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.[13] 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
image029
This, hypothesis, which is not justifiable by any electrodynamical
facts, supplies us then with that particular law of motion which has
been confirmed with great precision in recent years.
[13] The general theory of relativity renders it likely that the
electrical masses of an electron are held together by gravitational
forces.
The theory of relativity leads to the same law of motion, without
requiring any special hypothesis whatsoever as to the structure and the
behaviour of the electron. We arrived at a similar conclusion in
Section XIII in connection with the experiment of Fizeau, the result of
which is foretold by the theory of relativity without the necessity of
drawing on hypotheses as to the physical nature of the liquid.
The second class of facts to which we have alluded has reference to the
question whether or not the motion of the earth in space can be made
perceptible in terrestrial experiments. We have already remarked in
Section V that all attempts of this nature led to a negative result.
Before the theory of relativity was put forward, it was difficult to
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