ABSTRACT
The Schro¨dinger equation for a material system (atom or molecule) gives for the energy of the system a set of levels corresponding to stationary states. To be specific, only the ground level remains strictly stationary; for a system in an excited state, a probability to pass to the ground state appears, and such a transition is accompanied by radiation. So the concept of a purely mechanical system independent of radiation is approximate. We get a better approximation to reality if we consider an atom or a molecule together with radiation, i.e., in combination with the light quanta, of which the radiation is composed. While the energy of a mechanical system itself can be carried out by radiation or increased due to the absorption of light quanta, the energy of the total system composed of matter and light will remain unchanged, so this system is conservative.
