 100:61  000:30.
It is usual to allow a margin of safety of about twice the probable error
on either side of the mean. The evidence of the Principe plates is thus
just about sucient to rule out the possibility of the \half-de
ection,"
and the Sobral plates exclude it with practical certainty. The value of
the material found at Principe cannot be put higher than about one-sixth
of that at Sobral; but it certainly makes it less easy to bring criticism
against this conrmation of Einstein's theory seeing that it was obtained
independently with two dierent instruments at dierent places and with
dierent kinds of checks.
The best check on the results obtained with the 4-inch lens at Sobral is
the striking internal accordance of the measures for dierent stars. The
The predicted de
ection of light from innity to innity is just over 100:745, from
innity to the earth it is just under.
vii] WEIGHING LIGHT 109
theoretical de
ection should vary inversely as the distance from the sun's
centre; hence, if we plot the mean radial displacement found for each star
separately against the inverse distance, the points should lie on a straight
line. This is shown in Fig. 17 where the broken line shows the theoretical
Fig. 17.
.01
.02
.03
.04
.05
.06
.07
.08
.09
1′′.00
Distance 90′ 60′ 50′ 40′ 30′ 25′
Displacement
prediction of Einstein, the deviations being within the accidental errors of
the determinations. A line of half the slope representing the half-de
ection
would clearly be inadmissible.
Moreover, values of the de
ection were deduced from the measures in
right ascension and declination independently. These were in close agreement.
A diagram showing the relative positions of the stars is given in Fig. 18.
The square shows the limits of the plates used at Principe, and the
oblique rectangle the limits with the 4-inch lens at Sobral. The centre
of the sun moved from S to P in the 21
4 hours interval between totality
at the two stations; the sun is here represented for a time about midway
between. The stars measured on the Principe plates were Nos. 3, 4, 5, 6,
110 WEIGHING LIGHT [ch.
Fig. 18.
P S
1
2
3
4
5
6
7
8
9
10
11
12
13
N
S
E
W
10, 11; those at Sobral were 11, 10, 6, 5, 4, 2, 3 (in the order of the dots
from left to right in Fig. 17). None of these were fainter than 6m:0, the
brightest 1 Tauri (No. 4) being 4m:5.
It has been objected that although the observations establish a de
ection
of light in passing the sun equal to that predicted by Einstein, it is not
immediately obvious that this de
ection must necessarily be attributed to
the sun's gravitational eld. It is suggested that it may not be an essential
eect of the sun as a massive body, but an accidental eect owing to
the circumstance that the sun is surrounded by a corona which acts as
a refracting atmosphere. It would be a strange coincidence if this atmosphere
imitated the theoretical law in the exact quantitative way shown
in Fig. 17; and the suggestion appears to us far-fetched. However the obv
ii] WEIGHING LIGHT 111
jection can be met in a more direct way. We have already shown that the
gravitational eect on light is equivalent to that produced by a refracting
medium round the sun and have calculated the necessary refractive index.
At a height of 400; 000 miles above the surface the refractive index required
is 1:0000021. This corresponds to air at 1
140 atmosphere, hydrogen
at 1
70 atmosphere, helium at 1
20 atmospheric pressure. It seems obvious
that there can be no material of this order of density at such a distance
from the sun. The pressure on the sun's surface of the columns of material
involved would be of the order 10; 000 atmospheres; and we know
from spectroscopic evidence that there is no pressure of this order. If it is
urged that the mass could perhaps be supported by electrical forces, the
argument from absorption is even more cogent. The light from the stars
photographed during the eclipse has passed through a depth of at least
a million miles of material of this order of density|or say the equivalent
of 10; 000 miles of air at atmospheric density. We know to our cost what
absorption the earth's 5 miles of homogeneous atmosphere can eect. And
yet at the eclipse the stars appeared on the photographs with their normal
brightness. If the irrepressible critic insists that the material round the
sun may be composed of some new element with properties unlike any material
known to us, we may reply that the mechanism of refraction and of
absorption is the same, and there is a limit to the possibility of refraction
without appreciable absorption. Finally it would be necessary to arrange
that the density of the material falls o inversely as the distance from the
sun's centre in order to give the required variation of refractive index.
Several comets have been known to approach the sun within the limits
of distance here considered. If they had to pass through an atmosphere
of the density required to account for the displacement, they would have
suered enormous resistance. Dr Crommelin has shown that a study of
these comets sets an upper limit to the density of the corona, which makes
the refractive eect quite negligible.
Those who regard Einstein's law of gravitation as a natural deduction
from a theory based on the minimum of hypotheses will be satised to nd
that his remarkable prediction is quantitatively conrmed by observation,
and that no unforeseen cause has appeared to invalidate the test.
112 WEIGHING LIGHT
CHAPTER VIII
OTHER TESTS OF THE THEORY
The words of Mercury are harsh after the songs of Apollo.
Love's Labour's Lost.
We have seen that the swift-moving light-waves possess great advantages
as a means of exploring the non-Euclidean property of space. But there
is an old fable about the hare and the tortoise. The slow-moving planets
have qualities which must not be overlooked. The light-wave traverses the
region in a few minutes and makes its report; the planet plods on and on
for centuries going over the same ground again and again. Each time it
goes round it reveals a little about the space, and the knowledge slowly
accumulates.
According to Newton's law a planet moves round the sun in an ellipse,
and if there are no other planets disturbing it, the ellipse remains the same
for ever. According to Einstein's law the path is very nearly an ellipse,
but it does not quite close up; and in the next revolution the path has
advanced slightly in the same direction as that in which the planet was
moving. The orbit is thus an ellipse which very slowly revolves.
The exact prediction of Einstein's law is that in one revolution of the
planet the orbit will advance through a fraction of a revolution equal to
3v2=C2, where v is the speed of the planet and C the speed of light. The
earth has 1=10; 000 of the speed of light; thus in one revolution (one year)
the point where the earth is at greatest distance from the sun will move
on 3=100; 000; 000 of a revolution, or 000:038. We could not detect this
dierence in a year, but we can let it add up for a century at least. It
would then be observable but for one thing|the earth's orbit is very blunt,
very nearly circular, and so we cannot tell accurately enough which way
it is pointing and how its sharpest apses move. We can choose a planet
with higher speed so that the eect is increased, not only because v2 is
Appendix, Note 9.
114 OTHER TESTS OF THE THEORY [ch.
increased, but because the revolutions take less time; but, what is perhaps
more important, we need a planet with a sharp elliptical orbit, so that it
is easy to observe how its apses move round. Both these conditions are
fullled in the case of Mercury. It is the fastest of the planets, and the
predicted advance of the orbit amounts to 4300 per century; further the
eccentricity of its orbit is far greater than that of any of the other seven
planets.
Now an unexplained advance of the orbit of Mercury had long been
known. It had occupied the attention of Le Verrier, who, having successfully
predicted the planet Neptune from the disturbances of Uranus,
thought that the anomalous motion of Mercury might be due to an interior
planet, which was called Vulcan in anticipation. But, though thoroughly
sought for, Vulcan has never turned up. Shortly before Einstein arrived
at his law of gravitation, the accepted gures were as follows. The actual
observed advance of the orbit was 57400 per century; the calculated
perturbations produced by all the known planets amounted to 53200 per
century. The excess of 4200 per century remained to be explained. Although
the amount could scarcely be relied on to a second of arc, it was
at least thirty times as great as the probable accidental error.
The big discrepancy from the Newtonian gravitational theory is thus in
agreement with Einstein's prediction of an advance of 4300 per century.
The derivation of this prediction from Einstein's law can only be followed
by mathematical analysis; but it may be remarked that any slight
deviation from the inverse square law is likely to cause an advance or recession
of the apse of the orbit. That a particle, if it does not move in a
circle, should oscillate between two extreme distances is natural enough;
it could scarcely do anything else unless it had sucient speed to break
away altogether. But the interval between the two extremes will not in
general be half a revolution. It is only under the exact adjustment of the
inverse square law that this happens, so that the orbit closes up and the
next revolution starts at the same point. I do not think that any \simple
explanation" of this property of the inverse-square law has been given;
and it seems fair to remind those, who complain of the diculty of understanding
Einstein's prediction of the advance of the perihelion, that
the real trouble is that they have not yet succeeded in making clear to
the uninitiated this recondite result of the Newtonian theory. The slight
modications introduced by Einstein's law of gravitation upset this ne
adjustment, so that the oscillation between the extremes occupies slightly
more than a revolution. A simple example of this eect of a small deviation
from the inverse-square law was actually given by Newton.
viii] OTHER TESTS OF THE THEORY 115
It had already been recognised that the change of mass with velocity
may cause an advance of perihelion; but owing to the ambiguity of Newton's
law of gravitation the discussion was unsatisfactory. It was, however,
clear that the eect was too small to account for the motion of perihelion
of Mercury, the prediction being 1
2v2=C2, or at most v2=C2. Einstein's
theory is the only one which gives the full amount 3v2=C2.
It was suggested by Lodge that, this variation of mass with velocity
might account for the whole motion of the orbit of Mercury, if account
were taken of the sun's unknown absolute motion through the aether,
combining sometimes additively and sometimes negatively with the orbital
motion. In a discussion between him and the writer, it appeared that, if
the absolute motion were sucient to produce this eect on Mercury,
it must give observable eects for Venus and the Earth; and these do
not exist. Indeed from the close accordance of Venus and the Earth with
observation, it is possible to conclude that, either the sun's motion through
the aether is improbably small, or gravitation must conform to relativity,
in the sense of the restricted principle (p. 18), and conceal the eects of
the increase of mass with speed so far as an additive uniform motion is
concerned.
Unfortunately it is not possible to obtain any further test of Einstein's
law of gravitation from the remaining planets. We have to pass over Venus
and the Earth, whose orbits are too nearly circular to show the advance
of the apses observationally. Coming next to Mars with a moderately
eccentric orbit, the speed is very much smaller, and the predicted advance
is only 100:3 per century. Now the accepted gures show an observed
advance (additional to that produced by known causes) of 500 per century,
so that Einstein's correction improves the accordance of observation with
theory; but, since the result for Mars is in any case scarcely trustworthy
to 500 owing to the inevitable errors of observation, the improvement is
not very important. The main conclusion is that Einstein's theory brings
Mercury into line, without upsetting the existing good accordance of all
the other planets.
We have tested Einstein's law of gravitation for fast movement (light)
and for moderately slow movement (Mercury). For very slow movement
it agrees with Newton's law, and the general accordance of the latter with
observation can be transferred to Einstein's law. These tests appear to be
sucient to establish the law rmly. We can express it in this way.
Every particle or light-pulse moves so that the quantity s measured
along its track between two points has the maximum possible value, where
ds2 = 􀀀(1 􀀀 2m=r)􀀀1 dr2 􀀀 r2 d2 + (1 􀀀 2m=r) dt2:
116 OTHER TESTS OF THE THEORY [ch.
And the accuracy of the experimental test is sucient to verify the coef-
cients as far as terms of order m=r in the coecient of dr2, and as far as
terms of order m2=r2 in the coecient of dt2.
In this form the law appears to be rmly based on experiment, and
the revision or even the complete abandonment of the general ideas of
Einstein's theory would scarcely aect it.
These experimental proofs, that space in the gravitational eld of the
sun is non-Euclidean or curved, have appeared puzzling to those unfamiliar
with the theory. It is pointed out that the experiments show that physical
objects or loci are \warped" in the sun's eld; but it is suggested that
there is nothing to show that the space in which they exist is warped.
The answer is that it does not seem possible to draw any distinction
between the warping of physical space and the warping of physical objects
which dene space. If our purpose were merely to call attention to these
phenomena of the gravitational eld as curiosities, it would, no doubt, be
preferable to avoid using words which are liable to be misconstrued. But if
we wish to arrive at an understanding of the conditions of the gravitational
eld, we cannot throw over the vocabulary appropriate for that purpose,
merely because there may be some who insist on investing the words with
a metaphysical meaning which is clearly inappropriate to the discussion.
We come now to another kind of test. In the statement of the law of
gravitation just given, a quantity s is mentioned; and, so far as that statement
goes, s is merely an intermediary quantity dened mathematically.
But in our theory we have been identifying s with interval-length, measured
with an apparatus of scales and clocks; and it is very desirable to
test whether this identication can be conrmed|whether the geometry
of scales and clocks is the same as the geometry of moving particles and
light-pulses.
The question has been mooted whether we may not divide the present
theory into two parts. Can we not accept the law of gravitation in the form
suggested above as a self-contained result proved by observation, leaving
the further possibility that s is to be identied with interval-length open
to debate? The motive is partly a desire to consolidate our gains, freeing
them from the least taint of speculation; but perhaps also it is inspired by
the wish to leave an opening by which clock-scale geometry, i.e. the space
and time of ordinary perception, may remain Euclidean. Disregarding the
connection of s with interval-length, there is no object in attributing any
signicance of length to it; it can be regarded as a dynamical quantity
Appendix, Note 10.
viii] OTHER TESTS OF THE THEORY 117
like Action, and the new law of gravitation can be expressed after the traditional
manner without dragging in strange theories of space and time.
Thus interpreted, the law perhaps loses its theoretical inevitability; but
it remains strongly grounded on observation. Unfortunately for this proposal,
it is impossible to make a clean division of the theory at the point
suggested. Without some geometrical interpretation of s our conclusions
as to the courses of planets and light-waves cannot be connected with the
astronomical measurements which verify them. The track of a light-wave
in terms of the coordinates r, , t cannot be tested directly; the coordinates
aord only a temporary resting-place; and the measurement of the
displacement of the star-image on the photographic plate involves a reconversion
from the coordinates to s, which here appears in its signicance
as the interval in clock-scale geometry.
Thus even from the experimental standpoint, a rough correspondence
of the quantity s occurring in the law of gravitation with the clock-scale
interval is an essential feature. We have now to examine whether experimental
evidence can be found as to the exactness of this correspondence.
It seems reasonable to suppose that a vibrating atom is an ideal type of
clock. The beginning and end of a single vibration constitute two events,
and the interval ds between two events is an absolute quantity independent
of any mesh-system. This interval must be determined by the nature of
the atom; and hence atoms which are absolutely similar will measure by
their vibrations equal values of the absolute interval ds. Let us adopt the
usual mesh-system (r; ; t) for the solar system, so that
ds2 = 􀀀
􀀀1 dr2 􀀀 r2 d2 + 
 dt2:
Consider an atom momentarily at rest at some point in the solar system;
we say momentarily, because it must undergo the acceleration of the gravitational
eld where it is. If ds corresponds to one vibration, then, since
the atom has not moved, the corresponding dr and d will be zero, and
we have
ds2 = 
 dt2:
The time of vibration dt is thus 1=p
 times the interval of vibration ds.
Accordingly, if we have two similar atoms at rest at dierent points
in the system, the interval of vibration will be the same for both; but
the time of vibration will be proportional to the inverse square-root of 
,
118 OTHER TESTS OF THE THEORY [ch.
which diers for the two atoms. Since

 = 1 􀀀
2m
r
1=p
 = 1 +
m
r
; very approximately.
Take an atom on the surface of the sun, and a similar atom in a terrestrial
laboratory. For the rst, 1 + m=r = 1:00000212, and for the second
1 + m=r is practically 1. The time of vibration of the solar atom is thus
longer in the ratio 1:00000212, and it might be possible to test this by
spectroscopic examination.
There is one important point to consider. The spectroscopic examination
must take place in the terrestrial laboratory; and we have to test the
period of the solar atom by the period of the waves emanating from it
when they reach the earth. Will they carry the period to us unchanged?
Clearly they must. The rst and second pulse have to travel the same distance
(r), and they travel with the same velocity (dr=dt); for the velocity
of light in the mesh-system used is 1 􀀀 2m=r, and though this velocity
depends on r, it does not depend on t. Hence the dierence dt at one end
of the waves is the same as that at the other end.
Thus in the laboratory the light from a solar source should be of greater
period and greater wave-length (i.e. redder) than that from a corresponding
terrestrial source. Taking blue light of wave-length 4000 A, the solar
lines should be displaced 4000  :00000212, or 0:008 A towards the red
end of the spectrum.
The properties of a gravitational eld of force are similar to those of a
centrifugal eld of force; and it may be of interest to see how a corresponding
shift of the spectral lines occurs for an atom in a eld of centrifugal
force. Suppose that, as we rotate with the earth, we observe a very remote
atom momentarily at rest relative to our rotating axes. The case is just
similar to that of the solar atom; both are at rest relative to the respective
mesh-systems; the solar atom is in a eld of gravitational force, and
the other is in a eld of centrifugal force. The direction of the force is in
both cases the same|from the earth towards the atom observed. Hence
the atom in the centrifugal eld ought also to vibrate more slowly, and
show a displacement to the red in its spectral lines. It does, if the theory
hitherto given is right. We can abolish the centrifugal force by choosing
non-rotating axes. But the distant atom was at rest relative to the rotating
axes, that is to say, it was whizzing round with them. Thus from the
ordinary standpoint the atom has a large velocity relative to the observer,
and, in accordance with Chapter i, its vibrations slow down just as the
viii] OTHER TESTS OF THE THEORY 119
aviator's watch did. The shift of spectral lines due to a eld of centrifugal
force is only another aspect of a phenomenon already discussed.
The expected shift of the spectral lines on the sun, compared with the
corresponding terrestrial lines, has been looked for; but it has not been
found.
In estimating the importance of this observational result in regard to
the relativity theory, we must distinguish between a failure of the test and
a denite conclusion that the lines are undisplaced. The chief investigators
St John, Schwarzschild, Evershed, and Grebe and Bachem, seem to
be agreed that the observed displacement is at any rate less than that
predicted by the theory. The theory can therefore in no case claim support
from the present evidence. But something more must be established,
if the observations are to be regarded as in the slightest degree adverse
to the theory. If for instance the mean de
ection is found to be :004 instead
of :008 Angstrom units, the only possible conclusion is that there
are certain causes of displacement of the lines, acting in the solar atmosphere
and not yet identied. No one could be much surprised if this were
the case; and it would, of course, render the test nugatory. The case is
not much altered if the observed displacement is :002 units, provided the
latter quantity is above the accidental error of measurement; if we have
to postulate some unexplained disturbance, it may just as well produce a
displacement 􀀀:006 as +:002. For this reason Evershed's evidence is by
no means adverse to the theory, since he nds unexplained displacements
in any case. One set of lines measured by St John gave a mean displacement
of :0036 units; and this also shows that the test has failed. The
only evidence adverse to the theory, and not merely neutral, is a series
of measures by St John on 17 cyanogen lines, which he regarded as most
dependable. These gave a mean shift of exactly :000. If this stood alone
we should certainly be disposed to infer that the test had gone against
Einstein's theory, and that nothing had intervened to cast doubt on the
validity of the test. The writer is unqualied to criticise these mutually
contradictory spectroscopic conclusions; but he has formed the impression
that the last-mentioned result obtained by St John has the greatest weight
of any investigations up to the present.
It seems that judgment must be reserved; but it may be well to examine
A further paper by Grebe and Bachem (Zeitschrift fur Physik, 1920, p. 51), received
whilst this is passing through the press, makes out a case strongly favourable
for the Einstein displacement, and reconciles the discordant results found by most of
the investigators. But it may still be the best counsel to \wait and see," and I have
made no alteration in the discussion here given.
120 OTHER TESTS OF THE THEORY [ch.
how the present theory would stand if the verdict of this third crucial
experiment nally went against it.
It has become apparent that there is something illogical in the sequence
we have followed in developing the theory, owing to the necessity of proceeding
from the common ideas of space and time to the more fundamental
properties of the absolute world. We started with a denition of the
interval by measurements made with clocks and scales, and afterwards
connected it with the tracks of moving particles. Clearly this is an inversion
of the logical order. The simplest kind of clock is an elaborate
mechanism, and a material scale is a very complex piece of apparatus.
The best course then is to discover ds by exploration of space and time
with a moving particle or light-pulse, rather than by measures with scales
and clocks. On this basis by astronomical observation alone the formula
for ds in the gravitational eld of the sun has already been established.
To proceed from this to determine exactly what is measured by a scale
and a clock, it would at rst seem necessary to have a detailed theory of
the mechanisms involved in a scale and clock. But there is a short-cut
which seems legitimate. This short-cut is in fact the Principle of Equivalence.
Whatever the mechanism of the clock, whether it is a good clock
or a bad clock, the intervals it is beating must be something absolute; the
clock cannot know what mesh-system the observer is using, and therefore
its absolute rate cannot be altered by position or motion which is relative
merely to a mesh-system. Thus wherever it is placed, and however
it moves, provided it is not constrained by impacts or electrical forces, it
must always beat equal intervals as we have previously assumed. Thus
a clock may fairly be used to measure intervals, even when the interval
is dened in the new manner; any other result seems to postulate that it
pays heed to some particular mesh-system.
