I N T R O D U C T I O N T O V O L U M E 1 6 x l v conference he remarked, “I think that Mr. De Broglie is correct in investigating along these lines” (Doc. 77), without mentioning that he had been looking that way himself.[8] This comment occurs in the only substantial contribution by Einstein pub- lished in the conference proceedings (Solvay 1928, see Doc. 77).[9] His major con- cern in these remarks is to communicate fundamental misgivings about quantum theory, particularly regarding the “purely statistical way of thinking” mentioned in his letter to Lorentz on 17 June (Doc. 8). [10] Einstein begins by describing a simple experimental setup: electrons dif- fract through a small opening O, and then impinge uniformly across a large hemispherical photographic film, P (see Fig. 2 in Doc. 77). He then formulates two conceptions of what a quantum mechanical description of this setup amounts to. According to the first, quantum mechanics does not describe the individual electrons but only “the whole of an infinity of elementary processes,” whereas according to the second, quantum mechanics “claims to be a complete theory of individual processes.” Einstein argues that only the second conception establish- es conservation laws for elementary processes, predicts the results of the Bothe-Geiger experiments, and explains why alpha particles create nearly continuous lines in a cloud chamber.[11] Despite these advantages, however, Einstein still has objections to the second conception: If were simply regarded as the probability that at a certain point a given particle is found at a given time, it could happen that the same elementary process produces an action in two or several places on the screen. But the interpreta- tion according to which expresses the probability that this particle is found at a given point assumes an entirely peculiar mechanism of action at a distance, which prevents the wave continuously distributed in space from producing an ac- tion in two places on the screen. (Doc. 77) Einstein is arguing that as soon as a particle is detected at one point on the screen, some “peculiar mechanism of action at a distance” must immediately prevent the same particle being detected anywhere else. At the heart of this ar- gument is a concern with the non-separability of a quantum mechanical system— the fact that the parts of such a system cannot, in general, be completely specified independently of the whole, even when those parts are widely separated. Hence Einstein’s argument here appears to foreshadow the concern that motivated him in the famous “EPR” paper of 1935 (Einstein, Podolsky, and Rosen 1935).[12] The only other remark of Einstein’s that appears in the published proceedings is a brief question that he posed later in the general discussion (see Doc. 77).[13] This question highlights one of the main peculiarities of De Broglie’s theory—the fact that the corpuscles can follow very strange trajectories. Einstein’s question draws partic- ular attention to the fact that, in De Broglie’s view, photons not only sometimes move with a velocity less than c, but even come to a complete stop in the simple case of perpendicular reflection from a mirror.  2  2