Enrico Fermi

“ It's known that simple electrodynamic considerations [1] lead to the value for the electromagnetic mass of a spherical electricity-distribution of electrostatic energy , when denotes the speed of light. On the other hand, it's known that relativistic considerations for the mass of a system containing the energy give the value . Thus we stand before a contradiction between the two views, whose solution seems not unimportant to me, especially with respect to the great importance of the electromagnetic mass for general physics, as the foundation of the electron theory of matter. ”

“ However, if we apply the variation system , the integration field contracts to due to the same reasons.
Now it can be immediately seen, that variation is in contradiction with relativity theory, because it has no invariant characteristics against the world-transformation, and is based on the arbitrary space . On the other hand, variation has the desired invariant characteristics, and is always based on the proper-space, i.e., the space perpendicular to the world-tube, thus it is without doubt to be preferred before the previous one. ”

“ In this paper, Hamilton's principle will serve as a basis, being most useful for the treatment of a problem subjected to very complicated conditions - conditions of a different nature than those considered in ordinary mechanics, because our system must contract in the direction of motion according to relativity theory. ”