Three modes of escape from this conclusion seem to be left open. A clock
cannot pay any heed to the mesh-system used; but it may be aected by
the kind of space-time around ity. The terrestrial atom is in a eld of
gravitation so weak that the space-time may be considered practically

at; but the space-time round the solar atom is not 
at. It may happen
that the two atoms actually detect this absolute dierence in the world
around them and do not vibrate with the same interval ds|contrary to
Of course, there is always the possibility that this might be the case, though
it seems unlikely. The essential point of the relativity theory is that (contrary to
the common opinion) no experiments yet made have revealed any mesh-system of an
absolute character, not that experiments never will reveal such a system.
yAppendix, Note 11.
viii] OTHER TESTS OF THE THEORY 121
our assumption above. Then the prediction of the shift of the lines in
the solar spectrum is invalidated. Now it is very doubtful if an atom can
detect the curving of the region it occupies, because curvature is only
apparent when an extended region is considered; still an atom has some
extension, and it is not impossible that its equations of motion involve the
quantities B
 which distinguish gravitational from 
at space-time. An
apparently insuperable objection to this explanation is that the eect of
curvature on the period would almost certainly be represented by terms
of the form m2=r2, whereas to account for a negative result for the shift
of the spectral lines terms of much greater order of magnitude m=r are
needed.
The second possibility depends on the question whether it is possible for
an atom at rest on the sun to be precisely similar to one on the earth. If an
atom fell from the earth to the sun it would acquire a velocity of 610 km.
per sec., and could only be brought to rest by a systematic hammering by
other atoms. May not this have made a permanent alteration in its timekeeping
properties? It is true that every atom is continually undergoing
collisions, but it is just possible that the average solar atom has a dierent
period from the average terrestrial atom owing to this systematic dierence
in its history.
What are the two events which mark the beginning and end of an atomic
vibration? This question suggests a third possibility. If they are two absolute
events, like the explosions of two detonators, then the interval between
them will be a denite quantity, and our argument applies. But if, for example,
an atomic vibration is determined by the revolution of an electron
around a nucleus, it is not marked by any denite events. A revolution
means a return to the same position as before; but we cannot dene what
is the same position as before without reference to some mesh-system.
Hence it is not clear that there is any absolute interval corresponding to
the vibration of an atom; an absolute interval only exists between two
events absolutely dened.
It is unlikely that any of these three possibilities can negative the expected
shift of the spectral lines. The uncertainties introduced by them
are, so far as we can judge, of a much smaller order of magnitude. But it
will be realised that this third test of Einstein's theory involves rather more
complicated considerations than the two simple tests with light-waves and
the moving planet. I think that a shift of the Fraunhofer lines is a highly
probable prediction from the theory and I anticipate that experiment will
ultimately conrm the prediction; but it is not entirely free from guesswork.
These theoretical uncertainties are apart altogether from the great
122 OTHER TESTS OF THE THEORY [ch.
practical diculties of the test, including the exact allowance for the unfamiliar
circumstances of an absorbing atom in the sun's atmosphere.
Outside the three leading tests, there appears to be little chance of
checking the theory unless our present methods of measurement are greatly
improved. It is not practicable to measure the de
ection of light by any
body other than the sun. The apparent displacement of a star just grazing
the limb of Jupiter should be 000:017. A hundredth of a second of arc
is just about within reach of the most rened measurements with the
largest telescopes. If the observation could be conducted under the same
conditions as the best parallax measurements, the displacement could be
detected but not measured with any accuracy. The glare from the light of
the planet ruins any chance of success.
Most astronomers, who look into the subject, are entrapped sooner or
later by a fallacy in connection with double stars. It is thought that when
one component passes behind the other it will appear displaced from its
true position, like a star passing behind the sun; if the size of the occulting
star is comparable with that of the sun, the displacement should be of the
same order, 100:7. This would cause a very conspicuous irregularity in the
apparent orbit of a double star. But reference to p. 103 shows that an
essential point in the argument was the enormous ratio of the distance
QP of the star from the sun to the distance EF of the sun from the earth.
It is only in these conditions that the apparent displacement of the object
is equal to the de
ection undergone by its light. It is easy to see that
where this ratio is reversed, as in the case of the double star, the apparent
displacement is an extremely small fraction of the de
ection of the light.
It would be quite imperceptible to observation.
If two independent stars are seen in the same line of vision within
about 100, one being a great distance behind the other, the conditions
seem at rst more favourable. I do not know if any such pairs exist. It
would seem that we ought to see the more distant star not only by the
direct ray, which would be practically undisturbed, but also by a ray passing
round the other side of the nearer star and bent by it to the necessary
extent. The second image would, of course, be indistinguishable from that
of the nearer star; but it would give it additional brightness, which would
disappear in time when the two stars receded. But consider a pencil of
light coming past the nearer star; the inner edge will be bent more than
the outer edge, so that the divergence is increased. The increase is very
small; but then the whole divergence of a pencil from a source some hundred
billion miles away is very minute. It is easily calculated that the
increased divergence would so weaken the light as to make it impossible
viii] OTHER TESTS OF THE THEORY 123
to detect it when it reached us.
If two unconnected stars approached the line of sight still more closely,
so that one almost occulted the other, observable eects might be perceived.
When the proximity was such that the direct ray from the more
distant star passed within about 100 million kilometres of the nearer star,
it would begin to fade appreciably. The course of the ray would not yet be
appreciably de
ected, but the divergence of the pencil would be rapidly
increased, and less light from the star would enter our telescopes. The test
is scarcely likely to be an important one, since a suciently close approach
is not likely to occur; and in any case it would be dicult to feel condent
that the fading was not due to a nebulous atmosphere around the nearer
star.
The theory gives small corrections to the motion of the moon which
have been investigated by de Sitter. Both the axis of the orbit and its line
of intersection with the ecliptic should advance about 200 per century more
than the Newtonian theory indicates. Neither observation nor Newtonian
theory are as yet pushed to sucient accuracy to test this; but a comparatively
small increase in accuracy would make a comparison possible.
Since certain stars are perhaps ten times more massive than the sun,
without the radius being unduly increased, they should show a greater
shift of the spectral lines and might be more favourable for the third
crucial test. Unfortunately the predicted shift is indistinguishable from
that caused by a velocity of the star in the line-of-sight on Doppler's
principle. Thus the expected shift on the sun is equivalent to that caused
by a receding velocity of 0:634 kilometres per second. In the case of the sun
we know by other evidence exactly what the line-of-sight velocity should
be; but we have not this knowledge for other stars. The only indication
that could be obtained would be the detection of an average motion of
recession of the more massive stars. It seems rather unlikely that there
should be a real preponderance of receding motions among stars taken
indiscriminately from all parts of the sky; and the apparent eect might
then be attributed to the Einstein shift. Actually the most massive stars
(those of spectral type B) have been found to show an average velocity of
recession of about 4:5 km. per sec., which would be explained if the values
of m=r for these stars are about seven times greater than the value for the
sun|a quite reasonable hypothesis. This phenomenon was well-known to
astrophysicists some years before Einstein's theory was published. But
there are so many possible interpretations that no stress should be placed
on this evidence. Moreover the very diuse \giant" stars of type M have
Appendix, Note 12.
124 OTHER TESTS OF THE THEORY
also a considerable systematic velocity of recession, and for these m=r
must be much less than for the sun.
CHAPTER IX
MOMENTUM AND ENERGY
For spirits and men by dierent standards mete
The less and greater in the 
ow of time.
By sun and moon, primeval ordinances|
By stars which rise and set harmoniously|
By the recurring seasons, and the swing
This way and that of the suspended rod
Precise and punctual, men divide the hours,
Equal, continuous, for their common use.
Not so with us in the immaterial world;
But intervals in their succession
Are measured by the living thought alone
And grow or wane with its intensity.
And time is not a common property;
But what is long is short, and swift is slow
And near is distant, as received and grasped
By this mind and by that. Newman, Dream of Gerontius.
One of the most important consequences of the relativity theory is the
unication of inertia and gravitation.
The beginner in mechanics does not accept Newton's rst law of motion
without a feeling of hesitation. He readily agrees that a body at rest will
remain at rest unless something causes it to move; but he is not satised
that a body in motion will remain in uniform motion so long as it is not
interfered with. It is quite natural to think that motion is an impulse
which will exhaust itself, and that the body will nally come to a stop.
The teacher easily disposes of the arguments urged in support of this
view, pointing out the friction which has to be overcome when a train
or a bicycle is kept moving uniformly. He shows that if the friction is
diminished, as when a stone is projected across ice, the motion lasts for
126 MOMENTUM AND ENERGY [ch.
a longer time, so that if all interference by friction were removed uniform
motion might continue indenitely. But he glosses over the point that
if there were no interference with the motion|if the ice were abolished
altogether|the motion would be by no means uniform, but like that of a
falling body. The teacher probably insists that the continuance of uniform
motion does not require anything that can properly be called a cause.
The property is given a name inertia; but it is thought of as an innate
tendency in contrast to force which is an active cause. So long as forces
are conned to the thrusts and tensions of elementary mechanics, where
there is supposed to be direct contact of material, there is good ground for
this distinction; we can visualise the active hammering of the molecules
on the body, causing it to change its motion. But when force is extended
to include the gravitational eld the distinction is not so clear.
For our part we deny the distinction in this last case. Gravitational
force is not an active agent working against the passive tendency of inertia.
Gravitation and inertia are one. The uniform straight track is only relative
to some mesh-system, which is assigned by arbitrary convention. We
cannot imagine that a body looks round to see who is observing it and
then feels an innate tendency to move in that observer's straight line|
probably at the same time feeling an active force compelling it to move
some other way. If there is anything that can be called an innate tendency
it is the tendency to follow what we have called the natural track|the
longest track between two points. We might restate the rst law of motion
in the form \Every body tends to move in the track in which it actually
does move, except in so far as it is compelled by material impacts to follow
some other track than that in which it would otherwise move." Probably
no one will dispute this profound statement!
Whether the natural track is straight or curved, whether the motion is
uniform or changing, a cause is in any case required. This cause is in all
cases the combined inertia-gravitation. To have given it a name does not
excuse us from attempting an explanation of it in due time. Meanwhile
this identication of inertia and gravitation as arbitrary components of
one property explains why weight is always proportional to inertia. This
experimental fact veried to a very high degree of accuracy would otherwise
have to be regarded as a remarkable law of nature.
We have learnt that the natural track is the longest track between two
points; and since this is the only denable track having an absolute signi
cance in nature, we seem to have a sucient explanation of why an
undisturbed particle must follow it. That is satisfactory, so far as it goes,
but still we should naturally wish for a clearer picture of the cause|
ix] MOMENTUM AND ENERGY 127
inertia-gravitation|which propels it in this track.
It has been seen that the gravitational eld round a body involves a
kind of curvature of space-time, and accordingly round each particle there
is a minute pucker. Now at each successive instant a particle is displaced
continuously in time if not in space; and so in our four-dimensional representation
which gives a bird's-eye-view of all time, the pucker has the
form of a long groove along the track of the particle. Now such a groove
or pleat in a continuum cannot take an arbitrary course|as every dressmaker
knows. Einstein's law of gravitation gives the rule according to
which the curvatures at any point of space-time link on to those at surrounding
points; so that when a groove is started in any direction the rest
of its course can be forecasted. We have hitherto thought of the law of
gravitation as showing how the pucker spreads out in space, cf. Newton's
statement that the corresponding force weakens as the inverse square of
the distance. But the law of Einstein equally shows how the gravitational
eld spreads out in time, since there is no absolute distinction of time
and space. It can be deduced mathematically from Einstein's law that a
pucker of the form corresponding to a particle necessarily runs along the
track of greatest interval-length between two points.
The track of a particle of matter is thus determined by the interaction
of the minute gravitational eld, which surrounds and, so far as we know,
constitutes it, with the general space-time of the region. The various forms
which it can take, nd their explanation in the new law of gravitation.
The straight tracks of the stars and the curved tracks of the planets are
placed on the same level, and receive the same kind of explanation. The
one universal law, that the space-time continuum can be curved only in
the rst degree, is sucient to prescribe the forms of all possible grooves
crossing it.
The application of Einstein's law to trace the gravitational eld not only
through space but through time leads to a great unication of mechanics.
If we have given for a start a narrow slice of space-time representing the
state of the universe for a few seconds, with all the little puckers belonging
to particles of matter properly described, then step by step all space-time
can be linked on and the positions of the puckers shown at all subsequent
times (electrical forces being excluded). Nothing is needed for this except
the law of gravitation|that the curvature is only of the rst degree|and
there can thus be nothing in the predictions of mechanics which is not
comprised in the law of gravitation. The conservation of mass, of energy,
and of momentum must all be contained implicitly in Einstein's law.
It may seem strange that Einstein's law of gravitation should take over
128 MOMENTUM AND ENERGY [ch.
responsibility for the whole of mechanics; because in many mechanical
problems gravitation in the ordinary sense can be neglected. But inertia
and gravitation are unied; the law is also the law of inertia, and inertia
or mass appears in all mechanical problems. When, as in many problems,
we say that gravitation is negligible, we mean only that the interaction of
the minute puckers with one another can be neglected; we do not mean
that the interaction of the pucker of a particle with the general character
of the space-time in which it lies can be neglected, because this constitutes
the inertia of the particle.
The conservation of energy and the conservation of momentum in three
independent directions, constitute together four laws or equations which
are fundamental in all branches of mechanics. Although they apply when
gravitation in the ordinary sense is not acting, they must be deducible
like everything else in mechanics from the law of gravitation. It is a great
triumph for Einstein's theory that his law gives correctly these experimental
principles, which have generally been regarded as unconnected
with gravitation. We cannot enter into the mathematical deduction of
these equations; but we shall examine generally how they are arrived at.
It has already been explained that although the values of G are strictly
zero everywhere in space-time, yet if we take average values through a
small region containing a large number of particles of matter their average
or \macroscopic" values will not be zero. Expressions for these macroscopic
values can be found in terms of the number, masses and motions
of the particles. Since we have averaged the G, we should also average
the particles; that is to say, we replace them by a distribution of continuous
matter having equivalent properties. We thus obtain macroscopic
equations of the form
G = K;
where on the one side we have the somewhat abstruse quantities describing
the kind of space-time, and on the other side we have well-known physical
quantities describing the density, momentum, energy and internal stresses
of the matter present. These macroscopic equations are obtained solely
from the law of gravitation by the process of averaging.
By an exactly similar process we pass from Laplace's equation r2 = 0
to Poisson's equation for continuous matter r2 = 􀀀4, in the Newtonian
theory of gravitation.
When continuous matter is admitted, any kind of space-time becomes
possible. The law of gravitation instead of denying the possibility of cer-
It is the g's which are rst averaged, then the G are calculated by the formulae
in Note 5.
ix] MOMENTUM AND ENERGY 129
tain kinds, states what values of K, i.e. what distribution and motion of
continuous matter in the region, are a necessary accompaniment. This is
no contradiction with the original statement of the law, since that referred
to the case in which continuous matter is denied or excluded. Any set of
values of the potentials is now possible; we have only to calculate by the
formulae the corresponding values of G, and we at once obtain ten equations
giving the K which dene the conditions of the matter necessary
to produce these potentials. But suppose the necessary distribution of
matter through space and time is an impossible one, violating the laws of
mechanics! No, there is only one law of mechanics, the law of gravitation;
we have specied the distribution of matter so as to satisfy G = K,
and there can be no other condition for it to full. The distribution must
be mechanically possible; it might, however, be unrealisable in practice,
involving inordinately high or even negative density of matter.
In connection with the law for empty space, G = 0, it was noticed
that whereas this apparently forms a set of ten equations, only six of
them can be independent. This was because ten equations would suce
to determine the ten potentials precisely, and so x not only the kind of
space-time but the mesh-system. It is clear that we must preserve the
right to draw the mesh-system as we please; it is xed by arbitrary choice
not by a law of nature. To allow for the four-fold arbitrariness of choice,
there must be four relations always satised by the G, so that when six
of the equations are given the remaining four become tautological.
These relations must be identities implied in the mathematical denition
of G; that is to say, when the G have been written out in full according
to their denition, and the operations indicated by the identities carried
out, all the terms will cancel, leaving only 0 = 0. The essential point is
that the four relations follow from the mode of formation of the G from
their simpler constituents (g and their dierential coecients) and apply
universally. These four identical relations have actually been discovered.
When in continuous matter G = K clearly the same four relations
must exist between the K, not now as identities, but as consequences of
the law of gravitation, viz. the equality of G and K.
Thus the four dimensions of the world bring about a four-fold arbitrariness
of choice of mesh-system; this in turn necessitates four identical
relations between the G; and nally, in consequence of the law of gravitation,
these identities reveal four new facts or laws relating to the density,
energy, momentum or stress of matter, summarised in the expressions K.
Appendix, Note 13.
130 MOMENTUM AND ENERGY [ch.
These four laws turn out to be the laws of conservation of momentum
and energy.
The argument is so general that we can even assert that corresponding
to any absolute property of a volume of a world of four dimensions (in
this case, curvature), there must be four relative properties which are
conserved. This might be made the starting-point of a general inquiry
into the necessary qualities of a permanent perceptual world, i.e. a world
whose substance is conserved.
There is another law of physics which was formerly regarded as fundamental|
the conservation of mass. Modern progress has somewhat altered
our position with regard to it; not that its validity is denied, but it has
been reinterpreted, and has nally become merged in the conservation of
energy. It will be desirable to consider this in detail.
Fig. 19.
O
M
N
P
T
A
B B′
C
X
It was formerly supposed that the mass of a particle was a number
attached to the particle, expressing an intrinsic property, which remained
unaltered in all its vicissitudes. If m is this number, and u the velocity
of the particle, the momentum is mu; and it is through this relation,
coupled with the law of conservation of momentum that the mass m was
dened. Let us take for example two particles of masses m1 = 2 and
m2 = 3, moving in the same straight line. In the space-time diagram for
ix] MOMENTUM AND ENERGY 131
an observer S the velocity of the rst particle will be represented by a
direction OA (Fig. 19). The rst particle moves through a space MA in
unit time, so that MA is equal to its velocity referred to the observer S.
Prolonging the line OA to meet the second time-partition, NB is equal
to the velocity multiplied by the mass 2; thus the horizontal distance
NB represents the momentum. Similarly, starting from B and drawing
BC in the direction of the velocity of m2, prolonged through three timepartitions,
the horizontal progress from B represents the momentum of
the second particle. The length PC then represents the total momentum
of the system of two particles.
Suppose that some change of their velocities occurs, not involving any
transference of momentum from outside, e.g. a collision. Since the total
momentum PC is unaltered, a similar construction made with the new
velocities must again bring us to C; that is to say, the new velocities are
represented by the directions OB0, B0C, where B0 is some other point on
the line NB.
Fig. 20.
O
M
N
P
T1
A
B
B′
C
C′
X1
Now examine how this will appear to some other observer S1 in uniform
motion relative to S. His transformation of space and time has been
132 MOMENTUM AND ENERGY [ch.
described in Chapter iii and is represented in Fig. 20, which shows how his
time-partitions run as compared with those of S. The same actual motion
is, of course, represented by parallel directions in the two diagrams; but the
interpretation as a velocity MA is dierent in the two cases. Carrying the
velocity of m1 through two time-partitions, and of m2 through three timepartitions,
as before, we nd that the total momentum for the observer
S1 is represented by PC (Fig. 20); but making a similar construction with
the velocities after collision, we arrive at a dierent point C0. Thus whilst
momentum is conserved for the observer S, it has altered from PC to PC0
for the observer S1.
The discrepancy arises because in the construction the lines are prolonged
to meet partitions which are dierent for the two observers. The
rule for determining momentum ought to be such that both observers
make the same construction, independent of their partitions, so that both
arrive by the two routes at the same point C. Then it will not matter
if, through their dierent measures of time, one observer measures momentum
by horizontal progress and the other by oblique progress; both
will agree that the momentum has not been altered by the collision. To
describe such a construction, we must use the interval which is alike for
both observers; make the interval-length of OB equal to 2 units, and that
of BC equal to 3 units, disregarding the mesh-system altogether. Then
both observers will make the same diagram and arrive at the same point C
(dierent from C or C0 in the previous diagrams). Then if momentum is
conserved for one observer, it will be conserved for the other.
This involves a modied denition of momentum. Momentum must now
be the mass multiplied by the change of position x per lapse of interval
s, instead of per lapse of time t. Thus
momentum = m
x
s
instead of momentum = m
x
t
;
and the mass m still preserves its character as an invariant number associated
with the particle.
Whether the momentum as now dened is actually conserved or not,
is a matter for experiment, or for theoretical deduction from the law of
gravitation. The point is that with the original denition general conservation
is impossible, because if it held good for one observer it could not
hold for another. The new denition makes general conservation possible.
Actually this form of the momentum is the one deduced from the
ix] MOMENTUM AND ENERGY 133
law of gravitation through the identities already described. With regard
to experimental conrmation it is sucient at present to state that in
all ordinary cases the interval and the time are so nearly equal that such
experimental foundation as existed for the law of conservation of the old
momentum is just as applicable to the new momentum.
Thus in the theory of relativity momentum appears as an invariant mass
multiplied by a modied velocity x=s. The physicist, however, prefers
for practical purposes to keep to the old denition of momentum as mass
multiplied by the velocity x=t. We have
m
x
s
= m
t
s 

x
t
;
accordingly the momentum is separated into two factors, the velocity
x=t, and a mass M = mt=s, which is no longer an invariant for the
particle but depends on its motion relative to the observer's space and
time. In accordance with the usual practice of physicists the mass (unless
otherwise qualied) is taken to mean the quantity M.
