[12] As judged from a co-ordinate system moving with the body.
A direct comparison of this relation with experiment is not possible at
the present time (1920; see[Note], p. 48), owing to the fact that the
changes in energy _E_0 to which we can subject a system are not large
enough to make themselves perceptible as a change in the inertial mass
of the system.
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is too small in comparison with the mass _m_, which was present before
the alteration of the energy. It is owing to this circumstance that
classical mechanics was able to establish successfully the conservation
of mass as a law of independent validity.
[Note] The equation E = mc2 has been thoroughly proved time and again
since this time.
Let me add a final remark of a fundamental nature. The success of the
Faraday-Maxwell interpretation of electromagnetic action at a distance
resulted in physicists becoming convinced that there are no such things
as instantaneous actions at a distance (not involving an intermediary
medium) of the type of Newton s law of gravitation.
According to the theory of relativity, action at a distance with the
velocity of light always takes the place of instantaneous action at a
distance or of action at a distance with an infinite velocity of
transmission. This is connected with the fact that the velocity _c_
plays a fundamental role in this theory. In Part II we shall see in
what way this result becomes modified in the general theory of
relativity.
XVI.
EXPERIENCE AND THE SPECIAL THEORY OF RELATIVITY
36