D O C U M E N T S 1 9 9 , 2 0 0 F E B R U A R Y 1 9 2 6 2 1 7 199. From Pascual Jordan [Göttingen, after 18 February 1926][1] Dear Professor Einstein, Mr. Heisenberg showed me your letter and said that he had announced that a re- sponse to the last points you raised would be coming from me. Unfortunately I could not understand for certain in what way, according to your view, a problem should arise with the radiation fluctuations. (I do believe, though, that one definitely will not get any contradictions in the quantum-mechanical treatment of the wave problem, provided one always interprets the results of the formal calcu- lation in an appropriate manner.) As regards the localization of the wave energy, it can ¢always² be executed formally for the energy matrix without difficulty because we have assumed position and time coordinates as numbers the “energy of a state,” by contrast, is evidently not localizable. ¢On the contrary,² However, the squares, averaged over time, or higher powers of the energy fluctuation in one par- tial volume, become definable for the individual states.[2] That is why it does not appear to me to be doubtful that one must interpret it in this way: If for a particular state of the radiation in the total volume the energy contained in the small partial volume is taken out and measured numerous times, one obtains an “empirical en- ergy (= a function of time) in the partial volume” from this temporally variable number one can form the square and the higher powers (by usual multiplication) and average them over time. The theory asserts that these temporal averages are computable as the terms belonging to the relevant state of the diagonal matrices, that are obtained through the time-averaging of the partial volume’s squared or higher-powered energy matrix.— The classical theory, by contrast, contended that it could theoretically construct a temporally variable number that agreed with the above empirical energy number. With kind regards, sincerely yours, P. Jordan 200. To Léon Brillouin[1] 22 February 1926 Dear Mr. Brillouin, Both of your papers in the C[omptes] R[endus] were extraordinarily interesting to me.[2] What pertains to the derivation of refraction under the assumption of light quanta, however, is an incorrect assertion. The classical electrodynamic theory yields[3]