Using unaccelerated rectangular axes, we have by denition of s
s2 = t2 􀀀 x2 􀀀 y2 􀀀 z2;
so that
s
t2
= 1 􀀀 x
t 2
􀀀 y
t2
􀀀 z
t2
;
= 1 􀀀 u2;
where u is the resultant velocity of the particle (the velocity of light being
unity). Hence
M =
m
p(1 􀀀 u2)
:
Thus the mass increases as the velocity increases, the factor being the
same as that which determines the FitzGerald contraction.
The increase of mass with velocity is a property which challenges experimental
test. For success it is necessary to be able to experiment with high
velocities and to apply a known force large enough to produce appreciable
de
ection in the fast-moving particle. These conditions are conveniently
fullled by the small negatively charged particles emitted by radio-active
substances, known as  particles, or the similar particles which constitute
cathode rays. They attain speeds up to 0.8 of the velocity of light, for
which the increase of mass is in the ratio 1.66; and the negative charge
134 MOMENTUM AND ENERGY [ch.
enables a large electric or magnetic force to be applied. Modern experiments
fully conrm the theoretical increase of mass, and show that the
factor 1=p(1􀀀u2) is at least approximately correct. The experiment was
originally performed by Kaufmann; but much greater accuracy has been
obtained by recent modied methods.
Unless the velocity is very great the mass M may be written
m=p(1 􀀀 u2) = m + 1
2mu2:
Thus it consists of two parts, the mass when at rest, together with the
second term which is simply the energy of the motion. If we can say
that the term m represents a kind of potential energy concealed in the
matter, mass can be identied with energy. The increase of mass with
velocity simply means that the energy of motion has been added on. We
are emboldened to do this because in the case of an electrical charge the
electrical mass is simply the energy of the static eld. Similarly the mass
of light is simply the electromagnetic energy of the light.
In our ordinary units the velocity of light is not unity, and a rather
articial distinction between mass and energy is introduced. They are
measured by dierent units, and energy E has a mass E=C2 where C is
the velocity of light in the units used. But it seems very probable that
mass and energy are two ways of measuring what is essentially the same
thing, in the same sense that the parallax and distance of a star are two
ways of expressing the same property of location. If it is objected that they
ought not to be confused inasmuch as they are distinct properties, it must
be pointed out that they are not sense-properties, but mathematical terms
expressing the dividend and product of more immediately apprehensible
properties, viz. momentum and velocity. They are essentially mathematical
compositions, and are at the disposal of the mathematician.
This proof of the variation of mass with velocity is much more general
than that based on the electrical theory of inertia. It applies immediately
to matter in bulk. The masses m1 and m2 need not be particles; they can
be bodies of any size or composition. On the electrical theory alone, there
is no means of deducing the variation of mass of a planet from that of an
electron.
It has to be remarked that, although the inertial mass of a particle
only comes under physical measurement in connection with a change of
its motion, it is just when the motion is changing that the conception of
its mass is least denite; because it is at that time that the kinetic energy,
which forms part of the mass, is being passed on to another particle or
radiated into the surrounding eld; and it is scarcely possible to dene
ix] MOMENTUM AND ENERGY 135
the moment at which this energy ceases to be associated with the particle
and must be reckoned as broken loose. The amount of energy or mass in
a given region is always a denite quantity; but the amount attributable
to a particle is only denite when the motion is uniform. In rigorous work
it is generally necessary to consider the mass not of a particle but of a
region.
The motion of matter from one place to another causes an alteration of
the gravitational eld in the surrounding space. If the motion is uniform,
the eld is simply convected; but if the motion is accelerated, something
of the nature of a gravitational wave is propagated outwards. The velocity
of propagation is the velocity of light. The exact laws are not very simple
because we have seen that the gravitational eld modies the velocity of
light; and so the disturbance itself modies the velocity with which it is
propagated. In the same way the exact laws of propagation of sound are
highly complicated, because the disturbance of the air by sound modi-
es the speed with which it is propagated. But the approximate laws of
propagation of gravitation are quite simple and are the same as those of
electromagnetic disturbances.
After mass and energy there is one physical quantity which plays a very
fundamental part in modern physics, known as Action. Action here is
a very technical term, and is not to be confused with Newton's \Action
and Reaction." In the relativity theory in particular this seems in many
respects to be the most fundamental thing of all. The reason is not dif-
cult to see. If we wish to speak of the continuous matter present at
any particular point of space and time, we must use the term density.
Density multiplied by volume in space gives us mass or, what appears to
be the same thing, energy. But from our space-time point of view, a far
more important thing is density multiplied by a four-dimensional volume
of space and time; this is action. The multiplication by three dimensions
gives mass or energy; and the fourth multiplication gives mass or energy
multiplied by time. Action is thus mass multiplied by time, or energy
multiplied by time, and is more fundamental than either.
Action is the curvature of the world. It is scarcely possible to visualise
this statement, because our notion of curvature is derived from surfaces
of two dimensions in a three-dimensional space, and this gives too limited
an idea of the possibilities of a four-dimensional surface in space of ve or
more dimensions. In two dimensions there is just one total curvature, and
if that vanishes the surface is 
at or at least can be unrolled into a plane.
In four dimensions there are many coecients of curvature; but there is
one curvature par excellence, which is, of course, an invariant independent
136 MOMENTUM AND ENERGY [ch.
of our mesh-system. It is the quantity we have denoted by G. It does
not follow that if the curvature vanishes space-time is 
at; we have seen
in fact that in a natural gravitational eld space-time is not 
at although
there may be no mass or energy and therefore no action or curvature.
Wherever there is matter there is action and therefore curvature; and
it is interesting to notice that in ordinary matter the curvature of the
space-time world is by no means insignicant. For example, in water of
ordinary density the curvature is the same as that of space in the form
of a sphere of radius 570; 000; 000 kilometres. The result is even more
surprising if expressed in time units; the radius is about half-an-hour.
It is dicult to picture quite what this means; but at least we can predict
that a globe of water of 570; 000; 000 km. radius would have extraordinary
properties. Presumably there must be an upper limit to the possible size
of a globe of water. So far as I can make out a homogeneous mass of water
of about this size (and no larger) could exist. It would have no centre,
and no boundary, every point of it being in the same position with respect
to the whole mass as every other point of it|like points on the surface
of a sphere with respect to the surface. Any ray of light after travelling
for an hour or two would come back to the starting point. Nothing could
enter or leave the mass, because there is no boundary to enter or leave by;
in fact, it is coextensive with space. There could not be any other world
anywhere else, because there isn't an \anywhere else."
The mass of this volume of water is not so great as the most moderate
estimates of the mass of the stellar system. Some physicists have predicted
a distant future when all energy will be degraded, and the stellar universe
will gradually fall together into one mass. Perhaps then these strange
conditions will be realised!
The law of gravitation, the laws of mechanics, and the laws of the electromagnetic
eld have all been summed up in a single Principle of Least
Action. For the most part this unication was accomplished before the
advent of the relativity theory, and it is only the addition of gravitation
to the scheme which is novel. We can see now that if action is something
absolute, a conguration giving minimum action is capable of absolute
denition; and accordingly we should expect that the laws of the world
would be expressible in some such form. The argument is similar to that
by which we rst identied the natural tracks of particles with the tracks of
greatest interval-length. The fact that some such form of law is inevitable,
rather discourages us from seeking in it any clue to the structural details
It is rather curious that there is no action in space containing only light. Light
has mass (M) of the ordinary kind; but the invariant mass (m) vanishes.
ix] MOMENTUM AND ENERGY 137
of our world.
Action is one of the two terms in pre-relativity physics which survive
unmodied in a description of the absolute world. The only other survival
is entropy. The coming theory of relativity had cast its shadow before;
and physics was already converging to two great generalisations, the principle
of least action and the second law of thermodynamics or principle of
maximum entropy.
We are about to pass on to recent and more shadowy developments of
this subject; and this is an appropriate place to glance back on the chief
results that have emerged. The following summary will recall some of the
salient points.
1. The order of events in the external world is a four-dimensional order.
2. The observer either intuitively or deliberately constructs a system of
meshes (space and time partitions) and locates the events with respect to
these.
3. Although it seems to be theoretically possible to describe phenomena
without reference to any mesh-system (by a catalogue of coincidences),
such a description would be cumbersome. In practice, physics describes
the relations of the events to our mesh-system; and all the terms of elementary
physics and of daily life refer to this relative aspect of the world.
4. Quantities like length, duration, mass, force, etc. have no absolute
signicance; their values will depend on the mesh-system to which they
are referred. When this fact is realised, the results of modern experiments
relating to changes of length of rigid bodies are no longer paradoxical.
5. There is no fundamental mesh-system. In particular problems, and
more particularly in restricted regions, it may be possible to choose a meshsystem
which follows more or less closely the lines of absolute structure
in the world, and so simplify the phenomena which are related to it. But
the world-structure is not of a kind which can be traced in an exact way
by mesh-systems, and in any large region the mesh-system drawn must be
considered arbitrary. In any case the systems used in current physics are
arbitrary.
6. The study of the absolute structure of the world is based on the \interval"
between two events close together, which is an absolute attribute
of the events independent of any mesh-system. A world-geometry is constructed
by adopting the interval as the analogue of distance in ordinary
geometry.
7. This world-geometry has a property unlike that of Euclidean geometry
in that the interval between two real events may be real or imaginary.
138 MOMENTUM AND ENERGY [ch.
The necessity for a physical distinction, corresponding to the mathematical
distinction between real and imaginary intervals, introduces us to the
separation of the four-dimensional order into time and space. But this
separation is not unique, and the separation commonly adopted depends
on the observer's track through the four-dimensional world.
8. The geodesic, or track of maximum or minimum interval-length between
two distant events, has an absolute signicance. And since no other
kind of track can be dened absolutely, it is concluded that the tracks of
freely moving particles are geodesics.
9. In Euclidean geometry the geodesics are straight lines. It is evidently
impossible to choose space and time-reckoning so that all free particles in
the solar system move in straight lines. Hence the geometry must be
non-Euclidean in a eld of gravitation.
10. Since the tracks of particles in a gravitational eld are evidently
governed by some law, the possible geometries must be limited to certain
types.
11. The limitation concerns the absolute structure of the world, and
must be independent of the choice of mesh-system. This narrows down
the possible discriminating characters. Practically the only reasonable
suggestion is that the world must (in empty space) be \curved no higher
than the rst degree"; and this is taken as the law of gravitation.
12. The simplest type of hummock with this limited curvature has been
investigated. It has a kind of innite chimney at the summit, which we
must suppose cut out and lled up with a region where this law is not
obeyed, i.e. with a particle of matter.
13. The tracks of the geodesics on the hummock are such as to give
a very close accordance with the tracks computed by Newton's law of
gravitation. The slight dierences from the Newtonian law have been
experimentally veried by the motion of Mercury and the de
ection of
light.
14. The hummock might more properly be described as a ridge extending
linearly. Since the interval-length along it is real or time-like, the ridge
can be taken as a time-direction. Matter has thus a continued existence
in time. Further, in order to conform with the law, a small ridge must
always follow a geodesic in the general eld of space-time, conrming the
conclusion arrived at under (8).
15. The laws of conservation of energy and momentum in mechanics
can be deduced from this law of world-curvature.
16. Certain phenomena such as the FitzGerald contraction and the
variation of mass with velocity, which were formerly thought to depend
ix] MOMENTUM AND ENERGY 139
on the behaviour of electrical forces concerned, are now seen to be general
consequences of the relativity of knowledge. That is to say, length and
mass being the relations of some absolute thing to the observer's meshsystem,
we can foretell how these relations will be altered when referred
to another mesh-system.
140 MOMENTUM AND ENERGY
CHAPTER X
TOWARDS INFINITY
The geometer of to-day knows nothing about the nature of actually existing
space at an innite distance; he knows nothing about the properties of this
present space in a past or a future eternity. He knows, indeed, that the laws
assumed by Euclid are true with an accuracy that no direct experiment can
approach, not only in this place where we are, but in places at a distance from
us that no astronomer has conceived; but he knows this as of Here and Now;
beyond his range is a There and Then of which he knows nothing at present,
but may ultimately come to know more. W. K. Clifford (1873).
The great stumbling-block for a philosophy which denies absolute space is
the experimental detection of absolute rotation. The belief that the earth
rotates on its axis was suggested by the diurnal motions of the heavenly
bodies; this observation is essentially one of relative rotation, and, if the
matter rested there, no diculty would be felt. But we can detect the
same rotation, or a rotation very closely equal to it, by methods which
do not seem to bring the heavenly bodies into consideration; and such a
rotation is apparently absolute. The planet Jupiter is covered with cloud,
so that an inhabitant would probably be unaware of the existence of bodies
outside; yet he could quite well measure the rotation of Jupiter. By the
gyrocompass he would x two points on the planet|the north and south
poles. Then by Foucault's experiment on the change of the plane of motion
of a freely suspended pendulum, he would determine an angular velocity
about the poles. Thus there is certainly a denite physical constant, an
angular velocity about an axis, which has a fundamental importance for
the inhabitants of Jupiter; the only question is whether we are right in
giving it the name absolute rotation.
Contrast this with absolute translation. Here it is not a question of
giving the right name to a physical constant; the inhabitants of Jupiter
would nd no constant to name. We see at once that a relativity theory
of translation is on a dierent footing from a relativity theory of rotation.
142 TOWARDS INFINITY [ch.
The duty of the former is to explain facts; the duty of the latter is to
explain away facts.
Our present theory seems to make a start at tackling this problem, but
gives it up. It permits the observer, if he wishes, to consider the earth as
non-rotating, but surrounded by a eld of centrifugal force; all the other
bodies in the universe are then revolving round the earth in orbits mainly
controlled by this eld of centrifugal force. Astronomy on this basis is a
little cumbersome; but all the phenomena are explained perfectly. The
centrifugal force is part of the gravitational eld, and obeys Einstein's
law of gravitation, so that the laws of nature are completely satised
by this representation. One awkward question remains, What causes the
centrifugal force? Certainly not the earth which is here represented as nonrotating.
As we go further into space to look for a cause, the centrifugal
force becomes greater and greater, so that the more we defer the debt the
heavier the payment demanded in the end. Our present theory is like the
debtor who does not mind how big an obligation accumulates satised that
he can always put o the payment. It chases the cause away to innity,
content that the laws of nature|the relations between contiguous parts
of the world|are satised all the way.
One suggested loophole must be explored. Our new law of gravitation
admits that a rapid motion of the attracting body will aect the eld of
force. If the earth is non-rotating, the stars must be going round it with
terric speed. May they not in virtue of their high velocities produce
gravitationally a sensible eld of force on the earth, which we recognise
as the centrifugal eld? This would be a genuine elimination of absolute
rotation, attributing all eects indierently to the rotation of the earth the
stars being at rest, or to the revolution of the stars the earth being at rest;
nothing matters except the relative rotation. I doubt whether anyone will
persuade himself that the stars have anything to do with the phenomenon.
We do not believe that if the heavenly bodies were all annihilated it would
upset the gyrocompass. In any case, precise calculation shows that the
centrifugal force could not be produced by the motion of the stars, so far
as they are known.
We are therefore forced to give up the idea that the signs of the earth's
rotation|the protuberance of its equator, the phenomena of the gyrocompass,
etc.|are due to a rotation relative to any matter we can recognise.
The philosopher who persists that a rotation which is not relative to matter
is unthinkable, will no doubt reply that the rotation must then be
relative to some matter which we have not yet recognised. We have hitherto
been greatly indebted to the suggestions of philosophy in evolving
x] TOWARDS INFINITY 143
this theory, because the suggestions related to the things we know about;
and, as it turned out, they were conrmed by experiment. But as physicists
we cannot take the same interest in the new demand; we do not
necessarily challenge it, but it is outside our concern. Physics demands
of its scheme of nature something else besides truth, namely a certain
quality that we may call convergence. The law of conservation of energy
is only strictly true when the whole universe is taken into account; but
its value in physics lies in the fact that it is approximately true for a very
limited system. Physics is an exact science because the chief essentials
of a problem are limited to a few conditions; and it draws near to the
truth with ever-increasing approximation as it widens its purview. The
approximations of physics form a convergent series. History, on the other
hand, is very often like a divergent series; no approximation to its course
is reached until the last term of the innite series has been included in
the data of prediction. Physics, if it wishes to retain its advantage, must
take its own course, formulating those laws which are approximately true
for the limited data of sense, and extending them into the unknown. The
relativity of rotation is not approximately true for the data of sense, although
it may possibly be true when the unknown as well as the known
are included.
The same considerations that apply to rotation apply to acceleration,
although the diculty is less striking. We can if we like attribute to the
sun some arbitrary acceleration, balancing it by introducing a uniform
gravitational eld. Owing to this eld the rest of the stars will move with
the same acceleration and no phenomena will be altered. But then it
seems necessary to nd a cause for this eld. It is not produced by the
gravitation of the stars. Our only course is to pursue the cause further
and further towards innity; the further we put it away, the greater the
mass of attracting matter needed to produce it. On the other hand, the
earth's absolute acceleration does not intrude on our attention in the way
that its absolute rotation does.
We are vaguely conscious of a diculty in these results; but if we examine
it closely, the diculty does not seem to be a very serious one. The
theory of relativity, as we have understood it, asserts that our partitions
of space and time are introduced by the observer and are irrelevant to
To determine even roughly the earth's absolute acceleration we should need a
fairly full knowledge of the disturbing eects of all the matter in the universe. A similar
knowledge would be required to determine the absolute rotation accurately; but all the
matter likely to exist would have so small an eect, that we can at once assume that the
absolute rotation is very nearly the same as the experimentally determined rotation.
144 TOWARDS INFINITY [ch.
the laws of nature; and therefore the current quantities of physics, length,
duration, mass, force, etc., which are relative to these partitions, are not
things having an absolute signicance in nature. But we have never denied
that there are features of the world having an absolute signicance;
in fact, we have spent much time in nding such features. The geodesics
or natural tracks have been shown to have an absolute signicance; and it
is possible in a limited region of the world to choose space and time partitions
such that all geodesics become approximately straight lines. We may
call this a \natural" frame for that region, although it is not as a rule the
space and time adopted in practice; it is for example the space and time of
the observers in the falling projectile, not of Newton's super-observer. It
is capable of absolute denition, except that it is ambiguous in regard to
uniform motion. Now the rotation of the earth determined by Foucault's
pendulum experiment is the rotation referred to this natural frame. But
we must have misunderstood our own theory of relativity altogether, if
we think there is anything inadmissible in an absolute rotation of such a
kind.
Material particles and geodesics are both features of the absolute structure
of the world; and a rotation relative to geodesic structure does not
seem to be on any dierent footing from a velocity relative to matter.
There is, however, the striking feature that rotation seems to be relative
not merely to the local geodesic structure but to a generally accepted universal
frame; whereas it is necessary to specify precisely what matter a
velocity is measured with respect to. This is largely a question of how
much accuracy is needed in specifying velocities and rotations, respectively.
If in stating the speed of a  particle we do not mind an error
of 10; 000 kilometres a second, we need not specify precisely what star or
planet its velocity is referred to. The moon's (local) angular velocity is
sometimes given to fourteen signicant gures; I doubt if any universal
frame is well-dened enough for this accuracy. There is no doubt much
greater continuity in the geodesic structure in dierent parts of the world
than in the material structure; but the dierence is in degree rather than
in principle.
It is probable that here we part company from many of the continental
relativists, who give prominent place to a principle known as the law of
causality|that only those things are to be regarded as being in causal
connection which are capable of being actually observed. This seems to
be interpreted as placing matter on a plane above geodesic structure in
regard to the formulation of physical laws, though it is not easy to see in
what sense a distribution of matter can be regarded as more observable
x] TOWARDS INFINITY 145
than the eld of in
uence in surrounding space which makes us aware of
its existence. The principle itself is debateable; that which is observable
to us is determined by the accident of our own structure, and the law
of causality seems to impose our own limitations on the free interplay of
entities in the world outside us. In this book the tradition of Faraday and
Maxwell still rules our outlook; and for us matter and electricity are but
incidental points of complexity, the activity of nature being primarily in
the so-called empty spaces between.
The vague universal frame to which rotation is referred is called the in-
ertial frame. It is denite in the 
at space-time far away from all matter.
In the undulating country corresponding to the stellar universe it is not a
precise conception; it is rather a rude outline, arbitrary within reasonable
limits, but with the general course indicated. The reason for the term inertial
frame is of interest. We can quite freely use a mesh-system deviating
widely from the inertial frame (e.g. rotating axes); but we have seen that
there is a postponed debt to pay in the shape of an apparently uncaused
eld of force. But is there no debt to pay, even when the inertial frame is
used? In that case there is no gravitational or centrifugal force at innity;
but there is still inertia, which is of the same nature. The distinction between
force as requiring a cause and inertia as requiring no cause cannot
be sustained. We shall not become any more solvent by commuting our
debt into pure inertia. The debt is inevitable whatever mesh-system is
used; we are only allowed to choose the form it shall take.
