energy E[0], then its inertial mass increases by an amount
eq. 22: file eq22.gif
the inertial mass of a body is not a constant but varies according to the change in
the energy of the body. The inertial mass of a system of bodies can even be
regarded as a measure of its energy. The law of the conservation of the mass of a
system becomes identical with the law of the conservation of energy, and is only
valid provided that the system neither takes up nor sends out energy. Writing the
expression for the energy in the form
eq. 23: file eq23.gif
we see that the term mc^2, which has hitherto attracted our attention, is nothing
else than the energy possessed by the body ** before it absorbed the energy
E[0].
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.
eq. 22: file eq22.gif
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.
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