I N T R O D U C T I O N T O V O L U M E 1 4 l x i he drew a detailed sketch for his wife, Elsa (Docs. 8, 18). The BKS paper apparent- ly triggered instant debates among his Berlin colleagues. In mid-May, Max von Laue asked Einstein to give a report on Bohr’s new theory upon his return, and Ein- stein agreed (Doc. 250). He asked Elsa to send him the pertinent literature (Doc. 251) and prepared a colloquium, which he gave in Berlin on 28 May. The es- sence of his critique of the BKS theory is captured in Doc. 256. He reported in de- tail to Ehrenfest a few days afterward and repeated his main arguments against the BKS theory (Doc. 259). According to him, the Copenhagen group had “abolished the free quanta, but the quanta will not allow us to get rid of them” (Doc. 285). It is very likely that around the time of Einstein’s Berlin colloquium, the idea was conceived to put the BKS theory to a crucial experimental test that would also decide between Bohr’s theory and Compton’s interpretation of his effect based on Einstein’s light quantum hypothesis. The experiment was designed and carried out by Walther Bothe and Hans Geiger, and constituted one of the first precision coin- cidence experiments. Compton had explained the observed shift in wavelength of his scattered X-rays as the result of a collision process. This explanation implied that the scattering should produce recoil electrons, which Compton postulated but had not observed. Furthermore, Compton’s interpretation was based on the assumption that each col- lision is an elementary process in which both energy and momentum are conserved. The BKS theory, on the other hand, did not assume that energy and momentum conservation held for each elementary process but only on a statistical average. Bothe and Geiger now set out to detect the scattered X-ray quantum and the recoil electron simultaneously. They recorded events of each kind, projecting them on high-speed photographic film, and performed a statistical analysis in order to see whether coincidences occurred more frequently than allowed by chance. The setup and statistical nature of their experiment required an enormous amount of film and statistical data analysis, which is one of the reasons why Bothe and Geiger were very cautious and hesitant to announce their result. Yet the outcome was eagerly awaited, and rumors about a confirmation of Compton’s result had spread by the end of the year. Einstein, too, was cautious, and expressed doubts about the experiment in letters and conversations (see Docs. 358, 375, 398, 399). As late as 9 January 1925, Ehrenfest wrote to Einstein: “If Bothe-Geiger find ‘statistical independence’ between electron and scattered light quantum, it proves nothing. If, however, they find dependence, it is a triumph for Einstein over Bohr.— This time, I (exceptionally! [For God’s sake, don’t show this to your wife!!]) firmly believe you so I would be glad if dependence were made evident” (Doc. 417). At the end of 1924, Pascual Jordan published a paper, “On the Theory of Quantum Radiation,”[31] based on his Göttingen dissertation completed under the