The debt after all is a very harmless one. At innity we have the absolute
geodesics in space-time, and we have our own arbitrarily drawn
mesh-system. The relation of the geodesics to the mesh-system decides
whether our axes shall be termed rotating or non-rotating; and ideally it
is this relation that is determined when a so-called absolute rotation is
measured. No one could reasonably expect that there would be no determinable
relation. On the other hand uniform translation does not aect
the relation of the geodesics to the mesh-system|if they were straight
lines originally, they remain straight lines|thus uniform translation cannot
be measured except relative to matter.
We have been supposing that the conditions found in the remotest parts
of space accessible to observation can be extrapolated to innity; and that
there are still denite natural tracks in space-time far beyond the in
uence
of matter. Feelings of objection to this view arise in certain minds. It is
urged that as matter in
uences the course of geodesics it may well be
responsible for them altogether; so that a region outside the eld of action
of matter could have no geodesics, and consequently no intervals. All the
146 TOWARDS INFINITY [ch.
potentials would then necessarily be zero. Various modied forms of this
objection arise; but the main feeling seems to be that it is unsatisfactory to
have certain conditions prevailing in the world, which can be traced away
to innity and so have, as it were, their source at innity; and there is a
desire to nd some explanation of the inertial frame as built up through
conditions at a nite distance.
Now if all intervals vanished space-time would shrink to a point. Then
there would be no space, no time, no inertia, no anything. Thus a cause
which creates intervals and geodesics must, so to speak, extend the world.
We can imagine the world stretched out like a plane sheet; but then
the stretching cause|the cause of the intervals|is relegated beyond the
bounds of space and time, i.e. to innity. This is the view objected to,
though the writer does not consider that the objection has much force. An
alternative way is to in
ate the world from inside, as a balloon is blown
out. In this case the stretching force is not relegated to innity, and ruled
outside the scope of experiment; it is acting at every point of space and
time, curving the world to a sphere. We thus get the idea that space-time
may have an essential curvature on a great scale independent of the small
hummocks due to recognised matter.
It is not necessary to speculate whether the curvature is produced (as in
the balloon) by some pressure applied from a fth dimension. For us it will
appear as an innate tendency of four-dimensional space-time to curve. It
may be asked, what have we gained by substituting a natural curvature of
space-time for a natural stretched condition corresponding to the inertial
frame? As an explanation, nothing. But there is this dierence, that the
theory of the inertial frame can now be included in the dierential law of
gravitation instead of remaining outside and additional to the law.
It will be remembered that one clue by which we previously reached
the law of gravitation was that 
at space-time must be compatible with
it. But if space-time is to have a small natural curvature independent
of matter this condition is now altered. It is not dicult to nd the
necessary alteration of the law. It will contain an additional, and at
present unknown, constant, which determines the size of the world.
Spherical space is not very easy to imagine. We have to think of the
properties of the surface of a sphere|the two-dimensional case|and try to
conceive something similar applied to three-dimensional space. Stationing
ourselves at a point let us draw a series of spheres of successively greater
radii. The surface of a sphere of radius r should be proportional to r2;
but in spherical space the areas of the more distant spheres begin to fall
Appendix, Note 14.
x] TOWARDS INFINITY 147
below the proper proportion. There is not so much room out there as we
expected to nd. Ultimately we reach a sphere of biggest possible area,
and beyond it the areas begin to decrease. The last sphere of all shrinks
to a point|our antipodes. Is there nothing beyond this? Is there a kind of
boundary there? There is nothing beyond and yet there is no boundary.
On the earth's surface there is nothing beyond our own antipodes but
there is no boundary there.
The diculty is that we try to realise this spherical world by imagining
how it would appear to us and to our measurements. There has been
nothing in our experience to compare it with, and it seems fantastic. But
if we could get rid of the personal point of view, and regard the sphericity
of the world as a statement of the type of order of events outside us, we
should think that it was a simple and natural order which is as likely as
any other to occur in the world.
In such a world there is no diculty about accumulated debt at the
boundary. There is no boundary. The centrifugal force increases until
we reach the sphere of greatest area, and then, still obeying the law of
gravitation, diminishes to zero at the antipodes. The debt has paid itself
automatically.
We must not exaggerate what has been accomplished by this modication
of the theory. A new constant has been introduced into the law of
gravitation which gives the world a denite extension. Previously there
was nothing to x the scale of the world; it was simply given a priori that
it was innite. Granted extension, so that the intervals are not invariably
zero, we can determine geodesics everywhere, and hence mark out the
inertial frame.
Spherical space-time, that is to say a four-dimensional continuum of
space and imaginary time forming the surface of a sphere in ve dimensions,
has been investigated by Prof. de Sitter. If real time is used the world
is spherical in its space dimensions, but open towards plus and minus innity
in its time dimension, like an hyperboloid. This happily relieves us of
the necessity of supposing that as we progress in time we shall ultimately
come back to the instant we started from! History never repeats itself.
But in the space dimensions we should, if we went on, ultimately come
back to the starting point. This would have interesting physical results,
and we shall see presently that Einstein has a theory of the world in which
the return can actually happen; but in de Sitter's theory it is rather an
abstraction, because, as he says, \all the paradoxical phenomena can only
happen after the end or before the beginning of eternity."
The area is, of course, to be determined by measurement of some kind.
148 TOWARDS INFINITY [ch.
The reason is this. Owing to curvature in the time dimension, as we
examine the condition of things further and further from our starting
point, our time begins to run faster and faster, or to put it another way
natural phenomena and natural clocks slow down. The condition becomes
like that described in Mr H. G. Wells's story \The new accelerator."
When we reach half-way to the antipodal point, time stands still. Like
the Mad Hatter's tea party, it is always 6 o'clock; and nothing whatever
can happen however long we wait. There is no possibility of getting any
further, because everything including light has come to rest here. All that
lies beyond is for ever cut o from us by this barrier of time; and light can
never complete its voyage round the world.
That is what happens when the world is viewed from one station; but if
attracted by such a delightful prospect, we proceeded to visit this scene of
repose, we should be disappointed. We should nd nature there as active
as ever. We thought time was standing still, but it was really proceeding
there at the usual rate, as if in a fth dimension of which we had no
cognisance. Casting an eye back on our old home we should see that time
apparently had stopped still there. Time in the two places is proceeding in
directions at right angles, so that the progress of time at one point has no
relation to the perception of time at the other point. The reader will easily
see that a being conned to the surface of a sphere and not cognisant of
a third dimension, will, so to speak, lose one of his dimensions altogether
when he watches things occurring at a point 90° away. He regains it if he
visits the spot and so adapts himself to the two dimensions which prevail
there.
It might seem that this kind of fantastic world-building can have little
to do with practical problems. But that is not quite certain. May we
not be able actually to observe the slowing down of natural phenomena at
great distances from us? The most remote objects known are the spiral
nebulae, whose distances may perhaps be of the order a million light years.
If natural phenomena are slowed down there, the vibrations of an atom are
slower, and its characteristic spectral lines will appear displaced to the red.
We should generally interpret this as a Doppler eect, implying that the
nebula is receding. The motions in the line-of-sight of a number of nebulae
have been determined, chie
y by Prof. Slipher. The data are not so ample
as we should like; but there is no doubt that large receding motions greatly
preponderate. This may be a genuine phenomenon in the evolution of the
material universe; but it is also possible that the interpretation of spectral
displacement as a receding velocity is erroneous; and the eect is really
the slowing of atomic vibrations predicted by de Sitter's theory.
x] TOWARDS INFINITY 149
Prof. Einstein himself prefers a dierent theory of curved space-time.
His world is cylindrical|curved in the three space dimensions and straight
in the time dimension. Since time is no longer curved, the slowing of
phenomena at great distances from the observer disappears, and with it
the slight experimental support given to the theory by the observations of
spiral nebulae. There is no longer a barrier of eternal rest, and a ray of
light is able to go round the world.
In various ways crude estimates of the size of the world both on de Sitter's
and Einstein's hypotheses have been made; and in both cases the
radius is thought to be of the order 1013 times the distance of the earth
from the sun. A ray of light from the sun would thus take about 1000 million
years to go round the world; and after the journey the rays would
converge again at the starting point, and then diverge for the next circuit.
The convergent would have all the characteristics of a real sun so far as
light and heat are concerned, only there would be no substantial body
present. Thus corresponding to the sun we might see a series of ghosts
occupying the positions where the sun was 1000, 2000, 3000, etc., million
years ago, if (as seems probable) the sun has been luminous for so long.
It is rather a pleasing speculation that records of the previous states
of the sidereal universe may be automatically reforming themselves on
the original sites. Perhaps one or more of the many spiral nebulae are
really phantoms of our own stellar system. Or it may be that only a
proportion of the stars are substantial bodies; the remainder are optical
ghosts revisiting their old haunts. It is, however, unlikely that the light
rays after their long journey would converge with the accuracy which this
theory would require. The minute de
ections by the various gravitational
elds encountered on the way would turn them aside, and the focus would
be blurred. Moreover there is a likelihood that the light would gradually
be absorbed or scattered by matter diused in space, which is encountered
on the long journey.
It is sometimes suggested that the return of the light-wave to its starting
point can most easily be regarded as due to the force of gravitation,
there being sucient mass distributed through the universe to control its
path in a closed orbit. We should have no objection in principle to this
way of looking at it; but we doubt whether it is correct in fact. It is
quite possible for light to return to its starting point in a world without
gravitation. We can roll 
at space-time into a cylinder and join the edges;
its geometry will still be Euclidean and there will be no gravitation; but
a ray of light can go right round the cylinder and return to the starting
point in space. Similarly in Einstein's more complex type of cylinder
150 TOWARDS INFINITY [ch.
(three dimensions curved and one dimension linear), it seems likely that
the return of the light is due as much to the connectivity of his space, as
to the non-Euclidean properties which express the gravitational eld.
For Einstein's cylindrical world it is necessary to postulate the existence
of vast quantities of matter (not needed on de Sitter's theory) far in excess
of what has been revealed by our telescopes. This additional material may
either be in the form of distant stars and galaxies beyond our limits of
vision, or it may be uniformly spread through space and escape notice by
its low density. There is a denite relation between the average density
of matter and the radius of the world; the greater the radius the smaller
must be the average density.
Two objections to this theory may be urged. In the rst place, absolute
space and time are restored for phenomena on a cosmical scale. The ghost
of a star appears at the spot where the star was a certain number of million
years ago; and from the ghost to the present position of the star is a
denite distance|the absolute motion of the star in the meantime. The
world taken as a whole has one direction in which it is not curved; that
direction gives a kind of absolute time distinct from space. Relativity is
reduced to a local phenomenon; and although this is quite sucient for
the theory hitherto described, we are inclined to look on the limitation
rather grudgingly. But we have already urged that the relativity theory
is not concerned to deny the possibility of an absolute time, but to deny
that it is concerned in any experimental knowledge yet found; and it need
not perturb us if the conception of absolute time turns up in a new form
in a theory of phenomena on a cosmical scale, as to which no experimental
knowledge is yet available. Just as each limited observer has his own particular
separation of space and time, so a being coextensive with the world
might well have a special separation of space and time natural to him. It
is the time for this being that is here dignied by the title \absolute."
Secondly, the revised law of gravitation involves a new constant which
depends on the total amount of matter in the world; or conversely the
total amount of matter in the world is determined by the law of gravitation.
This seems very hard to accept|at any rate without some plausible
explanation of how the adjustment is brought about. We can see that, the
constant in the law of gravitation being xed, there may be some upper
limit to the amount of matter possible; as more and more matter is added
in the distant parts, space curves round and ultimately closes; the process
The ghost is not formed where the star is now. If two stars were near together
when the light left them their ghosts must be near together, although the stars may
now be widely separated.
x] TOWARDS INFINITY 151
of adding more matter must stop, because there is no more space, and we
can only return to the region already dealt with. But there seems nothing
to prevent a defect of matter, leaving space unclosed. Some mechanism
seems to be needed, whereby either gravitation creates matter, or all the
matter in the universe conspires to dene a law of gravitation.
Although this appears to the writer rather bewildering, it is welcomed by
those philosophers who follow the lead of Mach. For it leads to the result
that the extension of space and time depends on the amount of matter in
the world|partly by its direct eect on the curvature and partly by its
in
uence on the constant of the law of gravitation. The more matter there
is, the more space is created to contain it, and if there were no matter the
world would shrink to a point.
In the philosophy of Mach a world without matter is unthinkable. Matter
in Mach's philosophy is not merely required as a test body to display
properties of something already there, which have no physical meaning
except in relation to matter; it is an essential factor in causing those properties
which it is able to display. Inertia, for example, would not appear
by the insertion of one test body in the world; in some way the presence
of other matter is a necessary condition. It will be seen how welcome to
such a philosophy is the theory that space and the inertial frame come
into being with matter, and grow as it grows. Since the laws of inertia
are part of the law of gravitation, Mach's philosophy was summed up|
perhaps unconsciously|in the profound saying \If there were no matter
in the universe, the law of gravitation would fall to the ground."
No doubt a world without matter, in which nothing could ever happen,
would be very uninteresting; and some might deny its claim to be regarded
as a world at all. But a world uniformly lled with matter would be
equally dull and unprotable; so there seems to be little object in denying
the possibility of the former and leaving the latter possible.
The position can be summed up as follows:|in a space without absolute
features, an absolute rotation would be as meaningless as an absolute
translation; accordingly, the existence of an experimentally determined
quantity generally identied with absolute rotation requires explanation.
It was remarked on p. 36 that it would be dicult to devise a plan of
the world according to which uniform motion has no signicance but nonuniform
motion is signicant; but such a world has been arrived at|a
plenum, of which the absolute features are intervals and geodesics. In a
limited region this plenum gives a natural frame with respect to which an
acceleration or rotation (but not a velocity) capable of absolute denition
can be measured. In the case of rotation the local distortions of the
152 TOWARDS INFINITY
frame are of comparatively little account; and this explains why in practice
rotation appears to have reference to some world-wide inertial frame.
Thus absolute rotation does not indicate any logical 
aw in the theory
hitherto developed; and there is no need to accept any modication of our
views. Possibly there may be a still wider relativity theory, in which our
supposed plenum is to be regarded as itself an abstraction of the relations
of the matter distributed throughout the world, and not existent apart
from such matter. This seems to exalt matter rather unnecessarily. It may
be true; but we feel no necessity for it, unless experiment points that way.
It is with some such underlying idea that Einstein's cylindrical space-time
was suggested, since this cannot exist without matter to keep it stretched.
Now we freely admit that our assumption of perfect 
atness in the remote
parts of space was arbitrary, and there is no justication for insisting on it.
A small curvature is possible both conceptually and experimentally. The
arguments on both sides have hitherto been little more than prejudices,
which would be dissipated by any experimental or theoretical lead in one
direction. Weyl's theory of the electromagnetic eld, discussed in the next
chapter, assigns a denite function to the curvature of space; and this
considerably alters the aspect of the question. We are scarcely suciently
advanced to oer a nal opinion; but the conception of cylindrical spacetime
seems to be favoured by this new development of the theory.
Some may be inclined to challenge the right of the Einstein theory, at
least as interpreted in this book, to be called a relativity theory. Perhaps
it has not all the characteristics which have at one time or another been
associated with that name; but the reader, who has followed us so far, will
see how our search for an absolute world has been guided by a recognition
of the relativity of the measurements of physics. It may be urged that
our geodesics ought not to be regarded as fundamental; a geodesic has no
meaning in itself; what we are really concerned with is the relation of a
particle following a geodesic to all the other matter of the world and the
geodesic cannot be thought of apart from such other matter. We would
reply, \Your particle of matter is not fundamental; it has no meaning in
itself; what you are really concerned with is its `eld'|the relation of
the geodesics about it to the other geodesics in the world|and matter
cannot be thought of apart from its eld." It is all a tangle of relations;
physical theory starts with the simplest constituents, philosophical theory
with the most familiar constituents. They may reach the same goal; but
their methods are often incompatible.
CHAPTER XI
ELECTRICITY AND GRAVITATION
Thou shalt not have in thy bag divers weights, a great and a small.
Thou shalt not have in thine house divers measures, a great and a small.
But thou shalt have a perfect and just weight, a perfect and just measure shalt
thou have. Book of Deuteronomy.
The relativity theory deduces from geometrical principles the existence
of gravitation and the laws of mechanics of matter. Mechanics is derived
from geometry, not by adding arbitrary hypotheses, but by removing unnecessary
assumptions, so that a geometer like Riemann might almost have
foreseen the more important features of the actual world. But nature has
in reserve one great surprise|electricity.
Electrical phenomena are not in any way a mist in the relativity theory,
and historically it is through them that it has been developed. Yet we
cannot rest satised until a deeper unity between the gravitational and
electrical properties of the world is apparent. The electron, which seems
to be the smallest particle of matter, is a singularity in the gravitational
eld and also a singularity in the electrical eld. How can these two facts
be connected? The gravitational eld is the expression of some state of
the world, which also manifests itself in the natural geometry determined
with measuring appliances; the electric eld must also express some state
of the world, but we have not as yet connected it with natural geometry.
May there not still be unnecessary assumptions to be removed, so that
a yet more comprehensive geometry can be found, in which gravitational
and electrical elds both have their place?
There is an arbitrary assumption in our geometry up to this point,
which it is desirable now to point out. We have based everything on the
\interval," which, it has been said, is something which all observers, whatever
their motion or whatever their mesh-system, can measure absolutely,
agreeing on the result. This assumes that they are provided with identical
standards of measurement|scales and clocks. But if A is in motion relative
to B and wishes to hand his standards to B to check his measures,
154 ELECTRICITY AND GRAVITATION [ch.
he must stop their motion; this means in practice that he must bombard
his standards with material molecules until they come to rest. Is it fair to
assume that no alteration of the standard is caused by this process? Or
if A measures time by the vibrations of a hydrogen atom, and space by
the wave-length of the vibration, still it is necessary to stop the atom by
a collision in which electrical forces are involved?
The standard of length in physics is the length in the year 1799 of a
bar deposited at Paris. Obviously no interval is ever compared directly
with that length; there must be a continuous chain of intermediate steps
extending like a geodetic triangulation through space and time, rst along
the past history of the scale actually used, then through intermediate
standards, and nally along the history of the Paris metre itself. It may
be that these intermediate steps are of no importance|that the same
result is reached by whatever route we approach the standard; but clearly
we ought not to make that assumption without due consideration. We
ought to construct our geometry in such a way as to show that there
are intermediate steps, and that the comparison of the interval with the
ultimate standard is not a kind of action at a distance.
To compare intervals in dierent directions at a point in space and time
does not require this comparison with a distant standard. The physicist's
method of describing phenomena near a point P is to lay down for comparison
(1) a mesh-system, (2) a unit of length (some kind of material
standard), which can also be used for measuring time, the velocity of light
being unity. With this system of reference he can measure in terms of his
unit small intervals PP0 running in any direction from P, summarising
the results in the fundamental formula
ds2 = g11 dx2
1 + g22 dx2
2 + 
 
 
 + 2g12 dx1dx2 + 
 
 
 :
If now he wishes to measure intervals near a distant point Q, he must lay
down a mesh-system and a unit of measure there. He naturally tries to
simplify matters by using what he would call the same unit of measure at
P and Q, either by transporting a material rod or some equivalent device.
If it is immaterial by what route the unit is carried from P to Q, and
replicas of the unit carried by dierent routes all agree on arrival at Q,
this method is at any rate explicit. The question whether the unit at Q
dened in this way is really the same as that at P is mere metaphysics. But
if the units carried by dierent routes disagree, there is no unambiguous
means of identifying a unit at Q with the unit at P. Suppose P is an
event at Cambridge on March 1, and Q at London on May 1; we are
contemplating the possibility that there will be a dierence in the results
xi] ELECTRICITY AND GRAVITATION 155
of measures made with our standard in London on May 1, according as
the standard is taken up to London on March 1 and remains there, or
is left at Cambridge and taken up on May 1. This seems at rst very
improbable; but our reasons for allowing for this possibility will appear
presently. If there is this ambiguity the only possible course is to lay down
(1) a mesh-system lling all the space and time considered, (2) a denite
unit of interval, or gauge, at every point of space and time. The geometry
of the world referred to such a system will be more complicated than that
of Riemann hitherto used; and we shall see that it is necessary to specify
not only the 10 g's, but four other functions of position, which will be
found to have an important physical meaning.
The observer will naturally simplify things by making the units of gauge
at dierent points as nearly as possible equal, judged by ordinary comparisons.
But the fact remains that, when the comparison depends on the
route taken, exact equality is not denable; and we have therefore to admit
that the exact standards are laid down at every point independently.
It is the same problem over again as occurs in regard to mesh-systems.
We lay down particular rectangular axes near a point P; presently we
make some observations near a distant point Q. To what coordinates
shall the latter be referred? The natural answer is that we must use the
same coordinates as we were using at P. But, except in the particular
case of 
at space, there is no means of dening exactly what coordinates
at Q are the same as those at P. In many cases the ambiguity may be
too tri
ing to trouble us; but in exact work the only course is to lay down
a denite mesh-system extending throughout space, the precise route of
the partitions being necessarily arbitrary. We now nd that we have to
add to this by placing in each mesh a gauge whose precise length must
be arbitrary. Having done this the next step is to make measurements of
intervals (using our gauges). This connects the absolute properties of the
world with our arbitrarily drawn mesh-system and gauge-system. And so
by measurement we determine the g's and the new additional quantities,
which determine the geometry of our chosen system of reference, and at
the same time contain within themselves the absolute geometry of the
world|the kind of space-time which exists in the eld of our experiments.
