7 6 D O C U M E N T 7 0 O C T O B E R 1 9 2 7 This correspondence of quantized waves on the one hand and particles on the other can, by the way, be obtained also in the case of arbitrary interactions between the particles, as was shown in an article by O. Klein and me.[11] The “wave equa- tion” of the quantized waves (not to be confused with the four-dimensional Schrödinger wave equation) then becomes nonlinear. I hope that I will soon be able to send you the corrections to the articles by Klein and me. In the meantime, I remain with kind regards, respectfully yours, P. Jordan 70. From Paul Ehrenfest Leiden, 12 October 1927 Dear Einstein, Don’t suppose that I think about you as seldom as I write to you. On the contrary. But I always hesitate to burden you with letters when you must receive so many of them. However, today I simply MUST. Lorentz[1] would like a collection, as complete as possible, of the most important articles on quanta to be already available during the discussions in Brussels. There- fore I ask you, like all others, to send your recent articles (and especially the ones on quantum statistics) NOW to: Mr. Lefébure (Ch. Lefébure),[2] Institut internat. de physique Solvay, 43 Rue des Champs Élysées, Brussels My wife writes me that you have so many grave worries.[3] That was another reason not to burden you with scribblings. Dear Einstein! All I can tell you is this: there is a group of people in the world who are deeply fond of you, apart from all the physics braininess (it might also have been your brains in physics that FIRST attracted them to you). My sincerest good wishes, and may things soon look up for you and yours! Your P. Ehrenfest Would[4] it be possible to recommend Langevin-Weiss for the Nobel Prize?[5] The magnetons have become important.— [Naturally, I’m also thinking about Compton, Paschen, Stern, Sommerfeld[6] I’m sure you’ve heard that Einthoven died.[7]