Let us go back to less general and more precise problems, for example to the theory of radiation. Mr. Planck has devised an amendment to his first theory, and I would like to say a few words about it. According to his new ideas, the emission of light would be made suddenly by quanta, but the absorption would be continuous. He wished to escape the following difficulty, which, I know not why, seemed more embarrassing with regard to absorption. The light arrives on each resonator in a continuous way; if it can only be absorbed quantum by quantum, the energy must accumulate in a kind of antechamber of the resonator, until there is enough to enter. In the second theory, this difficulty disappears, but there is always a waiting room for the energy that comes out, since the ether can transmit it only in infinitely small fractions.
In the new theory, the resonators will retain a residual energy even at absolute zero. If we adopt M. Planck’s new way of seeing, we must then modify the relation between the energy of the radiant body and the intensity of its radiance. This radiation is no longer proportional to the energy, but only to the excess of this energy over the residual which remains at absolute zero.
Will I admit that I was not entirely satisfied with this new hypothesis? Mr. Planck speaks only of emission and absorption, and speaks of it as if the resonator were fixed; there is no question of energy exchange by collision or the Döppler-Fizeau principle; under these conditions, there can therefore be no tendency towards a final state, as I said above; the demonstration by which we seek to make known this final state is therefore only an optical illusion The author does not say whether collision exchanges are continuous like absorption, or discontinuous like emission, and when one wants to apply the general theory of exchanges by collisions, one does not find again the results of Mr. Planck. It is therefore appropriate to stick to his first ideas.