Having laid down a unit-gauge at every point, we can speak quite definitely
of the change in interval-length of a measuring-rod moved from
point to point, meaning, of course, the change compared with the unitgauges.
Let us take a rod of interval-length l at P, and move it successively
through the displacements dx1, dx2, dx3, dx4; and let the result be to increase
its length in terms of the gauges by the amount l. The change
156 ELECTRICITY AND GRAVITATION [ch.
depends as much on the dierence of the gauges at the two points as on
the behaviour of the rod; but there is no possibility of separating the two
factors. It is clear that  will not depend on l, because the change of length
must be proportional to the original length|unless indeed our whole idea
of measurement by comparison with a gauge is wrong. Further it will not
depend on the direction of the rod either in its initial or nal positions
because the interval-length is independent of direction. (Of course, the
space-length would change, but that is already taken care of by the g's.)
 can thus only depend on the displacements dx1, dx2, dx3, dx4, and we
may write it
 = 1 dx1 + 2 dx2 + 3 dx3 + 4 dx4;
so long as the displacements are small. The coecients 1, 2, 3, 4 apply
to the neighbourhood of P, and will in general be dierent in dierent
parts of space.
This indeed assumes that the result is independent of the order of the
displacements dx1, dx2, dx3, dx4|that is to say that the ambiguity of
the comparison by dierent routes disappears in the limit when the whole
route is suciently small. It is parallel with our previous implicit assumption
that although the length of the track from a point P to a distant
point Q depends on the route, and no denite meaning can be attached
to the interval between them without specifying a route, yet in the limit
there is a denite small interval between P and Q when they are su-
ciently close together.
To understand the meaning of these new coecients  let us brie
y recapitulate
what we understand by the g's. Primarily they are quantities
derived from experimental measurements of intervals, and describe the
geometry of the space and time partitions which the observer has chosen.
As consequential properties they describe the eld of force, gravitational,
centrifugal, etc., with which he perceives himself surrounded. They relate
to the particular mesh-system of the observer; and by altering his
mesh-system, he can alter their values, though not entirely at will. From
their values can be deduced intrinsic properties of the world|the kind of
space-time in which the phenomena occur. Further they satisfy a denite
condition|the law of gravitation|so that not all mathematically possible
space-times and not all arbitrary values of the g's are such as can occur
in nature.
All this applies equally to the 's, if we substitute gauge-system for
mesh-system, and some at present unknown force for gravitation. They
We refuse to contemplate the idea that when the metre rod changes its length to
two metres, each centimetre of it changes to three centimetres.
xi] ELECTRICITY AND GRAVITATION 157
can theoretically be determined by interval-measurement; but they will be
more conspicuously manifested to the observer through their consequential
property of describing some kind of eld of force surrounding him. The
's refer to the arbitrary gauge-system of the observer; but he cannot
by altering his gauge-system alter their values entirely at will. Intrinsic
properties of the world are contained in their values, unaected by any
change of gauge-system. Further we may expect that they will have to
satisfy some law corresponding to the law of gravitation, so that not all
arbitrary values of the 's are such as can occur in nature.
It is evident that the 's must refer to some type of phenomenon which
has not hitherto appeared in our discussion; and the obvious suggestion is
that they refer to the electromagnetic eld. This hypothesis is strengthened
when we recall that the electromagnetic eld is, in fact, specied at
every point by the values of four quantities, viz. the three components of
electromagnetic vector potential, and the scalar potential of electrostatics.
Surely it is more than a coincidence that the physicist needs just four more
quantities to specify the state of the world at a point in space, and four
more quantities are provided by removing a rather illogical restriction on
our system of geometry of natural measures.
[The general reader will perhaps pardon a few words addressed especially
to the mathematical physicist. Taking the ordinary unaccelerated
rectangular coordinates x, y, z, t, let us write F, G, H, 􀀀 for 1, 2, 3,
4, then
dl
l
=  = F dx + Gdy + H dx 􀀀 dt:
From which, by integration,
log l + const. = Z (F dx + Gdy + H dz 􀀀 dt):
The length l will be independent of the route taken if
F dx + Gdy + H dz 􀀀 dt
is a perfect dierential. The condition for this is
@H
@y 􀀀
@G
@z
= 0;
@F
@z 􀀀
@H
@x
= 0;
@G
@x 􀀀
@F
@y
= 0;
􀀀
@
@x 􀀀
@F
@t
= 0; 􀀀
@
@y 􀀀
@G
@t
= 0; 􀀀
@
@z 􀀀
@H
@t
= 0:
If F, G, H,  are the potentials of electromagnetic theory, these are precisely
the expressions for the three components of magnetic force and the
158 ELECTRICITY AND GRAVITATION [ch.
three components of electric force, given in the text-books. Thus the condition
that distant intervals can be compared directly without specifying
a particular route of comparison is that the electric and magnetic forces
are zero in the intervening space and time.
It may be noted that, even when the coordinate system has been de-
ned, the electromagnetic potentials are not unique in value; but arbitrary
additions can be made provided these additions form a perfect dierential.
It is just this 
exibility which in our geometrical theory appears in
the form of the arbitrary choice of gauge-system. The electromagnetic
forces on the other hand are independent of the gauge-system, which is
eliminated by \curling."]
It thus appears that the four new quantities appearing in our extended
geometry may actually be the four potentials of electromagnetic theory;
and further, when there is no electromagnetic eld our previous geometry
is valid. But in the more general case we have to adopt the more general
geometry in which there appear fourteen coecients, ten describing the
gravitational and four the electrical conditions of the world.
We ought now to seek the law of the electromagnetic eld on the same
lines as we sought for the law of gravitation, laying down the condition that
it must be independent of mesh-system and gauge-system since it seeks to
limit the possible kinds of world which can exist in nature. Happily this
presents no diculty, because the law expressed by Maxwell's equations,
and universally adopted, fulls the conditions. There is no need to modify
it fundamentally as we modied the law of gravitation. We do, however,
generalise it so that it applies when a gravitational eld is present at the
same time|not merely, as given by Maxwell, for 
at space-time. The
de
ection of electromagnetic waves (light) by a gravitational eld is duly
contained in this generalised law.
Strictly speaking the laws of gravitation and of the electromagnetic eld
are not two laws but one law, as the geometry of the g's and the 's is
one geometry. Although it is often convenient to separate them, they are
really parts of the general condition limiting the possible kinds of metric
that can occur in empty space.
It will be remembered that the four-fold arbitrariness of our meshsystem
involved four identities, which were found to express the conservation
of energy and momentum. In the new geometry there is a fth
arbitrariness, namely that of the selected gauge-system. This must also
give rise to an identity; and it is found that the new identity expresses the
law of conservation of electric charge.
A grasp of the new geometry may perhaps be assisted by a further comx
i] ELECTRICITY AND GRAVITATION 159
parison. Suppose an observer has laid down a line of a certain length and
in a certain direction at a point P, and he wishes to lay down an exactly
similar line at a distant point Q. If he is in 
at space there will be no
diculty; he will have to proceed by steps, a kind of triangulation, but
the route chosen is of no importance. We know denitely that there is
just one direction at Q parallel to the original direction at P; and it is in
ordinary geometry supposed that the length is equally determinate. But if
space is not 
at the case is dierent. Imagine a two-dimensional observer
conned to the curved surface of the earth trying to perform this task.
As he does not appreciate the third dimension he will not immediately
perceive the impossibility; but he will nd that the direction which he
has transferred to Q diers according to the route chosen. Or if he went
round a complete circuit he would nd on arriving back at P that the direction
he had so carefully tried to preserve on the journey did not agree
with that originally drawn. We describe this by saying that in curved
space, direction is not integrable; and it is this non-integrability of direction
which characterises the gravitational eld. In the case considered
the length would be preserved throughout the circuit; but it is possible
to conceive a more general kind of space in which the length which it
was attempted to preserve throughout the circuit, as well as the direction,
disagreed on return to the starting point with that originally drawn.
In that case length is not integrable; and the non-integrability of length
characterises the electromagnetic eld. Length associated with direction is
called a vector; and the combined gravitational and electric eld describe
that in
uence of the world on our measurements by which a vector carried
by physical measurement round a closed circuit changes insensibly into a
dierent vector.
The welding together of electricity and gravitation into one geometry
is the work of Prof. H. Weyl, rst published in 1918y. It appears to the
writer to carry conviction, although up to the present no experimental
test has been proposed. It need scarcely be said that the inconsistency
of length for an ordinary circuit would be extremely minutez, and the
ordinary manifestations of the electromagnetic eld are the consequential
It might be thought that if the observer preserved mentally the original direction in
three-dimensional space, and obtained the direction at any point in the two-dimensional
space by projecting it, there would be no ambiguity. But the three-dimensional space
in which a curved two-dimensional space is conceived to exist is quite arbitrary. A
two-dimensional observer cannot ascertain by any observation whether he is on a plane
or a cylinder, a sphere or any other convex surface of the same total curvature.
yAppendix, Note 15.
zI do not think that any numerical estimate has been made.
160 ELECTRICITY AND GRAVITATION [ch.
results of changes which would be imperceptible to direct measurement.
It will be remembered that the gravitational eld is likewise perceived by
the consequential eects, and not by direct interval-measurement.
But the theory does appear to require that, for example, the time of
vibration of an atom is not quite independent of its previous history. It
may be assumed that the previous histories of terrestrial atoms are so
much alike that there are no signicant dierences in their periods. The
possibility that the systematic dierence of history of solar and terrestrial
atoms may have an eect on the expected shift of the spectral lines on the
sun has already been alluded to. It seems doubtful, however, whether the
eect could attain the necessary magnitude.
It may seem dicult to identify these abstract geometrical qualities of
the world with the physical forces of electricity and magnetism. How,
for instance, can the change in the length of a rod taken round a circuit
in space and time be responsible for the sensations of an electric shock?
The geometrical potentials () obey the recognised laws of electromagnetic
potentials, and each entity in the physical theory|charge, electric force,
magnetic element, light, etc.|has its exact analogue in the geometrical
theory; but is this formal correspondence a sucient ground for identication?
The doubt which arises in our minds is due to a failure to recognise
the formalism of all physical knowledge. The suggestion \This is not the
thing I am speaking of, though it behaves exactly like it in all respects"
carries no physical meaning. Anything which behaves exactly like electricity
must manifest itself to us as electricity. Distinction of form is the only
distinction that physics can recognise; and distinction of individuality, if
it has any meaning at all, has no bearing on physical manifestations.
We can only explore the world with apparatus, which is itself part of the
world. Our idealised apparatus is reduced to a few simple types|a neutral
particle, a charged particle, a rigid scale, etc. The absolute constituents
of the world are related in various ways, which we have studied, to the
indications of these test-bodies. The main features of the absolute world
are so simple that there is a redundancy of apparatus at our disposal; and
probably all that there is to be known could theoretically be found out by
exploration with an uncharged particle. Actually we prefer to look at the
world as revealed by exploration with scales and clocks|the former for
measuring so-called imaginary intervals, and the latter for real intervals;
this gives us a unied geometrical conception of the world. Presumably,
we could obtain a unied mechanical conception by taking the moving
uncharged particle as standard indicator; or a unied electrical conception
by taking the charged particle. For particular purposes one test-body
xi] ELECTRICITY AND GRAVITATION 161
is generally better adapted than others. The gravitational eld is more
sensitively explored with a moving particle than a scale. Although the
electrical eld can theoretically be explored by the change of length of a
scale taken round a circuit, a far more sensitive way is to use a little bit of
the scale|an electron. And in general for practical eciency, we do not
use any simple type of apparatus, but a complicated construction built up
with a view to a particular experiment. The reason for emphasising the
theoretical interchangeability of test-bodies is that it brings out the unity
and simplicity of the world; and for that reason there is an importance in
characterising the electromagnetic condition of the world by reference to
the indications of a scale and clock, however inappropriate they may be
as practical test-bodies.
Weyl's theory opens up interesting avenues for development. The details
of the further steps involve dicult mathematics; but a general outline is
possible. As on Einstein's more limited theory there is at any point an
important property of the world called the curvature; but on the new
theory it is not an absolute quantity in the strictest sense of the word.
It is independent of the observer's mesh-system, but it depends on his
gauge. It is obvious that the number expressing the radius of curvature
of the world at a point must depend on the unit of length; so we cannot
say that the curvatures at two points are absolutely equal, because they
depend on the gauges assigned at the two points. Conversely the radius
of curvature of the world provides a natural and absolute gauge at every
point; and it will presumably introduce the greatest possible symmetry
into our laws if the observer chooses this, or some denite fraction of it,
as his gauge. He, so to speak, forces the world to be spherical by adopting
at every point a unit of length which will make it so. Actual rods as they
are moved about change their lengths compared with this absolute unit
according to the route taken, and the dierences correspond to the electromagnetic
eld. Einstein's curved space appears in a perfectly natural
manner in this theory; no part of space-time is 
at, even in the absence
of ordinary matter, for that would mean innite radius of curvature, and
there would be no natural gauge to determine, for example, the dimensions
of an electron|the electron could not know how large it ought to
be, unless it had something to measure itself against.
The connection between the form of the law of gravitation and the total
amount of matter in the world now appears less mysterious. The curvature
of space indirectly provides the gauge which we use for measuring the
amount of matter in the world.
Since the curvature is not independent of the gauge, Weyl does not
162 ELECTRICITY AND GRAVITATION [ch.
identify it with the most fundamental quantity in nature. There is, however,
a slightly more complicated invariant which is a pure number, and
this is taken to be Action. We can thus mark out a denite volume of
space and time, and say that the action within it is 5, without troubling
to dene coordinates or the unit of measure! It might be expected that
the action represented by the number 1 would have specially interesting
properties; it might, for instance, be an atom of action and indivisible.
Experiment has isolated what are believed to be units of action, which
at least in many phenomena behave as indivisible atoms called quanta;
but the theory, as at present developed, does not permit us to represent
the quantum of action by the number 1. The quantum is a very minute
fraction of the absolute unit.
When we come across a pure number having some absolute signicance
in the world it is natural to speculate on its possible interpretation. It
might represent a number of discrete entities; but in that case it must
necessarily be an integer, and it seems clear that action can have fractional
values. An angle is commonly represented as a pure number, but it has
not really this character; an angle can only be measured in terms of a
unit of angle, just as a length is measured in terms of a unit of length.
I can only think of one interpretation of a fractional number which can
have an absolute signicance, though doubtless there are others. The
number may represent the probability of something, or some function of a
probability. The precise function is easily found. We combine probabilities
by multiplying, but we combine the actions in two regions by adding; hence
the logarithm of a probability is indicated. Further, since the logarithm of
a probability is necessarily negative, we may identify action provisionally
with minus the logarithm of the statistical probability of the state of the
world which exists.
The suggestion is particularly attractive because the Principle of Least
Action now becomes the Principle of Greatest Probability. The law of
nature is that the actual state of the world is that which is statistically
most probable.
Weyl's theory also shows that the mass of a portion of matter is necessarily
positive; on the original theory no adequate reason is given why
negative matter should not exist. It is further claimed that the theory
shows to some extent why the world is four-dimensional. To the mathematician
it seems so easy to generalise geometry to n dimensions, that we
naturally expect a world of four dimensions to have an analogue in ve
dimensions. Apparently this is not the case, and there are some essential
Appendix, Note 16.
xi] ELECTRICITY AND GRAVITATION 163
properties, without which it could scarcely be a world, which exist only
for four dimensions. Perhaps this may be compared with the well-known
diculty of generalising the idea of a knot; a knot can exist in space of
any odd number of dimensions, but not in space of an even number.
Finally the theory suggests a mode of attacking the problem of how
the electric charge of an electron is held together; at least it gives an
explanation of why the gravitational force is so extremely weak compared
with the electric force. It will be remembered that associated with the
mass of the sun is a certain length, called the gravitational mass, which
is equal to 1:5 kilometres. In the same way the gravitational mass or
radius of an electron is 7 
 10􀀀56 cms. Its electrical properties are similarly
associated with a length 2
10􀀀13 cms., which is called the electrical radius.
The latter is generally supposed to correspond to the electron's actual
dimensions. The theory suggests that the ratio of the gravitational to the
electrical radius, 3 
 1042, ought to be of the same order as the ratio of
the latter to the radius of curvature of the world. This would require the
radius of space to be of the order 6 
 1029 cms., or 2 
 1011 parsecs., which
though somewhat larger than the provisional estimates made by de Sitter,
is within the realm of possibility.
164 ELECTRICITY AND GRAVITATION
CHAPTER XII
ON THE NATURE OF THINGS
Hippolyta. This is the silliest stu that ever I heard.
Theseus. The best in this kind are but shadows; and the worst are
no worse, if imagination amend them.
A Midsummer-Night's Dream.
The constructive results of the theory of relativity are based on two principles
which have been enunciated|the restricted principle of relativity,
and the principle of equivalence. These may be summed up in the statement
that uniform motion and elds of force are purely relative. In their
more formal enunciations they are experimental generalisations, which can
be admitted or denied; if admitted, all the observational results obtained
by us can be deduced mathematically without any reference to the views
of space, time, or force, described in this book. In many respects this is
the most attractive aspect of Einstein's work; it deduces a great number
of remarkable phenomena solely from two general principles, aided by a
mathematical calculus of great power; and it leaves aside as irrelevant
all questions of mechanism. But this mode of development of the theory
cannot be described in a non-technical book.
To avoid mathematical analysis we have had to resort to geometrical
illustrations, which run parallel with the mathematical development and
enable its processes to be understood to some extent. The question arises,
are these merely illustrations of the mathematical argument, or illustrations
of the actual processes of nature. No doubt the safest course is
to avoid the thorny questions raised by the latter suggestion, and to say
that it is quite sucient that the illustrations should correctly replace the
mathematical argument. But I think that this would give a misleading
view of what the theory of relativity has accomplished in science.
The physicist, so long as he thinks as a physicist, has a denite belief
in a real world outside him. For instance, he believes that atoms and
molecules really exist; they are not mere inventions that enable him to
166 ON THE NATURE OF THINGS [ch.
grasp certain laws of chemical combination. That suggestion might have
suced in the early days of the atomic theory; but now the existence of
atoms as entities in the real world of physics is fully demonstrated. This
condent assertion is not inconsistent with philosophic doubts as to the
meaning of ultimate reality.
When therefore we are asked whether the four-dimensional world may
not be regarded merely as an illustration of mathematical processes, we
must bear in mind that our questioner has probably an ulterior motive. He
has already a belief in a real world of three Euclidean dimensions, and he
hopes to be allowed to continue in this belief undisturbed. In that case our
answer must be denite; the real three-dimensional world is obsolete, and
must be replaced by the four-dimensional space-time with non-Euclidean
properties. In this book we have sometimes employed illustrations which
certainly do not correspond to any physical reality|imaginary time, and
an unperceived fth dimension. But the four-dimensional world is no mere
illustration; it is the real world of physics, arrived at in the recognised way
by which physics has always (rightly or wrongly) sought for reality.
I hold a certain object before me, and see an outline of the gure of
Britannia; another observer on the other side sees a picture of a monarch;
a third observer sees only a thin rectangle. Am I to say that the gure of
Britannia is the real object; and that the crude impressions of the other
observers must be corrected to make allowance for their positions? All
the appearances can be accounted for if we are all looking at a threedimensional
object|a penny|and no reasonable person can doubt that
the penny is the corresponding physical reality. Similarly, an observer on
the earth sees and measures an oblong block; an observer on another star
contemplating the same object nds it to be a cube. Shall we say that the
oblong block is the real thing, and that the other observer must correct
his measures to make allowance for his motion? All the appearances are
accounted for if the real object is four-dimensional, and the observers are
merely measuring dierent three-dimensional appearances or sections; and
it seems impossible to doubt that this is the true explanation. He who
doubts the reality of the four-dimensional world (for logical, as distinct
from experimental, reasons) can only be compared to a man who doubts
the reality of the penny, and prefers to regard one of its innumerable
appearances as the real object.
Physical reality is the synthesis of all possible physical aspects of nature.
An illustration may be taken from the phenomena of radiant-energy,
or light. In a very large number of phenomena the light coming from an
atom appears to be a series of spreading waves, extending so as to be
xii] ON THE NATURE OF THINGS 167
capable of lling the largest telescope yet made. In many other phenomena
the light coming from an atom appears to remain a minute bundle
of energy, all of which can enter and blow up a single atom. There may
be some illusion in these experimental deductions; but if not, it must be
admitted that the physical reality corresponding to light must be some
synthesis comprehending both these appearances. How to make this synthesis
has hitherto baed conception. But the lesson is that a vast number
of appearances may be combined into one consistent whole|perhaps all
appearances that are directly perceived by terrestrial observers|and yet
the result may still be only an appearance. Reality is only obtained when
all conceivable points of view have been combined.
