From Einstein's theorem to Bell's theorem: a history of quantum non-locality

Abstract

In this Einstein Year of Physics it seems appropriate to look at an important aspect of Einstein's work that is often down-played: his contribution to the debate on the interpretation of quantum mechanics. Contrary to physics ‘folklore’, Bohr had no defence against Einstein's 1935 attack (the EPR paper) on the claimed completeness of orthodox quantum mechanics. I suggest that Einstein's argument, as stated most clearly in 1946, could justly be called Einstein's reality – locality – completeness theorem, since it proves that one of these three must be false. Einstein's instinct was that completeness of orthodox quantum mechanics was the falsehood, but he failed in his quest to find a more complete theory that respected reality and locality. Einstein's theorem, and possibly Einstein's failure, inspired John Bell in 1964 to prove his reality – locality theorem. This strengthened Einstein's theorem (but showed the futility of his quest) by demonstrating that either reality or locality is a falsehood. This revealed the full non-locality of the quantum world for the first time.

Acknowledgments

This work was supported by the Australian Research Council. I gratefully acknowledge the assistance and advice of Sheldon Chow, Damian Pope, Nadine Wiseman, Dave Kielpinski, Rob Spekkens, and Travis Norsen.

Notes

^†The name of this book, The Einstein Decade, derives from its Part II, in which the author (a former research assistant of Einstein) gives synopses of all of Einstein's papers from 1905 to 1915. Part I (124 pages) is essentially a scientific biography of Einstein, and is not limited to these years. But Bose – Einstein condensation (1925) is the last scientific contribution discussed in any detail, and the entire debate with Bohr goes unmentioned.

^†One can perhaps avoid subjectivism by being totally impartial, and denying that there is a matter of fact even about one's own experience. This was the route taken already in 1957 by Everett 52: the relative-state or many-worlds interpretation.

^†To be scrupulous, there are perhaps four other ways that the correlations in such an experiment could be explained away. (1) One could simply “refuse to consider the correlations mysterious” 57. (2) One could deny that the experimenters have free will to choose the settings of their measurement devices at random, as required for a statistically valid Bell-experiment 54. (3) One could entertain the idea of backward-in-time causation 58. (4) One could conclude that ordinary (Boolean) logic is not valid in our Universe 59. I do not consider these escape routes because they seem to undercut the core assumptions necessary to undertake scientific experiments. Bell expressed similar sentiments: with regard to option (1) he said “Outside [the] peculiar context [of quantum philosophy], such an attitude would be dismissed as unscientific. The scientific attitude is that correlations cry out for explanation.” 56. With regard to option (2) he thought it was not worth considering unless it could be shown to have some theoretical justification: “When a theory…in which such conspiracies inevitably occur…is announced, I will not refuse to listen…” 60. In Bell's opinion, option (3) was the same as option (2): “I have not myself been able to make sense of backward causation. When I try to think of it I lapse quickly into fatalism”, as quoted in 58. Finally, of option (4), Bell said that “When one remembers the role of the apparatus, ordinary logic is just fine.” 61, and thought that a “full appreciation of this [role] would have aborted…most of ‘quantum logic’.” 62 (p. vii).

^†Shimony 70 has also used the phrase ‘passion at a distance’, as opposed to ‘action at a distance’, as a colourful way to explain uncontrollable, as opposed to controllable, non-locality. Other terms have also been used 71. Finally, it is interesting to note that Einstein's pejorative term ‘telepathy’ 3 has recently been resurrected (modified to ‘quantum pseudo-telepathy’) 72 for the cases where the usefulness of uncontrollable non-locality is most evident.

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H. M. Wiseman

Howard Wiseman did his BSc (1991) and PhD (1994) at the University of Queensland, followed by a post-doc at the University of Auckland. Since then he has held fellowships from the Australian Research Council, being currently Federation Fellow and Professor at Griffith University. His principal research areas are quantum feedback control, quantum information, and fundamental questions in quantum mechanics. The current article arose from a need to understand the background and implications of Bell's theorem, for satisfying personal curiosity, for motivating other research, and for teaching non-physicists.