1 3 6 D O C . 7 7 E S S E N C E O F Q U A N T U M M E C H A N I C S 77. The Essence of Quantum Mechanics [Einstein 1928] Dated 28 October 1927 Published 1928 In: Electrons et Photons, Rapports et discussions Solvay 1928, pp. 253–256. The Fifth Solvay Congress was held in Brussels, 24–29 October 1927, and the pro- ceedings were published in Solvay 1928. (See the Introduction, sec. I, for a brief overview of the history of the Solvay conferences.) Two remarks of Einstein’s appear in these proceedings, both in the general discussion at the conclusion of the Congress. Einstein submitted a written version of the first remark nearly a month after the end of the conference (see Doc. 76), whereas his second remark is a short question that he raised in the course of the discussion. Both are reproduced below and introduced with some editorial commentary. Einstein’s first, longer remark communicates his fundamental misgivings about quantum theory. The most important of these concerns the nonseparability of quantum mechanics: the fact that the state of one part of a quantum system cannot, in general, be completely specified independently of the rest of the system. Einstein also mentions a different misgiving he has with quantum mechanics, concerning the fact that the wave function is defined on a high-dimensional configuration space. Einstein claims that “objections of principle are opposed to this multi- dimensional representation.” In particular, Einstein claims that treating a configu- ration space representation as fundamental is at odds with both Bose-Einstein statistics and the idea that forces act locally (as in classical field theories).