That is why it has been necessary to give up the reality of the everyday
world of three dimensions. Until recently it comprised all the possible
appearances that had been considered. But now it has been discovered
that there are new points of view with new appearances; and the reality
must contain them all. It is by bringing in all these new points of view
that we have been able to learn the nature of the real world of physics.
Let us brie
y recapitulate the steps of our synthesis. We found one step
already accomplished. The immediate perception of the world with one
eye is a two-dimensional appearance. But we have two eyes, and these
combine the appearances of the world as seen from two positions; in some
mysterious way the brain makes the synthesis by suggesting solid relief,
and we obtain the familiar appearance of a three-dimensional world. This
suces for all possible positions of the observer within the parts of space
hitherto explored. The next step was to combine the appearances for
all possible states of uniform motion of the observer. The result was to
add another dimension to the world, making it four-dimensional. Next
the synthesis was extended to include all possible variable motions of the
observer. The process of adding dimensions stopped, but the world became
non-Euclidean; a new geometry called Riemannian geometry was
adopted. Finally the points of view of observers varying in size in any way
were added; and the result was to replace the Riemannian geometry by a
still more general geometry described in the last chapter.
The search for physical reality is not necessarily utilitarian, but it has
been by no means protless. As the geometry became more complex, the
physics became simpler; until nally it almost appears that the physics
has been absorbed into the geometry. We did not consciously set out
to construct a geometrical theory of the world; we were seeking physical
reality by approved methods, and this is what has happened.
Is the point now reached the ultimate goal? Have the points of view
168 ON THE NATURE OF THINGS [ch.
of all conceivable observers now been absorbed? We do not assert that
they have. But it seems as though a denite task has been rounded o,
and a natural halting-place reached. So far as we know, the dierent
possible impersonal points of view have been exhausted|those for which
the observer can be regarded as a mechanical automaton, and can be
replaced by scientic measuring-appliances. A variety of more personal
points of view may indeed be needed for an ultimate reality; but they
can scarcely be incorporated in a real world of physics. There is thus
justication for stopping at this point but not for stopping earlier.
It may be asked whether it is necessary to take into account all conceivable
observers, many of whom, we suspect, have no existence. Is not
the real world that which comprehends the appearances to all real observers?
Whether or not it is a tenable hypothesis that that which no one
observes does not exist, science uncompromisingly rejects it. If we deny
the rights of extra-terrestrial observers, we must take the side of the Inquisition
against Galileo. And if extra-terrestrial observers are admitted,
the other observers, whose results are here combined, cannot be excluded.
Our inquiry into the nature of things is subject to certain limitations
which it is important to realise. The best comparison I can oer is with
a future antiquarian investigation, which may be dated about the year
5000 a.d. An interesting nd has been made relating to a vanished civilisation
which 
ourished about the twentieth century, namely a volume
containing a large number of games of chess, written out in the obscure
symbolism usually adopted for that purpose. The antiquarians, to whom
the game was hitherto unknown, manage to discover certain uniformities;
and by long research they at last succeed in establishing beyond doubt
the nature of the moves and rules of the game. But it is obvious that no
amount of study of the volume will reveal the true nature either of the
participants in the game|the chessmen|or the eld of the game|the
chess-board. With regard to the former, all that is possible is to give arbitrary
names distinguishing the chessmen according to their properties; but
with regard to the chess-board something more can be stated. The material
of the board is unknown, so too are the shapes of the meshes|whether
squares or diamonds; but it is ascertainable that the dierent points of
the eld are connected with one another by relations of two-dimensional
order, and a large number of hypothetical types of chess-board satisfying
these relations of order can be constructed. In spite of these gaps in
their knowledge, our antiquarians may fairly claim that they thoroughly
understand the game of chess.
The application of this analogy is as follows. The recorded games are
xii] ON THE NATURE OF THINGS 169
our physical experiments. The rules of the game, ascertained by study of
them, are the laws of physics. The hypothetical chess-board of 64 squares
is the space and time of some particular observer or player; whilst the more
general relations of two-fold order, are the absolute relations of order in
space-time which we have been studying. The chessmen are the entities of
physics|electrons, particles, or point-events; and the range of movement
may perhaps be compared to the elds of relation radiating from them|
electric and gravitational elds, or intervals. By no amount of study of the
experiments can the absolute nature or appearance of these participants
be deduced; nor is this knowledge relevant, for without it we may yet learn
\the game" in all its intricacy. Our knowledge of the nature of things must
be like the antiquarians' knowledge of the nature of chessmen, viz. their
nature as pawns and pieces in the game, not as carved shapes of wood.
In the latter aspect they may have relations and signicance transcending
anything dreamt of in physics.
It is believed that the familiar things of experience are very complex; and
the scientic method is to analyse them into simpler elements. Theories
and laws of behaviour of these simpler constituents are studied; and from
these it becomes possible to predict and explain phenomena. It seems
a natural procedure to explain the complex in terms of the simple, but
it carries with it the necessity of explaining the familiar in terms of the
unfamiliar.
There are thus two reasons why the ultimate constituents of the real
world must be of an unfamiliar nature. Firstly, all familiar objects are of
a much too complex character. Secondly, familiar objects belong not to
the real world of physics, but to a much earlier stage in the synthesis of
appearances. The ultimate elements in a theory of the world must be of
a nature impossible to dene in terms recognisable to the mind.
The fact that he has to deal with entities of unknown nature presents
no diculty to the mathematician. As the mathematician in the Prologue
explained, he is never so happy as when he does not know what he is
talking about. But we ourselves cannot take any interest in the chain of
reasoning he is producing, unless we can give it some meaning|a meaning,
which we nd by experiment, it will bear. We have to be in a position to
make a sort of running comment on his work. At rst his symbols bring
no picture of anything before our eyes, and we watch in silence. Presently
we can say \Now he is talking about a particle of matter". . . \Now he is
talking about another particle". . . \Now he is saying where they will be at
a certain time of day". . . \Now he says that they will be in the same spot at
a certain time." We watch to see.|\Yes. The two particles have collided.
170 ON THE NATURE OF THINGS [ch.
For once he is speaking about something familiar, and speaking the truth,
although, of course, he does not know it." Evidently his chain of symbols
can be interpreted as describing what occurs in the world; we need not,
and do not, form any idea of the meaning of each individual symbol; it is
only certain elaborate combinations of them that we recognise.
Thus, although the elementary concepts of the theory are of undened
nature, at some later stage we must link the derivative concepts to the
familiar objects of experience.
We shall now collect the results arrived at in the previous chapters
by successive steps, and set the theory out in more logical order. The
extension in Chapter xi will not be considered here, partly because it
would increase the diculty of grasping the main ideas, partly because it
is less certainly established.
In the relativity theory of nature the most elementary concept is the
point-event. In ordinary language a point-event is an instant of time at a
point of space; but this is only one aspect of the point-event, and it must
not be taken as a denition. Time and space|the familiar terms|are
derived concepts to be introduced much later in our theory. The rst
simple concepts are necessarily undenable, and their nature is beyond
human understanding. The aggregate of all the point-events is called the
world. It is postulated that the world is four-dimensional, which means
that a particular point-event has to be specied by the values of four
variables or coordinates, though there is entire freedom as to the way in
which these four identifying numbers are to be assigned.
The meaning of the statement that the world is four-dimensional is not
so clear as it appears at rst. An aggregate of a large number of things has
in itself no particular number of dimensions. Consider, for example, the
words on this page. To a casual glance they form a two-dimensional distribution;
but they were written in the hope that the reader would regard
them as a one-dimensional distribution. In order to dene the number of
dimensions we have to postulate some ordering relation; and the result
depends entirely on what this ordering relation is|whether the words are
ordered according to sense or to position on the page. Thus the statement
that the world is four-dimensional contains an implicit reference to some
ordering relation. This relation appears to be the interval, though I am
not sure whether that alone suces without some relation corresponding
to proximity. It must be remembered that if the interval s between two
events is small, the events are not necessarily near together in the ordinary
sense.
Between any two neighbouring point-events there exists a certain relax
ii] ON THE NATURE OF THINGS 171
tion known as the interval between them. The relation is a quantitative
one which can be measured on a denite scale of numerical values. But
the term \interval" is not to be taken as a guide to the real nature of
the relation, which is altogether beyond our conception. Its geometrical
properties, which we have dwelt on so often in the previous chapters, can
only represent one aspect of the relation. It may have other aspects associated
with features of the world outside the scope of physics. But in
physics we are concerned not with the nature of the relation but with the
number assigned to express its intensity; and this suggests a graphical
representation, leading to a geometrical theory of the world of physics.
What we have here called the world might perhaps have been legitimately
called the aether; at least it is the universal substratum of things
which the relativity theory gives us in place of the aether.
We have seen that the number expressing the intensity of the intervalrelation
can be measured practically with scales and clocks. Now, I think it
is improbable that our coarse measures can really get hold of the individual
intervals of point-events; our measures are not suciently microscopic
for that. The interval which has appeared in our analysis must be a
macroscopic value; and the potentials and kinds of space deduced from
it are averaged properties of regions, perhaps small in comparison even
with the electron, but containing vast numbers of the primitive intervals.
We shall therefore pass at once to the consideration of the macroscopic
interval; but we shall not forestall later results by assuming that it is
measurable with a scale and clock. That property must be introduced in
its logical order.
Consider a small portion of the world. It consists of a large (possibly
innite) number of point-events between every two of which an interval
exists. If we are given the intervals between a point A and a sucient
number of other points, and also between B and the same points, can
we calculate what will be the interval between A and B? In ordinary
geometry this would be possible; but, since in the present case we know
nothing of the relation signied by the word interval, it is impossible to
predict any law a priori. But we have found in our previous work that
there is such a rule, expressed by the formula
ds2 = g11 dx2
1 + g22 dx2
2 + 
 
 
 + 2g12 dx1dx2 + 
 
 
 :
This means that, having assigned our identication numbers (x1, x2, x3,
There is also a qualitative distinction into two kinds, ultimately identied as timelike
and space-like, which for mathematical treatment are distinguished by real and
imaginary numbers.
172 ON THE NATURE OF THINGS [ch.
x4) to the point-events, we have only to measure ten dierent intervals
to enable us to determine the ten coecients, g11, etc., which in a small
region may be considered to be constants; then all other intervals in this
region can be predicted from the formula. For any other region we must
make fresh measures, and determine the coecients for a new formula.
I think it is unlikely that the individual interval-relations of point-events
follow any such denite rule. A microscopic examination would probably
show them as quite arbitrary, the relations of so-called intermediate points
being not necessarily intermediate. Perhaps even the primitive interval is
not quantitative, but simply 1 for certain pairs of point-events and 0 for
others. The formula given is just an average summary which suces for
our coarse methods of investigation, and holds true only statistically. Just
as statistical averages of one community may dier from those of another,
so may this statistical formula for one region of the world dier from that
of another. This is the starting point of the innite variety of nature.
Perhaps an example may make this clearer. Compare the point-events
to persons, and the intervals to the degree of acquaintance between them.
There is no means of forecasting the degree of acquaintance between A
and B from a knowledge of the familiarity of both with C, D, E, etc. But
a statistician may compute in any community a kind of average rule. In
most cases if A and B both know C, it slightly increases the probability
of their knowing one another. A community in which this correlation was
very high would be described as cliquish. There may be dierences among
communities in this respect, corresponding to their degree of cliquishness;
and so the statistical laws may be the means of expressing intrinsic dierences
in communities.
Now comes the diculty which is by this time familiar to us. The ten g's
are concerned, not only with intrinsic properties of the world, but with our
arbitrary system of identication-numbers for the point-events; or, as we
have previously expressed it, they describe not only the kind of space-time,
but the nature of the arbitrary mesh-system that is used. Mathematics
shows the way of steering through this diculty by xing attention on
expressions called tensors, of which B
 and G are examples.
A tensor does not express explicitly the measure of an intrinsic quality
of the world, for some kind of mesh-system is essential to the idea of
measurement of a property, except in certain very special cases where the
property is expressed by a single number termed an invariant, e.g. the
interval, or the total curvature. But to state that a tensor vanishes, or
that it is equal to another tensor in the same region, is a statement of
intrinsic property, quite independent of the mesh-system chosen. Thus
xii] ON THE NATURE OF THINGS 173
by keeping entirely to tensors, we contrive that there shall be behind our
formulae an undercurrent of information having reference to the intrinsic
state of the world.
In this way we have found two absolute formulae, which appear to be
fully conrmed by observation, namely
in empty space, G = 0;
in space containing matter, G = K;
where K contains only physical quantities which are perfectly familiar
to us, viz. the density and state of motion of the matter in the region.
I think the usual view of these equations would be that the rst expresses
some law existing in the world, so that the point-events by natural
necessity tend to arrange their relations in conformity with this equation.
But when matter intrudes it causes a disturbance or strain of the natural
linkages; and a rearrangement takes place to the extent indicated by the
second equation.
But let us examine more closely what the equation G = 0 tells us.
We have been giving the mathematician a free hand with his indenable
intervals and point-events. He has arrived at the quantity G; but as
yet this means to us|absolutely nothing. The pure mathematician left
to himself never \deviates into sense." His work can never relate to the
familiar things around us, unless we boldly lay hold of some of his symbols
and give them an intelligible meaning|tentatively at rst, and then
denitely as we nd that they satisfy all experimental knowledge. We
have decided that in empty space G vanishes. Here is our opportunity.
In default of any other suggestion as to what the vanishing of G might
mean, let us say that the vanishing of G means emptiness; so that G,
if it does not vanish, is a condition of the world which distinguishes space
said to be occupied from space said to be empty. Hitherto G was merely
a formal outline to be lled with some undened contents; we are as far
as ever from being able to explain what those contents are; but we have
now given a recognisable meaning to the completed picture, so that we
shall know it when we come across it in the familiar world of experience.
The two equations are accordingly merely denitions|denitions of the
way in which certain states of the world (described in terms of the inde-
nables) impress themselves on our perceptions. When we perceive that a
certain region of the world is empty, that is merely the mode in which our
senses recognise that it is curved no higher than the rst degree. When
we perceive that a region contains matter we are recognising the intrinsic
curvature of the world; and when we believe we are measuring the mass
174 ON THE NATURE OF THINGS [ch.
and momentum of the matter (relative to some axes of reference) we are
measuring certain components of world-curvature (referred to those axes).
The statistical averages of something unknown, which have been used to
describe the state of the world, vary from point to point; and it is out of
these that the mind has constructed the familiar notions of matter and
emptiness.
The law of gravitation is not a law in the sense that it restricts the possible
behaviour of the substratum of the world; it is merely the denition
of a vacuum. We need not regard matter as a foreign entity causing a
disturbance in the gravitational eld; the disturbance is matter. In the
same way we do not regard light as an intruder in the electromagnetic
eld, causing the electromagnetic force to oscillate along its path; the oscillations
constitute the light. Nor is heat a 
uid causing agitation of the
molecules of a body; the agitation is heat.
This view, that matter is a symptom and not a cause, seems so natural
that it is surprising that it should be obscured in the usual presentation
of the theory. The reason is that the connection of mathematical analysis
with the things of experience is usually made, not by determining what
matter is, but by what certain combinations of matter do. Hence the
interval is at once identied with something familiar to experience, namely
the thing that a scale and a clock measure. However advantageous that
may be for the sake of bringing the theory into touch with experiment
at the outset, we can scarcely hope to build up a theory of the nature of
things if we take a scale and clock as the simplest unanalysable concepts.
The result of this logical inversion is that by the time the equation G =
K is encountered, both sides of the equation are well-dened quantities.
Their necessary identity is overlooked, and the equation is regarded as a
new law of nature. This is the fault of introducing the scale and clock
prematurely. For our part we prefer rst to dene what matter is in terms
of the elementary concepts of the theory; then we can introduce any kind
of scientic apparatus; and nally determine what property of the world
that apparatus will measure.
Matter dened in this way obeys all the laws of mechanics, including
conservation of energy and momentum. Proceeding with a similar development
of Weyl's more general theory of the combined gravitational and
electrical elds, we should nd that it has the familiar electrical and optical
properties. It is purely gratuitous to suppose that there is anything
else present, controlling but not to be identied with the relations of the
fourteen potentials (g's and 's).
There is only one further requirement that can be demanded from matx
ii] ON THE NATURE OF THINGS 175
ter. Our brains are constituted of matter, and they feel and think|or at
least feeling and thinking are closely associated with motions or changes of
the matter of the brain. It would be dicult to say that any hypothesis as
to the nature of matter makes this process less or more easily understood;
and a brain constituted out of dierential coecients of g's can scarcely
be said to be less adapted to the purposes of thought than one made, say,
out of tiny billiard balls! But I think we may even go a little beyond this
negative justication. The primary interval relation is of an undened
nature, and the g's contain this undenable element. The expression G
is therefore of dened form, but of undened content. By its form alone
it is tted to account for all the physical properties of matter; and physical
investigation can never penetrate beneath the form. The matter of the
brain in its physical aspects is merely the form; but the reality of the brain
includes the content. We cannot expect the form to explain the activities
of the content, any more than we can expect the number 4 to explain the
activities of the Big Four at Versailles.
Some of these views of matter were anticipated with marvellous foresight
by W. K. Cliord forty years ago. Whilst other English physicists
were distracted by vortex-atoms and other will-o'-the-wisps, Cliord was
convinced that matter and the motion of matter were aspects of spacecurvature
and nothing more. And he was no less convinced that these
geometrical notions were only partial aspects of the relations of what he
calls \elements of feeling."|\The reality corresponding to our perception
of the motion of matter is an element of the complex thing we call feeling.
What we might perceive as a plexus of nerve-disturbances is really in itself
a feeling; and the succession of feelings which constitutes a man's consciousness
is the reality which produces in our minds the perception of the
motions of his brain. These elements of feeling have relations of nextness
or contiguity in space, which are exemplied by the sight-perceptions of
contiguous points; and relations of succession in time which are exempli
ed by all perceptions. Out of these two relations the future theorist
has to build up the world as best he may. Two things may perhaps help
him. There are many lines of mathematical thought which indicate that
distance or quantity may come to be expressed in terms of position in the
wide sense of the analysis situs. And the theory of space-curvature hints at
a possibility of describing matter and motion in terms of extension only."
(Fortnightly Review, 1875.)
The equation G = K is a kind of dictionary explaining what the
dierent components of world-curvature mean in terms ordinarily used in
176 ON THE NATURE OF THINGS [ch.
mechanics. If we write it in the slightly modied, but equivalent, form
G 􀀀 1
2gG = 􀀀8T;
we have the following scheme of interpretation
T11; T12; T13; T14
T22; T23; T24
T33; T34
T44
=
p11 + u2; p12 + uv; p13 + uw; 􀀀u;
p22 + v2; p23 + vw; 􀀀v;
p33 + w2; 􀀀w;
:
Here we are using the partitions of space and time adopted in ordinary
mechanics;  is the density of the matter, u, v, w its component velocities,
and p11, p12, : : : p33, the components of the internal stresses which are
believed to be analysable into molecular movements.
Now the question arises, is it legitimate to make identications on such a
wholesale scale? Having identied T44 as density, can we go on to identify
another quantity T34 as density multiplied by velocity? It is as though we
identied one \thing" as air, and a quite dierent \thing" as wind. Yes,
it is legitimate, because we have not hitherto explained what is to be the
counterpart of velocity in our scheme of the world; and this is the way
we choose to introduce it. All identications are at this stage provisional,
being subject to subsequent test by observation.
A denition of the velocity of matter in some such terms as \wind divided
by air," does not correspond to the way in which motion primarily
manifests itself in our experience. Motion is generally recognised by the
disappearance of a particle at one point of space and the appearance of an
apparently identical particle at a neighbouring point. This manifestation
of motion can be deduced mathematically from the identifying denition
here adopted. Remembering that in physical theory it is necessary to
proceed from the simple to the complex, which is often opposed to the
instinctive desire to proceed from the familiar to the unfamiliar, this inversion
of the order in which the manifestations of motion appear need
occasion no surprise. Permanent identity of particles of matter (without
which the ordinary notion of velocity fails) is a very familiar idea, but it
appears to be a very complex feature of the world.
A simple instance may be given where the familiar kinematical conception
of motion is insucient. Suppose a perfectly homogeneous continuous
ring is rotating like a wheel, what meaning can we attach to its motion?
The kinematical conception of motion implies change|disappearance at
one point and reappearance at another point|but no change is detectable.
The state at any one moment is the same as at a previous moment, and
xii] ON THE NATURE OF THINGS 177
the matter occupying one position now is indistinguishable from the matter
in the same position a moment ago. At the most it can only dier in
a mysterious non-physical quality|that of identity; but if, as most physicists
are willing to believe, matter is some state in the aether, what can
we mean by saying that two states are exactly alike, but are not identical?
Is the hotness of the room equal to, but not identical with, its
hotness yesterday? Considered kinematically, the rotation of the ring appears
to have no meaning; yet the revolving ring diers mechanically from
a stationary ring. For example, it has gyrostatic properties. The fact
that in nature a ring has atomic and not continuous structure is scarcely
relevant. A conception of motion which aords a distinction between a
rotating and non-rotating continuous ring must be possible; otherwise this
would amount to an a priori proof that matter is atomic. According to
the conception now proposed, velocity of matter is as much a static quality
as density. Generally velocity is accompanied by changes in the physical
state of the world, which aord the usual means of recognising its existence;
but the foregoing illustration shows that these symptoms do not
always occur.
