1 3 4 D O C U M E N T 1 1 2 N O V E M B E R 1 9 2 5 It would certainly be very nice if I could sit with you again by the fireside and chat, but I’m afraid your lectures will not permit you an excursion to Kiel during the semester!? Best wishes for the New Year, and I wish for myself that it may bring you to visit me soon. Sincerely yours, Anschütz 112. From Werner Heisenberg Göttingen, 16 November 1925 Dear Professor, Your very friendly letter to Jordan delighted me [1] and since I feel somewhat re- sponsible for the trouble caused by this new theory, I would like to reply to you about the objections in your letter. First, as regards the final point, half-integral quantization of the HCl bands, this quantization already appears at the end of my paper[2] (a reprint of which I enclose herewith I am very embarrassed that I haven’t ¢done² sent it already). ¢However² I entirely agree with your objection to the zero-point oscillations. But I also simply believe that the exposition in our manuscript up to now is entirely wrong (in the “final” version, the proofs of which you will probably be receiving in about two weeks, we wrote this part differently as well.)[3] The hν-term in the fluctuation for- mula definitely does not result from interferences with the zero-point radiation the reason for the occurrence of the zero-point energy was, of course, the vanishing of the zero-point radiation (comp., e.g., p. 886 in the encl. paper).[4] Hence, it evidently cannot be a question of interferences with zero-point radiation, because that particular radiation is not present. Rather, the reason for both effects (for which surely hardly any empirical doubt exists, either) seems to me to be the altered ge- ometry that is the basis of the attempted new theory.[5] Nevertheless, I don’t think that Nernst’s “sulphuric ether” must follow with absolute certainty from this theory.[6] That’s because the very occurrence of zero-point energy (comp., e.g., (23) of the encl. pap.) is initially purely formal. In any event, its physical meaning has not yet been completely clarified, and one can at least hope that the terrible consequences you described so vividly can be evaded. I also fully agree with your other objection, namely, that in a final theory a closer assignment of terms to radiation quantities would have to be carried out than has been attempted in our theory with the “diversity of all transitions.”[7] But there cer- tainly still is room in the formalism of this theory for such an assignment and if one reflects about radiation and coupling phenomena, there seem to be ways avail-