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
we see that the term mc2, which has hitherto attracted our attention, is nothing else than the energy possessed by the body[12] before it absorbed the energy E0.
[ [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 E0 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.
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.