Einstein's angelic thoughts

Einstein's persistent misgivings about quantum mechanics are generally viewed today in the way that Max Born viewed them: "In his later years, Einstein could no longer take in new ideas in physics that contradicted his own firmly held philosophical convictions." However, Einstein's critique provided perceptive physical observations that teased out a number of subtle features and yielded rich insights. A striking example is the notion of entanglement that originated from the Einstein-Podolsky-Rosen (EPR) paper. The discovery of Bell's theorem has made the EPR argument empirically relevant. Apart from its deep-seated conceptual ramifications, quantum entanglement now underpins quantum communications and quantum computation.

Einstein's position was a logically consistent defence of realism, meaning that objects exist "out there" with definite values of their properties, irrespective of whether they are observed. Because the standard framework of quantum mechanics was not "realist", Einstein found it unacceptable. Subsequent studies have revealed that a consistent alternative interpretation of quantum mechanics based on "realism" is possible, such as David Bohm's model. In view of these developments and renewed interest in the foundations of quantum mechanics, an appraisal of Einstein's critique of quantum mechanics from a fresh perspective is called for.

Jagdish Mehra's central theme is intended to be Einstein's view versus quantum theoretical orthodoxy, but what he actually provides is mainly a sketchy historical overview. He does not try to relate Einstein's views to modern studies on quantum mechanics, nor does he attempt a comprehensive review. Still, the book brings together a number of key features of Einstein's arguments and philosophical notions. A carefully compiled list of references and the lucidity of the writing add to its utility.

Chapter one sets the stage by briefly tracing developments in the initial years of quantum mechanics. From chapter two onwards, Einstein's early views and his role in the discovery of Schrödinger's equation are discussed. The relevant Einstein-Schrödinger letters of that period are interesting, for instance Schrödinger's remark that "the whole thing would certainly have not originated" if Einstein had not stressed the importance of de Broglie's idea of wave-particle duality in his work on a quantum theory for an ideal gas. Mehra proceeds to discuss the Bohr-Einstein debate and Einstein's gradual shift from doubting the consistency of quantum mechanics to questioning its "completeness", which culminated in the EPR paper. However, Mehra provides no new insight. For example, recent research into the history of the EPR argument has shown that the published version was written by Podolsky and not seen by Einstein prior to its publication. Einstein later felt that the points he wanted to make were not adequately presented. Einstein's own version can be found in his letters to Schrödinger and Karl Popper, and the clearest account is in his letter of 1952 to his friend Michele Besso. Mehra ignores all this.

Chapter six concerns exchanges between Einstein and Born. Their disagreements on questions related to causality and determinism are brought out. However, Mehra overlooks an important conversation between Heisenberg and Einstein in 1926 that is reported in Heisenberg's memoir, Physics and Beyond . This indicates that soon after Heisenberg's discovery, Einstein was able to discern a number of conceptual conundrums in the new mechanics. Apart from pointing out that they were "speaking of what we know about nature and no longer about what nature really does", Einstein anticipated the quantum measurement problem: how to analyse quantum-mechanically the occurrence of a definite outcome in a measurement. Einstein's insistence on a realist description of an individual event comes out in his telling remark to Heisenberg: "If your theory is right, you will have to tell me sooner or later what the atom does when it passes from one stationary state to the next."

The last chapter dealing with Einstein's philosophy is instructive. It studies how Einstein's views evolved from special relativity when he was influenced by Mach to his later views closer to those of Kant. Mehra points out that Einstein's disagreement with Mach arose from his work on Brownian motion, which was opposed by Mach. Mehra discusses a significant meeting between Einstein and Mach in 1913 when Einstein clarified what he meant by calling the atomic hypothesis "economical". "Logical economy" entailed that "the observable properties should be derived from as few assumptions as possible". In his later years, a unified description of a range of phenomena was also a key requirement of Einstein.

Machian positivism, in the sense that the only valid propositions are those that concern observable phenomena, is associated with special relativity. However, after inventing general relativity without any empirical motivation, Einstein became suspicious of positivism, and before the advent of quantum mechanics he was fully committed to realism. In fact, Einstein told Popper in 1950 that he regretted no mistake of his so much as his earlier belief in positivism. Mehra rightly argues that Einstein's later belief in "playing" with theories because they "are free inventions of the mind" has a Kantian flavour. The delicate balance in Einstein between mathematics and operationalism based on his ideas of causality and realism is not easy to explain, though Mehra makes a valiant attempt to do so concisely. Relevant portions from Einstein's articles are quoted, including a beautiful piece from his Herbert Spencer lecture at Oxford in 1933. However, the discussion ends rather abruptly without a proper conclusion.

Pauli once remarked: "One should no more rack one's brain about the problem of whether something one cannot know anything about exists, than about the ancient question of how many angels are able to sit on the point of a needle. But it seems to me that Einstein's questions are ultimately always of this kind." Mehra's disappointing book may nevertheless stimulate a few readers to take a closer look at Einstein's own views in order to judge Pauli's provocative assessment for themselves.

Dipankar Home is professor of physics, Bose Institute, Calcutta, India.