This denition of velocity enables us to understand why velocity except
in reference to matter is meaningless, whereas acceleration and rotation
have a meaning. The philosophical argument, that velocity through space
is meaningless, ceases to apply as soon as we admit any kind of structure
or aether in empty regions; consequently the problem is by no means so
simple as is often supposed. But our denition of velocity is dynamical, not
kinematical. Velocity is the ratio of certain components of T, and only
exists when T44 is not zero. Thus matter (or electromagnetic energy) is the
only thing that can have a velocity relative to the frame of reference. The
velocity of the world-structure or aether, where the T vanish, is always of
the indeterminate form 00. On the other hand acceleration and rotation
are dened by means of the g and exist wherever these exist; so that
the acceleration and rotation of the world-structure or aether relative to
the frame of reference are determinate. Notice that acceleration is not
dened as change of velocity; it is an independent entity, much simpler
and more universal than velocity. It is from a comparison of these two
entities that we ultimately obtain the denition of time.
This nally resolves the diculty encountered in Chapter x|the ap-
Even in Newtonian mechanics we speak of the \eld of acceleration," and think
of it as existing even when there is no test body to display the acceleration. In the
present theory this eld of acceleration is described by the g. There is no such thing
as a \eld of velocity" in empty space; but there is in a material ocean.
178 ON THE NATURE OF THINGS [ch.
parent dierence in the Principle of Relativity as applied to uniform and
non-uniform motion. Fundamentally velocity and acceleration are both
static qualities of a region of the world (referred to some mesh-system).
Acceleration is a comparatively simple quality present wherever there is
geodesic structure, that is to say everywhere. Velocity is a highly complex
quality existing only where the structure is itself more than ordinarily
complicated, viz. in matter. Both these qualities commonly give physical
manifestations, to which the terms acceleration and velocity are more particularly
applied; but it is by examining their more fundamental meaning
that we can understand the universality of the one and the localisation of
the other.
It has been shown that there are four identical relations between the
ten qualities of a piece of matter here identied, which depend solely on
the way the G were by denition constructed out of simpler elements.
These four relations state that, provided the mesh-system is drawn in one
of a certain number of ways, mass (or energy) and momentum will be
conserved. The conservation of mass is of great importance; matter will
be permanent, and for every particle disappearing at any point a corresponding
mass will appear at a neighbouring point; the change consists
in the displacement of matter, not its creation or destruction. This gives
matter the right to be regarded, not as a mere assemblage of symbols,
but as the substance of a permanent world. But the permanent world so
found demands the partitioning of space-time in one of a certain number
of ways, viz. those discussed in Chapter iii; from these a particular space
and time are selected, because the observer wishes to consider himself,
or some arbitrary body, at rest. This gives the space and time used for
ordinary descriptions of experience. In this way we are able to introduce
perceptual space and time into the four-dimensional world, as derived
concepts depending on our desire that the new-found matter should be
permanent.
I think it is now possible to discern something of the reason why the
world must of necessity be as we have described it. When the eye surveys
the tossing waters of the ocean, the eddying particles of water leave little
impression; it is the waves that strike the attention, because they have
a certain degree of permanence. The motion particularly noticed is the
motion of the wave-form, which is not a motion of the water at all. So the
mind surveying the world of point-events looks for the permanent things.
When the kind of space-time is such that a strict partition of this kind is impossible,
strict conservation does not exist; but we retain the principle as formally satised
by attributing energy and momentum to the gravitational eld.
xii] ON THE NATURE OF THINGS 179
The simpler relations, the intervals and potentials, are transient, and are
not the stu out of which mind can build a habitation for itself. But the
thing that has been identied with matter is permanent, and because of
its permanence it must be for mind the substance of the world. Practically
no other choice was possible.
It must be recognised that the conservation of mass is not exactly equivalent
to the permanence of matter. If a loaf of bread suddenly transforms
into a cabbage, our surprise is not diminished by the fact that there may
have been no change of weight. It is not very easy to dene this extra
element of permanence required, because we accept as quite natural apparently
similar transformations|an egg into an omelette, or radium into
lead. But at least it seems clear that some degree of permanence of one
quality, mass, would be the primary property looked for in matter, and
this gives sucient reason for the particular choice.
We see now that the choice of a permanent substance for the world of
perception necessarily carries with it the law of gravitation, all the laws of
mechanics, and the introduction of the ordinary space and time of experience.
Our whole theory has really been a discussion of the most general
way in which permanent substance can be built up out of relations; and it
is the mind which, by insisting on regarding only the things that are permanent,
has actually imposed these laws on an indierent world. Nature
has had very little to do with the matter; she had to provide a basis|
point-events; but practically anything would do for that purpose if the
relations were of a reasonable degree of complexity. The relativity theory
of physics reduces everything to relations; that is to say, it is structure,
not material, which counts. The structure cannot be built up without material;
but the nature of the material is of no importance. We may quote a
passage from Bertrand Russell's Introduction to Mathematical Philosophy.
\There has been a great deal of speculation in traditional philosophy
which might have been avoided if the importance of structure, and the dif-
culty of getting behind it, had been realised. For example it is often said
that space and time are subjective, but they have objective counterparts;
or that phenomena are subjective, but are caused by things in themselves,
which must have dierences inter se corresponding with the dierences in
the phenomena to which they give rise. Where such hypotheses are made,
it is generally supposed that we can know very little about the objective
counterparts. In actual fact, however, if the hypotheses as stated were
correct, the objective counterparts would form a world having the same
structure as the phenomenal world. . . . In short, every proposition having
a communicable signicance must be true of both worlds or of neither: the
180 ON THE NATURE OF THINGS [ch.
only dierence must lie in just that essence of individuality which always
eludes words and baes description, but which for that very reason is
irrelevant to science."
This is how our theory now stands.|We have a world of point-events
with their primary interval-relations. Out of these an unlimited number of
more complicated relations and qualities can be built up mathematically,
describing various features of the state of the world. These exist in nature
in the same sense as an unlimited number of walks exist on an open moor.
But the existence is, as it were, latent unless someone gives a signicance
to the walk by following it; and in the same way the existence of any one
of these qualities of the world only acquires signicance above its fellows,
if a mind singles it out for recognition. Mind lters out matter from the
meaningless jumble of qualities, as the prism lters out the colours of the
rainbow from the chaotic pulsations of white light. Mind exalts the permanent
and ignores the transitory; and it appears from the mathematical
study of relations that the only way in which mind can achieve her object
is by picking out one particular quality as the permanent substance of the
perceptual world, partitioning a perceptual time and space for it to be
permanent in, and, as a necessary consequence of this Hobson's choice,
the laws of gravitation and mechanics and geometry have to be obeyed.
Is it too much to say that mind's search for permanence has created the
world of physics? So that the world we perceive around us could scarcely
have been other than it is?
The last sentence possibly goes too far, but it illustrates the direction
in which these views are tending. With Weyl's more general theory of
interval-relations, the laws of electrodynamics appear in like manner to
depend merely on the identication of another permanent thing|electric
charge. In this case the identication is due, not to the rudimentary
instinct of the savage or the animal, but the more developed reasoningpower
of the scientist. But the conclusion is that the whole of those laws
of nature which have been woven into a unied scheme|mechanics, gravitation,
electrodynamics and optics|have their origin, not in any special
mechanism of nature, but in the workings of the mind.
\Give me matter and motion," said Descartes, \and I will construct the
universe." The mind reverses this. \Give me a world|a world in which
This summary is intended to indicate the direction in which the views suggested by
the relativity theory appear to me to be tending, rather than to be a precise statement
of what has been established. I am aware that there are at present many gaps in
the argument. Indeed the whole of this part of the discussion should be regarded as
suggestive rather than dogmatic.
xii] ON THE NATURE OF THINGS 181
there are relations|and I will construct matter and motion."
Are there then no genuine laws in the external world? Laws inherent
in the substratum of events, which break through into the phenomena
otherwise regulated by the despotism of the mind? We cannot foretell
what the nal answer will be; but, at present, we have to admit that there
are laws which appear to have their seat in external nature. The most
important of these, if not the only law, is a law of atomicity. Why does
that quality of the world which distinguishes matter from emptiness exist
only in certain lumps called atoms or electrons, all of comparable mass?
Whence arises this discontinuity? At present, there seems no ground for
believing that discontinuity is a law due to the mind; indeed the mind
seems rather to take pains to smooth the discontinuities of nature into
continuous perception. We can only suppose that there is something in
the nature of things that causes this aggregation into atoms. Probably our
analysis into point-events is not nal; and if it could be pushed further to
reach something still more fundamental, then atomicity and the remaining
laws of physics would be seen as identities. This indeed is the only kind
of explanation that a physicist could accept as ultimate. But this more
ultimate analysis stands on a dierent plane from that by which the pointevents
were reached. The world may be so constituted that the laws of
atomicity must necessarily hold; but, so far as the mind is concerned, there
seems no reason why it should have been constituted in that way. We can
conceive a world constituted otherwise. But our argument hitherto has
been that, however the world is constituted, the necessary combinations
of things can be found which obey the laws of mechanics, gravitation and
electrodynamics, and these combinations are ready to play the part of the
world of perception for any mind that is tuned to appreciate them; and
further, any world of perception of a dierent character would be rejected
by the mind as unsubstantial.
If atomicity depends on laws inherent in nature, it seems at rst dicult
to understand why it should relate to matter especially; since matter is not
of any great account in the analytical scheme, and owes its importance
to irrelevant considerations introduced by the mind. It has appeared,
however, that atomicity is by no means conned to matter and electricity;
the quantum, which plays so great a part in recent physics, is apparently
an atom of action. So nature cannot be accused of connivance with mind
in singling out matter for special distinction. Action is generally regarded
as the most fundamental thing in the real world of physics, although the
mind passes it over because of its lack of permanence; and it is vaguely
believed that the atomicity of action is the general law, and the appearance
182 ON THE NATURE OF THINGS
of electrons is in some way dependent on this. But the precise formulation
of the theory of quanta of action has hitherto baed physicists.
There is a striking contrast between the triumph of the scientic mind in
formulating the great general scheme of natural laws, nowadays summed
up in the principle of least action, and its present defeat by the newly discovered
but equally general phenomena depending on the laws of atomicity
of quanta. It is too early to cry failure in the latter case; but possibly the
contrast is signicant. It is one thing for the human mind to extract from
the phenomena of nature the laws which it has itself put into them; it
may be a far harder thing to extract laws over which it has had no control.
It is even possible that laws which have not their origin in the mind
may be irrational, and we can never succeed in formulating them. This
is, however, only a remote possibility; probably if they were really irrational
it would not have been possible to make the limited progress that
has been achieved. But if the laws of quanta do indeed dierentiate the
actual world from other worlds possible to the mind, we may expect the
task of formulating them to be far harder than anything yet accomplished
by physics.
The theory of relativity has passed in review the whole subject-matter
of physics. It has unied the great laws, which by the precision of their
formulation and the exactness of their application have won the proud
place in human knowledge which physical science holds to-day. And yet,
in regard to the nature of things, this knowledge is only an empty shell|a
form of symbols. It is knowledge of structural form, and not knowledge of
content. All through the physical world runs that unknown content, which
must surely be the stu of our consciousness. Here is a hint of aspects
deep within the world of physics, and yet unattainable by the methods of
physics. And, moreover, we have found that where science has progressed
the farthest, the mind has but regained from nature that which the mind
has put into nature.
We have found a strange foot-print on the shores of the unknown. We
have devised profound theories, one after another, to account for its origin.
At last, we have succeeded in reconstructing the creature that made the
foot-print. And Lo! it is our own.
APPENDIX
MATHEMATICAL NOTES
The references marked \Report" are to the writer's \Report on the Relativity
Theory of Gravitation" for the Physical Society of London (Fleetway
Press), where fuller mathematical details are given.
Probably the most complete treatise on the mathematical theory of the
subject is H. Weyl's Raum, Zeit, Materie (Julius Springer, Berlin).
Note 1 (p. 17).
It is not possible to predict the contraction rigorously from the universally
accepted electromagnetic equations, because these do not cover the
whole ground. There must be other forces or conditions which govern the
form and size of an electron; under electromagnetic forces alone it would
expand indenitely. The old electrodynamics is entirely vague as to these
forces.
The theory of Larmor and Lorentz shows that if any system at rest in
the aether is in equilibrium, a similar system in uniform motion through
the aether, but with all lengths in the direction of motion diminished in
FitzGerald's ratio, will also be in equilibrium so far as the dierential
equations of the electromagnetic eld are concerned. There is thus a
general theoretical agreement with the observed contraction, provided the
boundary conditions at the surface of an electron behave in the same way.
The latter suggestion is conrmed by experiments on isolated electrons in
rapid motion (Kaufmann's experiment). It turns out that this requires
an electron to suer the same kind of contraction as a material rod; and
thus, although the theory throws light on the adjustments involved in
material contraction, it can scarcely be said to give an explanation of the
occurrence of contraction generally.
184 APPENDIX
Note 2 (p. 43).
Suppose a particle moves from (x1; y1; z1; t1) to (x2; y2; z2; t2), its velocity
u is given by
u2 =
(x2 􀀀 x1)2 + (y2 􀀀 y1)2 + (z2 􀀀 z1)2
(t2 􀀀 t1)2 :
Hence from the formula for s2
s = (t2 􀀀 t1)p(1 􀀀 u2):
(We omit a p􀀀1, as the sign of s2 is changed later in the chapter.)
If we take t1 and t2 to be the start and nish of the aviator's cigar
(Chapter i), then as judged by a terrestrial observer, t2􀀀t1 = 60 minutes, p(1 􀀀 u2) = FitzGerald contraction = 1
2 .
As judged by the aviator,
t2 􀀀 t1 = 30 minutes; p(1 􀀀 u2) = 1:
Thus for both observers s = 30 minutes, verifying that it is an absolute
quantity independent of the observer.
Note 3 (p. 44).
The formulae of transformation to axes with a dierent orientation are
x = x0 cos  􀀀  0 sin ; y = y0; z = z0;  = x0 sin  +  0 cos ;
where  is the angle turned through in the plane x.
Let u = i tan , so that cos  = (1 􀀀 u2)􀀀1
2 = , say. The formulae
become
x = (x0 􀀀 iu 0); y = y0; z = z0;  = ( 0 + iux0);
or, reverting to real time by setting i = t,
x = (x0 􀀀 ut0); y = y0; z = z0; t = (t0 􀀀 ux0);
which gives the relation between the estimates of space and time by two
dierent observers.
The factor  gives in the rst equation the FitzGerald contraction, and
in the fourth equation the retardation of time. The terms ut0 and ux0
correspond to the changed conventions as to rest and simultaneity.
A point at rest, x = const., for the rst observer corresponds to a point
moving with velocity u, x0 􀀀 ut0 = const., for the second observer. Hence
their relative velocity is u.
MATHEMATICAL NOTES 185
Note 4 (p. 73).
The condition for 
at space in two dimensions is
@
@x1  g12
g11p(g11g22 􀀀 g2
12)
@g11
@x2 􀀀
1
p(g11g22 􀀀 g2
12)
@g22
@x1 
+
@
@x2  2
p(g11g22 􀀀 g2
12)
@g12
@x1 􀀀
1
p(g11g22 􀀀 g2
12)
@g11
@x2
􀀀
g12
g11p(g11g22 􀀀 g2
12)
@g11
@x1 = 0:
Note 5 (p. 80).
Let g be the determinant of four rows and columns formed with the
elements g.
Let g be the minor of g, divided by g.
Let the \3-index symbol" f; g denote
1
2g @g
@x
+
@g
@x 􀀀
@g
@x 
summed for values of  from 1 to 4. There will be 40 dierent 3-index
symbols.
Then the Riemann-Christoel tensor is
B
 = f; gf; g 􀀀 f; gf; g +
@
@x f; g 􀀀
@
@x f; g;
the terms containing  being summed for values of  from 1 to 4.
The \contracted" Riemann-Christoel tensor G can be reduced to
G = 􀀀
@
@x f; g + f; gf; g
+
@2
@x@x
logp􀀀g 􀀀 f; g
@
@x
logp􀀀g;
where in accordance with a general convention in this subject, each term
containing a sux twice over ( and ) must be summed for the values
1, 2, 3, 4 of that sux.
The curvature G = gG, summed in accordance with the foregoing
convention.
186 APPENDIX
Note 6 (p. 87).
The electric potential due to a charge e is
 =
e
r(1 􀀀 vr=C);
where vr is the velocity of the charge in the direction of r, C the velocity
of light, and the square bracket signies antedated values. To the rst
order of vr=C, the denominator is equal to the present distance r, so
the expression reduces to e=r in spite of the time of propagation. The
foregoing formula for the potential was found by Lienard and Wiechert.
Note 7 (p. 88).
It is found that the following scheme of potentials rigorously satises
the equations G = 0, according to the values of G in Note 5,
􀀀1=
 0 0 0
􀀀x1
2 0 0
􀀀x1
2 sin2 x2
2 0


where 
 = 1 􀀀 =x1 and  is any constant (see Report, § 28). Hence
these potentials describe a kind of space-time which can occur in nature
referred to a possible mesh-system. If  = 0, the potentials reduce to
those for 
at space-time referred to polar coordinates; and, since in the
applications required  will always be extremely small, our coordinates
can scarcely be distinguished from polar coordinates. We can therefore
use the familiar symbols r, , , t, instead of x1, x2, x3, x4. It must,
however, be remembered that the identication with polar coordinates is
only approximate; and, for example, an equally good approximation is
obtained if we write x1 = r + 1
2, a substitution often used instead of
x1 = r since it has the advantage of making the coordinate-velocity of
light more symmetrical.
We next work out analytically all the mechanical and optical properties
of this kind of space-time, and nd that they agree observationally with
those existing round a particle at rest at the origin with gravitational
mass 1
2. The conclusion is that the gravitational eld here described is
produced by a particle of mass 1
2|or, if preferred, a particle of matter
at rest is produced by the kind of space-time here described.
MATHEMATICAL NOTES 187
Note 8 (p. 90).
Setting the gravitational constant equal to unity, we have for a circular
orbit
m=r2 = v2=r;
so that m = v2r:
The earth's speed, v, is approximately 30 km. per sec., or 1
10000 in terms
of the velocity of light. The radius of its orbit, r, is about 1:5 
 108 km.
Hence, m, the gravitational mass of the sun is approximately 1:5 km.
The radius of the sun is 697; 000 kms., so that the quantity 2m=r occurring
in the formulae is, for the sun's surface, :00000424 or 000:87.
Note 9 (p. 113).
See Report, §§ 29, 30. The general equations of a geodesic are
d2x
ds2 + f; g
dx
ds
dx
ds
= 0 ( = 1; 2; 3; 4):
From the formula for the line-element
ds2 = 􀀀
􀀀1 dr2 􀀀 r2 d2 + 
 dt2; (1)
we calculate the three-index symbols and it is found that two of the equations
of the geodesic take the rather simple form
d2
ds2 +
2
r 

dr
ds
d
ds
= 0;
d2t
ds2 +
d(log 
)
dr 

dr
ds
dt
ds
= 0;
which can be integrated giving
r2 d
ds
= h; (2)
dt
ds
=
c


; (3)
where h and c are constants of integration.
Eliminating dt and ds from (1), (2) and (3), we have
h
r2
dr
d2
+
h2
r2 = c2 􀀀 1 +
2m
r
+
2mh2
r3 ;
188 APPENDIX
or writing u = 1=r,
du
d2
+ u2 =
c2 􀀀 1
h2 +
2mu
h2 + 2mu3:
Dierentiating with respect to 
d2u
d2 + u =
m
h2 + 3mu2;
which gives the equation of the orbit in the usual form in particle dynamics.
It diers from the equation of the Newtonian orbit by the small term
3mu2, which is easily shown to give the motion of perihelion.
The track of a ray of light is also obtained from this formula, since
by the principle of equivalence it agrees with that of a material particle
moving with the speed of light. This case is given by ds = 0, and therefore
h = 1. The dierential equation for the path of a light-ray is thus
d2u
d2 + u = 3mu2:
An approximate solution is
u =
cos 
R
+
m
R2 (cos2  + 2 sin2 );
neglecting the very small quantity m2=R2. Converting to Cartesian coordinates,
this becomes
x = R 􀀀
m
R
x2 + 2y2
p(x2 + y2)
:
The asymptotes of the light-track are found by taking y very large
compared with x, giving
x = R 
2m
R
y
so that the angle between them is 4m=R.
Note 10 (p. 116).
Writing the line element in the form
ds2 = 􀀀1 + a
m
r
+ 
 
 
 dr2 􀀀 r2 d2 + 1 + b
m
r
+ c
m2
r2 + 
 
 
dt2;
MATHEMATICAL NOTES 189
the approximate Newtonian attraction xes b equal to 􀀀2; then the observed
de
ection of light xes a equal to +2; and with these values the
observed motion of Mercury xes c equal to 0.
