Friedrich Hasenöhrl

“ If a radiating surface moves with uniform velocity in the same direction as the radiation emitted by it, then work must be constantly applied to overcome the pressure exerted by the latter. Now, this work is transformed into radiation energy as well, so that more energy of that amount is emanated from a surface moving in this sense, than from a stationary one. However, if the surface is moving in the opposite direction of the emanated radiation, then the latter steadily performs work, thus less energy of that amount is emanated from the moving surface than from a stationary one. ”

“ In the same way, as the electromagnetic mass is proportional to the static energy of the stationary electron, also the apparent mass caused by radiation is proportional to the energy content of the stationary cavity. Namely, the proportionality factor is in both cases of the same order of magnitude. Now, since the heat content of every body partly consists of radiation energy, then every body must possess such an apparent mass depending on the temperature, and which is added to the mass in the ordinary sense. ”

“ The second summand, however, represents the radiation energy gained from mechanical work. Since no work is performed at uniform motion as we have seen, this must be a work which must be performed when our system is accelerated.
One can easily determine the amount of this energy quantity in the following way. Let us imagine a system in absolute rest, and the radiation of both black surfaces and is somehow hindered: space is thus totally free of radiation. Now, in one instant the radiation from and shall be left free, and the system shall simultaneously brought to velocity . ”