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Unformatted text preview: A Universal Download Edition What is the Theory of Relativity?
The London Times, November 28, 1919 I GLADLY accede to the request of your colleague to write something for The Times on
relativity. After the lamentable breakdown of the old active intercourse between men of learning,
I welcome this opportunity of expressing my feelings of joy and gratitude toward the
astronomers and physicists of England. It is thoroughly in keeping with the great and proud
traditions of scientific work in your country that eminent scientists should have spent much time
and trouble, and your scientific institutions have spared no expense, to test the implications of a
theory which was perfected and published during the war in the land of your enemies. Even
though the investigation of the influence of the gravitational field of the sun on light rays is a
purely objective matter, I cannot forbear to express my personal thanks to my English colleagues
for their work; for without it I could hardly have lived to see the most important implication of
my theory tested.
We can distinguish various kinds of theories in physics. Most of them are constructive. They
attempt to build up a picture of the more complex phenomena out of the materials of a relatively
simple formal scheme from which they start out. Thus the kinetic theory of gases seeks to reduce
mechanical, thermal, and diffusional processes to movements of molecules -- i.e., to build them
up out of the hypothesis of molecular motion. When we say that we have succeeded in
understanding a group of natural processes, we invariably mean that a constructive theory has
been found which covers the processes in question.
Along with this most important class of theories there exists a second, which I will call
"principle-theories." These employ the analytic, not the synthetic, method. The elements which
form their basis and starting-point are not hypothetically constructed but empirically diseovered
ones, general characteristics of natural processes, principles that give rise to mathematically
formulated criteria which the separate processes or the theoretical representations of them have
to satisfy. Thus the science of thermodynamics seeks by analytical means to deduce necessary
conditions, which separate events have to satisfy, from the universally experienced fact that
perpetual motion is impossible.
The advantages of the constructive theory are completeness, adaptability, and clearness, those of
the principle theory are logical perfection and security of the foundations.
The theory of relativity belongs to the latter class. In order to grasp its nature, one needs first of
all to become acquainted with the principles on which it is based. Before I go into these,
however, I must observe that the theory of relativity resembles a building consisting of two WWW.UDownloadBooks.Com 1 A Universal Download Edition
separate stories, the special theory and the general theory. The special theory, on which the
general theory rests, applies to all physical phenomena with the exception of gravitation; the
general theory provides the law of gravitation and its relations to the other forces of nature.
It has, of course, been known since the days of the ancient Greeks that in order to describe the
movement of a body, a second body is needed to which the movement of the first is referred. The
movement of a vehicle is considered in reference to the earth's surface, that of a planet to the
totality of the visible fixed stars. In physics the body to which events are spatially referred is
called the coordinate system. The laws of the mechanics of Galileo and Newton, for instance, can
only be formulated with the aid of a coordinate system.
The state of motion of the coordinate system may not, however, be arbitrarily chosen, if the laws
of mechanics are to be valid (it must be free from rotation and acceleration). A coordinate system
which is admitted in mechanics is called an "inertial system." The state of motion of an inertial
system is according to mechanics not one that is determined uniquely by nature. On the contrary,
the following definition holds good: a coordinate system that is moved uniformly and in a
straight line relative to an inertial system is likewise an inertial system.
By the "special principle of relativity" is meant the generalization of this definition to include
any natural event whatever: thus, every universal law of nature which is valid in relation to a
coordinate system C, must also be valid, as it stands, in relation to a coordinate system C', which
is in uniform translatory motion relatively to C.
The second principle, on which the special theory of relativity rests, is the "principle of the
constant velocity of light in vacuo." This principle asserts that light in vacuo always has a
definite velocity of propagation (independent of the state of motion of thc observer or of the
source of the light). The confidence which physicists place in this principle springs from the
successes achieved by the electrodynamics of Maxwell and Lorentz.
Both the above-mentioned principles are powerfully supported by experience, but appear not to
be logically reconcilable. The special theory of relativity finally succeeded in reconciling them
logically by a modification of kinematics -- i.e., of the doctrine of the laws relating to space and
time (from the point of view of physics). It became clear that to speak of the simultaneity of two
events had no meaning except in relation to a given coordinate system, and that the shape of
measuring devices and the speed at which clocks move depend on their state of motion with
respect to the coordinate system.
