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Einstein

Relativity: The Special and General Theory
Albert Einstein: Relativity
Part I: The Special Theory of Relativity
Experience and the Special Theory of Relativity
To what extent is the special theory of relativity supported by experience ? This question is not
easily answered for the reason already mentioned in connection with the fundamental experiment
of Fizeau. The special theory of relativity has crystallised out from the Maxwell−Lorentz theory of
electromagnetic phenomena. Thus all facts of experience which support the electromagnetic theory
also support the theory of relativity. As being of particular importance, I mention here the fact that
the theory of relativity enables us to predict the effects produced on the light reaching us from the
fixed stars. These results are obtained in an exceedingly simple manner, and the effects indicated,
which are due to the relative motion of the earth with reference to those fixed stars are found to be
in accord with experience. We refer to the yearly movement of the apparent position of the fixed
stars resulting from the motion of the earth round the sun (aberration), and to the influence of the
radial components of the relative motions of the fixed stars with respect to the earth on the colour of
the light reaching us from them. The latter effect manifests itself in a slight displacement of the
spectral lines of the light transmitted to us from a fixed star, as compared with the position of the
same spectral lines when they are produced by a terrestrial source of light (Doppler principle). The
experimental arguments in favour of the Maxwell−Lorentz theory, which are at the same time
arguments in favour of the theory of relativity, are too numerous to be set forth here. In reality they
limit the theoretical possibilities to such an extent, that no other theory than that of Maxwell and
Lorentz has been able to hold its own when tested by experience.
But there are two classes of experimental facts hitherto obtained which can be represented in the
Maxwell−Lorentz theory only by the introduction of an auxiliary hypothesis, which in itself —
i.e. without making use of the theory of relativity — appears extraneous.
It is known that cathode rays and the so−called ²−rays emitted by radioactive substances consist of
negatively electrified particles (electrons) of very small inertia and large velocity. By examining the
deflection of these rays under the influence of electric and magnetic fields, we can study the law of
motion of these particles very exactly.
In the theoretical treatment of these electrons, we are faced with the difficulty that electrodynamic
theory of itself is unable to give an account of their nature. For since electrical masses of one sign
repel each other, the negative electrical masses constituting the electron would necessarily be
scattered under the influence of their mutual repulsions, unless there are forces of another kind
operating between them, the nature of which has hitherto remained obscure to us.1) If we now
assume that the relative distances between the electrical masses constituting the electron remain
unchanged during the motion of the electron (rigid connection in the sense of classical mechanics),
we arrive at a law of motion of the electron which does not agree with experience. Guided by purely
formal points of view, H. A. Lorentz was the first to introduce the hypothesis that the form of the
electron experiences a contraction in the direction of motion in consequence of that motion. the
contracted length being proportional to the expression
This, hypothesis, which is not justifiable by any electrodynamical facts, supplies us then with that
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