Max von Laue

“ In general, however, the exerted radiation of a resting body has a resulting momentum and accordingly exercises a motion drive, as is can be most easily seen by the example of a cavity (of perfectly reflecting substance) filled with radiation, having a small opening only at a single point. But if the body is isotropic, homogeneous and limited by a convex surface everywhere, it suffers a normal pressure from the radiation which is everywhere the same, not resulting in a net force. If one such body is at rest in the primed system, then it has a constant speed in relation to the unprimed system. ”

“ The relativity theory was already used by Lorentz [3] for calculating the field of an oscillating dipole, but without specifying the radiated energy and momentum, as well as the back-reaction of radiation on the dipole. Now, both theories agree as regards the electromagnetic equations, as well as in the expression for the ponderomotive force. The difference lies solely in the form which they attribute to the moving charges; because one theory assumes that they are not affected by motion, and in the other theory they contract in the direction of velocity. ”

“ The older theory needs a force whose work covers a portion of the energy radiation to maintain uniform motion. According to the theory of relativity, however, it completely stems from the energy of the light source.
The given values for the energy and momentum radiation are valid, since they depend only on the acceleration of the moving electron, even if it doesn't oscillate together with a dipole, but is moving in any other way. ”