But the old physics, including the laws of motion of Galileo and Newton, did not fit in with the
suggested relativist kinematics. From the latter, general mathematical conditions issued, to which
natural laws had to conform, if the above-mentioned two principles were really to apply. To
these, physics had to be adapted. In particular, scientists arrived at a new law of motion for
(rapidly moving) mass points, which was admirably confirmed in the case of electrically charged
particles. The most important upshot of the special theory of relativity concerned the inert
masses of corporeal systems. It turned out that the inertia of a system necessarily depends on its
energy-content, and this led straight to the notion that inert mass is simply latent energy. The WWW.UDownloadBooks.Com 2 A Universal Download Edition
principle of the conservation of mass lost its independence and became fused with that of the
conservation of energy.
The special theory of relativity, which was simply a systematic development of the
electrodynamics of Maxwell and Lorentz, pointed beyond itself, however. Should the
independence of physical laws of the state of motion of the coordinate system be restricted to the
uniform translatory motion of coordinate systems in respect to each other? What has nature to do
with our coordinate systems and their state of motion? If it is nccessary for the purpose of
describing nature, to make use of a coordinate system arbitrarily introduced by us, then the
choice of its state of motion ought to be subject to no restriction; the laws ought to be entirely
independent of this choice (general principle of relativity).
The establishment of this general principle of relativity is made easier by a fact of experience
that has long been known, namely, that the weight and the inertia of a body are controlled by the
same constant (equality of inertial and gravitational mass). Imagine a coordinate system which is
rotating uniformly with respect to an inertial system in the Newtonian manner.
The centrifugal forces which manifest themselves in relation to this system must, according to
Newton's teaching, be regarded as effects of inertia. But these centrifugal forces are, exactly like
the forces of gravity, proportional to the masses of the bodies.
Ought it not to be possible in this case to regard the coordinate system as stationary and the
centrifugal forces as gravitational forces? This seems the obvious view, but classical mechanics
This hasty consideration suggests that a general theory of relativity must supply the laws of
gravitation, and the consistent following up of the idea has justified our hopes.
But the path was thornier than one might suppose, because it demanded the abandonment of
Euclidean geometry. This is to say, the laws according to which solid bodies may be arranged in
space do not completely accord with the spatial laws attributed to bodies by Euclidean geometry.
This is what we mean when we talk of the "curvature of space." The fundamental concepts of the
"straight line," the "plane," etc., thereby lose their precise significance in physics.
In the general theory of relativity the doctrine of space and time, or kinematics, no longer figures
as a fundamental independent of the rest of physics. The geometrical behaviour of bodies and the
motion of clocks rather depend on gravitational fields, which in their turn are produced by
The new theory of gravitation diverges considerably, as regards principles, from Newton's
theory. But its practical results agree so nearly with those of Newton's theory that it is difficult to
find criteria for distinguishing them which are accessible to experience. Such have been
discovered so far:
1. In the revolution of the ellipses of the planetary orbits round the sun (confirmed in the
case of Mercury). WWW.UDownloadBooks.Com 3 A Universal Download Edition
2. In the curving of light rays by the action of gravitational fields (confirmed by the English
photography of eclipses).
3. In a displacement of the spectral lines toward the red end of the spectrum in the case of
light transmitted to us from stars of considerable magnitude (unconfirmed so far). *
The chief attraction of the theory lies in its logical completeness. If a single one of the
conclusions drawn from it proves wrong, it must be given up; to modify it without destroying the
whole structure seems to be impossible.
Let no one suppose, however, that the mighty work of Newton can really be superseded by this
or any other theory. His great and lucid ideas will retain their unique significance for all time as
the foundation of our whole modern conceptual structure in the sphere of natural philosophy.
Note: Some of the statements in your paper concerning my life and person owe their origin to the
lively imagination of the writer. Here is yet another application of the principle of relativity for
the delectation of the reader: today I am described in Germany as a "German savant," and in
England as a "Swiss Jew." Should it ever be my fate to be represented as a bête noire, I should,
on the contrary, become a "Swiss Jew" for the Germans and a "German savant" for the English.
footnote: * this criterion has since been confirmed WWW.UDownloadBooks.Com 4 ...
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This note was uploaded on 09/14/2009 for the course PHYSICS 101 taught by Professor Jamesbrown during the Spring '09 term at Washington State University .
- Spring '09
- Theory Of Relativity