D O C U M E N T 4 0 0 N O V E M B E R 1 9 2 6 6 1 9 While therefore I can not regard classical radiation as reversible in the strict sense in which I am using the word, I can not so easily dispose of your argument regarding “negative Einstrahlung.” While I do not believe that there is any energy in the undulating field of radiation, the existence of such a field can not be ignored, for it is by means of this field that we can predict all of the phenomena of interfer- ence. Yet I think of the field as existing independently of the transfer of light quanta, and indeed at times when there is no emission. The exchange of energy through light quanta may be regarded as coming within the scope of mechanics the field is non-mechanical. When we have set up the equations for the field we are able to predict the probability that a certain quantum exchange will occur, and these equations are something like the actuarial tables used by a life insurance company. No one would say that such a table causes the death of a man, but from the table we can calculate how many deaths will occur in a year in a given city. But even admitting that the radiation field is not the carrier of energy, but only serves to specify the probability of quantum processes (this probability being entire- ly analogous to the intensity of radiation in the classical theory), there would still be a phenomenon relating to the probabilities quite analogous to the “Einstrahlung” and “Ausstrahlung” of energy, and it might be argued that these have to be taken into consideration just as you have done. I have the feeling, however, that this will not prove to be the case. I formerly thought of the process by which an excited atom gives up its energy by radiation to some other atom, which in turn becomes excited, as entirely independent of all other similar processes, so that the probability of this transfer (with a given optical arrangement) would be determined by the condition of these atoms alone. Or, in other words, that the only fields that we have to take into account are the fields of the emitting and absorbing atom. However, this assumption proves to be not quite tenable. It leads directly to the Wien distribution law, the actual departure from which may therefore be regarded as due to the mutual influence of several radiation processes. I believe, however, that it is possible to find conditions of equilibrium, such as those which you, in your 1912 paper, call “aussergewöhnlich,”[1] where at a given temperature the concentration of radiant energy may be diminished without limit, and as the concentration diminishes the distribution should come nearer and nearer to the Wien distribution law. I have given the briefest outline of this idea in a paper which I have just sent to “Nature” and a copy of which I enclose herewith.[2] It is pretty extreme quantum theory, but I think you may find it amusing, and the extended thermodynamics which I discuss there might be built up quite without regard to the “photon” hypothesis. I am also sending you separately a copy of my Silliman Lectures,[3] which deals with many subjects in which you have been interested, and one chapter contains my latest views regarding the problem of entropy and probability.