To insert an arbitrary coecient of r2 d2 would merely vary the coordinate
system. We cannot arrive at any intrinsically dierent kind of
space-time in that way. Hence, within the limits of accuracy mentioned,
the expression found by Einstein is completely determinable by observation.
It may be mentioned that the line-element
ds2 = 􀀀dr2 􀀀 r2 d2 + (1 􀀀 2m=r) dt2;
gives one-half the observed de
ection of light, and one-third the motion of
perihelion of Mercury. As both these can be obtained on older theories,
taking account of the variation of mass with velocity, the coecient 
􀀀1
of dr2 is the essentially novel point in Einstein's theory.
Note 11 (p. 120).
It is often supposed that by the Principle of Equivalence any invariant
property which holds outside a gravitational eld also holds in a gravitational
eld; but there is necessarily some limitation on this equivalence.
Consider for instance the two invariant equations
ds2 = 1;
ds2(1 + k4B
B
 ) = 1;
where k is some constant having the dimensions of a length. Since B

vanishes outside a gravitational eld, if one of these equations is true the
other will be. But they cannot both hold in a gravitational eld, since
there B
B
 does not vanish, and is in fact equal to 24m2=r6. (I believe
that the numerical factor 24 is correct; but there are 65,536 terms in the
expression, and the terms which do not vanish have to be picked out.)
This ambiguity of the Principle of Equivalence is referred to in Report,
§§ 14, 27; and an enunciation is given which makes it denite. The enunciation
however is merely an explicit statement, and not a defence, of the
assumptions commonly made in applying the principle.
So far as general reasoning goes there seems no ground for choosing ds2
rather than ds2 (1+24k4m2=r6), or any similar expression, as the constant
character in the vibration of an atom.
190 APPENDIX
Note 12 (p. 123).
Let two rays diverging from a point at a distance R pass at distances
r and r + dr from a star of mass m. The de
ection being 4m=r, their
divergence will be increased by 4mdr=r2. This increase will be equal to the
original divergence dr=R if r = p4mR. Take for instance 4m = 10 km.,
R = 1015 km., then r = 108 km. So that the divergence of the light will
be doubled, when the actual de
ection of the ray is only 10􀀀7, or 000:02. In
the case of a star seen behind the sun the added divergence has no time
to take eect; but when the light has to travel a stellar distance after the
divergence is produced, it becomes weakened by it. Generally in stellar
phenomena the weakening of the light should be more prominent than the
actual de
ection.
Note 13 (p. 129).
The relations are (Report, § 39)
G
 = 1
2
@G
@x
( = 1; 2; 3; 4);
where G
 is the (contracted) covariant derivative of G
, or gG.
I doubt whether anyone has performed the laborious task of verifying
these identities by straightforward algebra.
Note 14 (p. 146).
The modied law for spherical space-time is in empty space
G = g:
In cylindrical space-time, matter is essential. The law in space occupied
by matter is
G 􀀀 1
2g(G 􀀀 2) = 􀀀8T;
the term 2 being the only modication. Spherical space-time of radius R
is given by  = 3=R2; cylindrical space-time by  = 1=R2 provided matter
of average density  = 1=4R2 is present. (See Report, §§ 50, 51.)
The total mass of matter in the cylindrical world is 1
2R. This must be
enormous, seeing that the sun's mass is only 11
2 kilometres.
HISTORICAL NOTE 191
Note 15 (p. 159).
Weyl's theory is given in Berlin. Sitzungsberichte, 30 May, 1918; An-
nalen der Physik, Bd. 59 (1919), p. 101.
Note 16 (p. 162).
The argument is rather more complicated than appears in the text,
where the distinction between action-density and action in a region, curvature
and total curvature in a region, has not been elaborated. Taking a
denitely marked out region in space and time, its measured volume will
be increased 16-fold by halving the gauge. Therefore for action-density we
must take an expression which will be diminished 16-fold by halving the
gauge. Now G is proportional to 1=R2, where R is the radius of curvature,
and so is diminished 4-fold. The invariant B
B
 has the same gaugedimensions
as G2; and hence when integrated through a volume gives a
pure number independent of the gauge. In Weyl's theory this is only the
gravitational part of the complete invariant
(B
 􀀀 1
2g
 F)(B
 􀀀 1
2g
F);
which reduces to
B
B
 + FF:
The second term gives actually the well-known expression for the actiondensity
of the electromagnetic eld, and this evidently strengthens the
identication of this invariant with action-density.
Einstein's theory, on the other hand, creates a diculty here, because
although there may be action in an electromagnetic eld without electrons,
the curvature is zero.
HISTORICAL NOTE
Before the Michelson-Morley experiment the question had been widely
discussed whether the aether in and near the earth was carried along by
the earth in its motion, or whether it slipped through the interstices between
the atoms. Astronomical aberration pointed decidedly to a stagnant
aether; but the experiments of Arago and Fizeau on the eect of motion
of transparent media on the velocity of light in those media, suggested a
partial convection of the aether in such cases. These experiments were
192 APPENDIX
rst-order experiments, i.e. they depended on the ratio of the velocity of
the transparent body to the velocity of light. The Michelson-Morley experiment
is the rst example of an experiment delicate enough to detect
second-order eects, depending on the square of the above ratio; the result,
that no current of aether past terrestrial objects could be detected,
appeared favourable to the view that the aether must be convected by the
earth. The diculty of reconciling this with astronomical aberration was
recognised.
An attempt was made by Stokes to reconcile mathematically a convection
of aether by the earth with the accurately veried facts of astronomical
aberration; but his theory cannot be regarded as tenable. Lodge
investigated experimentally the question whether smaller bodies carried
the aether with them in their motion, and showed that the aether between
two whirling steel discs was undisturbed.
The controversy, stagnant versus convected aether, had now reached
an intensely interesting stage. In 1895, Lorentz discussed the problem
from the point of view of the electrical theory of light and matter. By
his famous transformation of the electromagnetic equations, he cleared
up the diculties associated with the rst-order eects, showing that
they could all be reconciled with a stagnant aether. In 1900, Larmor
carried the theory as far as second-order eects, and obtained an exact
theoretical foundation for FitzGerald's hypothesis of contraction, which
had been suggested in 1892 as an explanation of the Michelson-Morley
experiment. The theory of a stagnant aether was thus reconciled with
all observational results; and henceforward it held the eld.
Further second-order experiments were performed by Rayleigh and
Brace on double refraction (1902, 1904), Trouton and Noble on a
torsional eect on a charged condenser (1903), and Trouton and
Rankine on electric conductivity (1908). All showed that the earth's
motion has no eect on the phenomena. On the theoretical side,
Lorentz (1902) showed that the indierence of the equations of the
electromagnetic eld to any velocity of the axes of reference, which
he had previously established to the rst order, and Larmor to the
second order, was exact to all orders. He was not, however, able to
establish with the same exactness a corresponding transformation for
bodies containing electrons.
Both Larmor and Lorentz had introduced a \local time" for the
moving system. It was clear that for many phenomena this local
time would replace the \real" time; but it was not suggested that
the observer in the moving system would be deceived into thinking
HISTORICAL NOTE 193
that it was the real time. Einstein, in 1905 founded the modern
principle of relativity by postulating that this local time was the
time for the moving observer; no real or absolute time existed, but
only the local times, dierent for dierent observers. He showed that
absolute simultaneity and absolute location in space are inextricably
bound together, and the denial of the latter carries with it the denial
of the former. By realising that an observer in the moving system
would measure all velocities in terms of the local space and time of
that system, Einstein removed the last discrepancies from Lorentz's
transformation.
The relation between the space and time coordinates in two systems
in relative motion was now obtained immediately from the principles of
space and time-measurement. It must hold for all phenomena provided
they do not postulate a medium which can serve as a standard for
absolute location and simultaneity. The previous deduction of these
formulae by lengthy transformation of the electromagnetic equations
now appears as a particular case; it shows that electromagnetic
phenomena have no reference to a medium with such properties.
The combination of the local spaces and times of Einstein into an
absolute space-time of four dimensions is the work of Minkowski (1908).
Chapter iii is largely based on his researches. Much progress was
made in the four-dimensional vector-analysis of the world; but the
whole problem was greatly simplied when Einstein and Grossmann
introduced for this purpose the more powerful mathematical calculus
of Riemann, Ricci, and Levi-Civita.
In 1911, Einstein put forward the Principle of Equivalence, thus turning
the subject towards gravitation for the rst time. By postulating that
not only mechanical but optical and electrical phenomena in a eld of
gravitation and in a eld produced by acceleration of the observer were
equivalent, he deduced the displacement of the spectral lines on the sun
and the displacement of a star during a total eclipse. In the latter case,
however, he predicted only the half-de
ection, since he was still working
with Newton's law of gravitation. Freundlich at once examined plates
obtained at previous eclipses, but failed to nd sucient data; he also
prepared to observe the eclipse of 1914 in Russia with this object, but was
stopped by the outbreak of war. Another attempt was made by the Lick
Observatory at the not very favourable eclipse of 1918. Only preliminary
results have been published; according to the information given, the probable
accidental error of the mean result (reduced to the sun's limb) was
about 100:6, so that no conclusion was permissible.
194 APPENDIX
The principle of equivalence opened up the possibility of a general theory
of relativity not conned to uniform motion, for it pointed a way out
of the objections which had been urged against such an extension from the
time of Newton. At rst the opening seemed a very narrow one, merely
indicating that the objections could not be considered nal until the possibilities
of complications by gravitation had been more fully exhausted.
By 1913, Einstein had surmounted the main diculties. His theory in a
complete form was published in 1915; but it was not generally accessible
in England until a year or two later. As this theory forms the main
subject-matter of the book, we may leave our historical survey at this
point.
INDEX
Absolute, approached through the
relative, 73
Absolute acceleration, 62, 143, 177
Absolute past and future, 46
Absolute rotation, 141, 151, 177
Absolute simultaneity, 11, 46
Absolute time, in cylindrical world,
150
Acceleration
a simpler quality than velocity,
178
modies FitzGerald contraction,
67
Action, 135
atomicity of, 181
on Weyl's theory, 162
Action, Principle of Least, 136, 162
Addition of velocities, 53
Aether
a plenum with geodesic structure,
151
identied with the \world", 171
non-material nature of, 34
stagnant, 191
Articial elds of force, 58
Atom, vibrating on sun, 118
Atomicity
law of, 181
of Action, 162
Aviator, space and time-reckoning of,
20{24
Bending of light
eect on star's position, 103
observational results, 108
theory of, 98, 188
Beta particles, 54, 133
Brain, constitution of, 175
Brazil, eclipse expedition to, 107
Causality, law of, 144
Causation and free will, 46
Centrifugal Force
compared with gravitation, 37, 59
debt at innity, 145
not caused by stars, 142
vibrating atom in eld of, 118
Chess, analogy of, 168
Christoel, 79
Circle in non-Euclidean space, 95
Cliord, 69, 141, 175
Cliquishness, 172
Clock
aected by velocity, 53
on sun, 66, 118
perfect, 11
recording proper-time, 64
Clock-scale, 52
Clock-scale geometry, not
fundamental, 66, 120, 174
Coincidences, 78
Comets
motion through coronal medium,
111
radiation-pressure in, 102
Conservation
of electric charge, 158
of energy and momentum, 127
of mass, 130, 179
Content contrasted with structural
form, 175, 182
Continuous matter, 82, 128
Contraction, FitzGerald, 17, 49
Convergence of physical
approximations, 143
Coordinate velocity, 98
Coordinates, 69
Corona, refraction by, 110
Cottingham, 104
Crommelin, 104, 111
196 INDEX
Curvature
degrees of, 81
identied with action, 135
merely illustrative, 76
of a globe of water, 136
of space and time, 146
on Weyl's theory, 161
perception of, 173
Cylinder and plane, indistinguishable
in two dimensions, 73
Cylindrical world, Einstein's, 149,
190
Davidson, 104
de Sitter, 123, 147, 163
De
ection of light
eect on star's position, 103
observational results, 108
theory of, 100, 189
Density, eect of motion on, 56
Displacement of spectral lines, 118
in nebulae, 148
in stars, 123
Displacement of star-images, 103,
106
Double stars and Einstein eect, 122
Duration, not inherent in external
world, 30
Eclipse, observations during, 104
Ehrenfest's paradox, 67
Electrical theory of inertia, 55
Electricity and gravitation, 153
Electromagnetic potentials and
forces, 157
Electron
dimensions of, 161
geometry inside, 82
inertia of, 56
Kaufmann's experiment on, 56,
134
singularity in eld, 153
Electron, gravitational mass of, 163
\Elsewhere", 46
Emptiness, perception of, 174
Energy
conservation of, 127
identied with mass, 134
inertia of, 55, 134
weight of radio-active, 102
Entropy, 137
Eotvos torsion-balance, 102
Equivalence
Principle of, 68, 120, 193
Euclidean geometry, 1, 43, 65
Euclidean space of ve dimensions,
75
Event, denition of, 41, 170
Evershed, 119
Extension in four dimensions, 33, 42
Feeling, elements of, 175
Field of velocity, 177
Fields of force
articial, 58
due to disturbance of observer, 62
electromagnetic, 157
relativity of, 60
FitzGerald Contraction, 17
consequences of, 19
modied by acceleration, 67
FitzGerald contraction
relativity explanation of, 52
Flat space in two dimensions, 72
Flat space-time, 74
at innity, 75
conditions for, 79
Flatsh, analogy of, 87
Flatland, 52
Force
compared with inertia, 126
electromagnetic, 157
elementary conception of, 57
elds of, 58
relativity of, 38, 60, 68
Form contrasted with content, 175,
182
INDEX 197
Formalism of knowledge, 160
Foucault's pendulum, 141
Four-dimensional order, 32, 51, 170
Four-dimensional space-time
geometry of, 41, 74
reality of, 166
Fourth dimension, 12
Frame, inertial, 145
Frames of reference, \right" and
\wrong", 37
Free will, 46
Freundlich, 193
Future, absolute, 46
Galilean potentials, 74
Gauge
eect on observations, 30
provided by radius of space, 161
Gauge-system, 155
Geodesic
absolute signicance of, 63, 138
denition of, 68
in regions at innity, 145
motion of particles in, 127, 138
Geodesic structure
absolute character of, 144, 151
acceleration of, 178
Geometrical conception of the world,
160, 167
Geometry
Euclidean, 1
hyperbolic, 43
Lobatchewskian, 1, 8
natural, 2
non-Euclidean, or Riemannian, 6,
65, 76, 80
non-Riemannian, 155
semi-Euclidean, 43
Ghosts of stars, 149
Globe of water, limit to size of, 136
Gravitation
propagation with velocity of light,
86, 135
relativity for uniform motion, 19,
115
Gravitation, Einstein's law of
dierential formula, 81
integrated formula for a particle,
88
macroscopic equations, 129, 176
Gravitation, Newton's law of
ambiguity of, 85
approximation to Einstein's law,
95
de
ection of light, 100, 103
Gravitational eld of Sun, 90
de
ection of light, 100, 108, 189
displacement of spectral lines, 118
motion of perihelion, 114, 187
Newtonian attraction, 93
result of observational
verication, 116
Grebe and Bachem, 119
Greenwich, Royal Observatory, 104
Gyrocompass, 141
Hummock in space-time, 89
Hurdles, analogy of counts of, 96
Hyperbolic geometry, 43
Identities connecting G, 129
Identity, permanent, 35, 176
Imaginary intervals, 138, 171
Imaginary time, 43, 166
Inertia
compared with force, 126
electrical theory of, 55
in regions at innity, 146
innite, 51
Mach's views, 151
of light, 101
relativity theory of, 128
Inertia-gravitation, 126
Inertial frame, 145
Innity, conditions at, 145
Integrability of length and direction,
159
198 INDEX
Interval, 42, 137, 171
general expression for, 74
practical measurement of, 52, 67
Interval-length
geometrical signicance essential,
116
identied with proper time, 64
tracks of maximum, 63, 138
zero for velocity of light, 64
Invariant mass, 133
of light, 136
Jupiter, de
ection of light by, 122
Kaufmann's experiment, 56, 134
Kinds of space, 73
Laplace's equation, 88, 128
Larmor, 17, 192
Le Verrier, 114
Length
denition of, 2
eect of motion on, 17
relativity of, 30
Levi-Civita, 79
Lift, accelerated, 58
Light
coordinate velocity of, 98
mass of, 56, 101, 136
voyage round the world, 149
weight of, 102
Light, bending of, 98, 103, 108, 188
Light, velocity of
an absolute velocity, 54
importance of, 54
system moving with, 23, 51
Lobatchewsky, 1, 8
Lodge, 29, 115, 192
Longest tracks, 63
Lorentz, 17, 192
Mach's philosophy, 151
Macroscopic
equations, 82, 128
interval, 171
Map of sun's gravitational eld, 91
Mass
conservation of, 130, 178
electrical theory of, 55
gravitational, 90
identied with energy, 134
invariant, 133
variation with velocity, 133
Mass of light, 56, 101, 136
Mathematics, Russell's description
of, 179
Matter
continuous, 82
denition of a particle, 89
extensional relations of, 7
gravitational equations in, 129
perception of, 174
physical and psychological
aspects, 175
Mercury, perihelion of, 113, 115
Mesh-systems, 70
irrelevance to laws of nature, 78
Michelson-Morley experiment, 16
Minkowski, 27, 193
Mirror, distortion by moving, 20
Momentum
conservation of, 130
of light, 102
redenition of, 132
Moon, motion of, 85, 123
Motion
insuciency of kinematical
conception, 176
Newton's rst law, 125
Natural frame, 144
Natural gauge, 161
Natural geometry, 2
Natural tracks, 63
INDEX 199
Nebulae, atomic vibrations in, 148
Newton
absolute rotation, 37
bending of light, 101
law of gravitation, 85
law of motion, 125
relativity for uniform motion, 36
super-observer, 61
Non-Euclidean geometry, 6, 65, 76,
80
Non-Riemannian geometry, 155
Observer and observed, 27
Observer, an unsymmetrical object,
52
Orbits under Einstein's law, 113
Order and dimensions, 13, 170
Ordering of events in external world,
31, 49, 168
Past, absolute, 46
Perceptions, as crude measures, 9,
13, 28
Perihelia of planets, motions of, 113
Permanence of matter, 179
Permanent identity, 35, 176
Permanent perceptual world, 130,
180
Poincare, 8
Point-event, 41, 170
Potentials, 72
Galilean values, 74
Potentials, electromagnetic, 157
Principe, eclipse expedition to, 104
Principle of Equivalence, 68, 120, 193
Principle of Least Action, 136, 162
Principle of Relativity (restricted),
18
Probability, a pure number, 162
Projectile, Jules Verne's, 58
Propagation of Gravitation, 86, 135
Proper-length, 10
Proper-time, 64
Pucker in space-time, 76
Quanta, 55, 162, 167, 182
Radiation-pressure, 101
Real world of physics, 33, 156
Receding velocities
of spiral nebulae, 148
Receding velocities
of B-type stars, 123
Re
ection by moving mirror, 20
Refracting medium equivalent to
gravitational eld, 99
Refraction of light in corona, 110
Relativity
of length and duration, 30
of motion, 34
restricted Principle of, 18
Relativity of Force, 38, 68
Relativity of rotation, 141, 144
Relativity of size, 29
Relativity, Newtonian, 36
Relativity, standpoint of, 24
Repulsion of light proceeding
radially, 94, 99
Retardation of time, 21, 50
in centrifugal eld, 118
in spherical world, 147
Ricci, 79
Riemann, 2, 79, 153
Riemann-Christoel tensor, 80
Riemannian, or non-Euclidean,
geometry, 6, 65, 76, 80
Rigid scale, denition of, 3
Rotation of a continuous ring, 176
Rotation, absolute, 141, 151, 177
Russell, 12, 179
Semi-Euclidean geometry, 65
Simultaneity, 10, 46
Sobral, eclipse expedition to, 107
Space
conventional, 8
kinds of, 73
meaning of, 3, 7, 13
relativity of, 30
200 INDEX
Space-like intervals, 54, 171
Space-time, 41
due to Minkowski, 193
partitions of, 49
Spherical space-time, 147
St John, 119
Standard metre, comparison with,
154
Stresses in continuous matter, 176
Structure opposed to content, 179,
182
Structure, geodesic
absolute character of, 144, 151
acceleration of, 178
behaviour at innity, 145
Super-observer, Newton's, 61
Synthesis of appearances, 28, 166
Tensors, 80, 172
Thomson, J. J., 55
Time
\standing still", 23, 170
absolute, 150
depends on observer's track, 34,
52
for moving observer, 21
imaginary, 43
measurement of, 11
past and future, 46
Time-like intervals, 54, 171
Tracks, natural, 63
Vacuum, dened by law of
gravitation, 174
Vector, non-integrable on Weyl's
theory, 159
Velocity
static character, 177
Velocity of gravitation, 86, 135
Velocity of light
importance of, 54
in gravitational eld, 99
system moving with, 23, 51
Velocity, addition-law, 53
Velocity, denition of, 176
Warping of space, 7, 116
Wave-front, slewing of, 99
Weight
of light, 98, 102
of radio-active energy, 102
proportional to inertia, 126
vanishes inside free projectile, 59
Weyl, 159
World, 170, 171
World-line, 78
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