D O C U M E N T 1 3 2 D E C E M B E R 1 9 2 5 1 5 3 sirable. Then I will contact Mr. Keesom[3] about possibly doing the experiment in the spring in Leyden. With warm regards, yours sincerely, M. Wolfke 131. Obituary for Willem H. Julius [Einstein 1926o] Dated 14 December 1925 Published April 1926 In: Astrophysical Journal 63 (1926): 196–198 [See documentary edition for English text.] 132. From Pascual Jordan Göttingen, Bunsenstr. 9, 15 December 1925 Dear Professor Einstein, Due to an illness I was prevented for a rather long time from thanking you cor- dially for your kind letter.[1] I’ve been healthy again now for about a week, but I have further delayed replying because I still wanted to reconsider the zero-point en- ergy. But I have not managed to do so and am not going to have time to work more intensely on it for the time being because I am very busy (I am a collaborator on a book by Franck on electron collisions, etc.) [2] I am hoping to have more time again at the end of January. My opinion of the zero-point energy of a cavity is roughly this, that it is actually just a formal computation quantity without any direct phys- ical meaning only the thermal energy related to T = 0 is physically definable the fluctuations, which are formally calculated as the fluctuation square of thermal + zero-point energy, are of course identical to the fluctuations of the thermal energy.[3] The calculation of the radiation pressure on the cavity walls would, ac- cording to our formulas, perhaps also yield a zero-point portion but that merely shows, I think, that we have been interpreting the formulas of electrodynamics somewhat clumsily the real physical quantities of the field are ¢probably² certainly
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