On the Inertia of Energy Required by the Relativity Principle, A. Einstein, Annalen der Physik 23 (1907): 371-384
The principle of relativity, in combination with Maxwell's equations, leads to the conclusion that the inertia of a body increases or decreases with its energy content in a completely determined way. That is to say, if one observers a body that emits a certain radiation simultaneously in two opposite directions, and if one examines this process from two coordinate systems which move relative to each other, one of which is at rest relative to the body, and if one applies--from both coordinate systems--the energy principle to the process, one arrives at the result that to an increase in the body's energy DE there must always correspond an increase in the mass DE = V2 where V denotes the velocity of light.
The circumstance that the special case discussed there necessitates an assumption of such extraordinary generality (about the dependence of the inertia on the energy) demands that the necessity and justification of this assumption be examined in a more general way. Especially, the question the question arises: Do not other special cases lead to conclusions that are incompatible with the one mentioned above? A first step in this respect I took last year by showing that the above assumption resolves the contradiction between electrodynamics and the principle of the constancy of the motion of the center of gravity (at least as far as the terms of first order are concerned).
The general answer to the question posed is not yet possible because we do not yet have a complete world view that would correspond to the principle of relativity. Rather, we must limit ourselves to the special cases we can handle at present without arbitrariness from the standpoint of relativistic electrodynamics. We are going to consider two such cases; in the first of these, the system whose inertial mass we shall examine consists of a rigid, electrified body, and in the second case it consists of a number of uniformly moving mass points which do not exert any forces on each other.