On the Dynamics of Moving Systems.
By Max Planck.
Introduction.
Recent researches in the field of thermal radiation lead to the conclusion (from the experimental as well as from the theoretical side), that a system devoid of ponderable matter and consisting only of electromagnetic radiation, obeys the basic laws of mechanics as well as the two laws of thermodynamics in such a complete way, that for all consequences drawn from it one has nothing left to wish for. Thus it has become necessary, that a number of ideas and laws which are usually considered as fixed and self evident conditions of all theoretical speculations in that field, are subject to a principle revision, and further consideration shows that some of the simplest and most important of them can in future only be characterized as approximations (though far reaching and practically very important), but in no way can be considered as exactly valid. Some examples will substantiate this in detail.
We are accustomed to regard the whole energy of a moving ponderable body as additively composed of a part, which varies only (regardless of the internal state of the body) with its speed: the energy of the kinetic motion, and a second part which (regardless of the speed) only depends on the internal state, namely on the density, temperature and chemical composition: the internal energy of the body. This decomposition is from now on, even principally, not allowed in any single case. For every ponderable body contains in its interior a specifiable finite amount of energy in the form of radiant heat, and if the body is imparted a certain velocity, then the heat radiation is also set into motion. As regards moving
