D O C U M E N T S 3 8 9 , 3 9 0 O C T O B E R 1 9 2 6 3 7 7 389. To Emil Rupp [Berlin,] 21 October 1926 Dear Mr. Rupp, I am submitting your work today.[1] Assuming that you wouldn’t mind, I took the liberty of making a few corrections to your manuscript regarding the theoretical interpretation. The experiments don’t show a finite “time of decay” but a finite “time of creation of the elementary interference field.”[2] Nowadays it is certain that the wave aspect and the energy aspect have to be clearly separated, wherein only the latter shows instantaneous character. For the sake of clarity, I deemed these corrections as absolutely necessary. If you do not agree with them, we have to dis- cuss this matter at the proof stage. Kind regards, your A. Einstein Translators’ note: Based on a translation by Doris Lonk and Tilman Sauer. 390. From Wilhelm Lenz[1] Aarau, Pension Aders, 21 October 1926 Dear Professor, At my request, Mr. Pauli has recently, during the scientists’ convention and later again in Holland,[2] spoken to you about a few difficulties that had arisen in the analysis of the equilibrium between radiation and matter in your closed world. [The note was prematurely sent to press against my wishes and has, in the interim, prob- ably already appeared in the Physikalische Zeitschrift.][3] I am very grateful for your considerations, which have been communicated to me by Mr. Pauli by letter. Meanwhile, I have encountered other problems whose solution would presumably significantly simplify the entire equilibrium calculation procedure. You had omit- ted the elastic stresses, etc. in setting up the tensor in eq. (102) in Four Lectures 1922.[4] If my equilibrium condition is right, then the tensor of the Maxw[ell] stresses have the same numerical value as the tensor of a smeared pon- derable mass taken solely into account by you and in (104), at least in the diagonal, the pressure components of the radiation must stay. The following seems more important to me than this, however: If gravitation is a general property of matter, then an internal pressure must arise in the world because of Newton’s force of grav- itation (as it would in a Van der Waals gas through the Van der Waals forces). I Tμν
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