Conservation of Momentum and the Doppler Principle

SINCE on the relativity theory mass and energy are related by the equation Mc 2 = W , the different energy states of an atom may be thought of as being different mass states of the atom. For example, on the quantum theory when an atom changes from an energy state W p , to an energy state W q , a quantum hv is radiated such that hv = W p - W q , ( W p being supposed greater than W q ), and hence hv may be supposed to be produced by a loss of mass in the atom as a whole. We shall therefore consider an atom to have a mass M p before the emission of a quantum hv and a mass M q after the emission. Each of these masses consists of the rest masses of the various parts of the atom together with the mass due to the kinetic and potential energies of the various parts. Further, although parts of the atom may be moving relatively to each other, we shall suppose that M p and M q are the rest masses of the atom as a whole in the two states.