John Stewart Bell FRS[1] (28 June 1928 – 1 October 1990) was a British physicist from Northern Ireland (Ulster), and the originator of Bell's theorem, a significant theorem in quantum physics regarding hidden variable theories.
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John Bell was born in Belfast, Northern Ireland. When he was 11 years old, he decided to be a scientist, and at 16 graduated from Belfast Technical High School. Bell then attended the Queen's University of Belfast, and obtained a bachelor's degree in experimental physics in 1948, and one in mathematical physics a year later. He went on to complete a Ph.D. in physics at the University of Birmingham in 1956, specialising in nuclear physics and quantum field theory. In 1954 he married Mary Ross, also a physicist, whom he had met while working on accelerator physics at Malvern, UK.[2]
Bell's career began with the UK Atomic Energy Research Establishment, near Harwell, Oxfordshire, known as AERE or Harwell Laboratory. After several years he moved to the European Center for Nuclear Research (CERN, Conseil Européen pour la Recherche Nucléaire). Here he worked almost exclusively on theoretical particle physics and on accelerator design, but found time to pursue a major avocation, investigating the foundations of quantum theory. He was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1987.[3]
In 1964, after a year's leave from CERN that he spent at Stanford University, the University of Wisconsin–Madison and Brandeis University, he wrote a paper entitled "On the Einstein-Podolsky-Rosen Paradox".[4] In this work, he showed that carrying forward EPR's analysis[5] permits one to derive the famous Bell's theorem. The resultant inequality, derived from certain assumptions, is violated by quantum theory.
There is some disagreement regarding what Bell's inequality—in conjunction with the EPR analysis—can be said to imply. Bell held that not only local hidden variables, but any and all local theoretical explanations must conflict with the predictions of quantum theory: "It is known that with Bohm's example of EPR correlations, involving particles with spin, there is an irreducible nonlocality."[6] According to an alternative interpretation, not all local theories in general, but only local hidden variables theories (or "local realist" theories) have shown to be incompatible with the predictions of quantum theory.
Bell's interest in hidden variables was motivated by the existence in the formalism of quantum mechanics of a "movable boundary" between the quantum system and the classical apparatus:
A possibility is that we find exactly where the boundary lies. More plausible to me is that we will find that there is no boundary. ... The wave functions would prove to be a provisional or incomplete description of the quantum-mechanical part, of which an objective account would become possible. It is this possibility, of a homogeneous account of the world, which is for me the chief motivation of the study of the so-called 'hidden variable' possibility.[7]
Bell was impressed that in the formulation of Bohm’s nonlocal hidden variable theory, no such boundary is needed, and it was this which sparked his interest in the field of research. Bell also criticized the standard formalism of quantum mechanics on the grounds of lack of physical precision:
For the good books known to me are not much concerned with physical precision. This is clear already from their vocabulary. Here are some words which, however legitimate and necessary in application, have no place in a formulation with any pretension to physical precision: system, apparatus, environment, microscopic, macroscopic, reversible, irreversible, observable, information, measurement. .... On this list of bad words from good books, the worst of all is 'measurement'.[8]
But if he were to thoroughly explore the viability of Bohm's theory, Bell needed to answer the challenge of the so-called impossibility proofs against hidden variables. Bell addressed these in a paper entitled "On the Problem of Hidden Variables in Quantum Mechanics".[9] (Bell had actually written this paper before his paper on the EPR paradox, but it did not appear until two years later, in 1966, due to publishing delays.[10]) Here he showed that John von Neumann’s argument[11] does not prove the impossibility, as it was claimed. The argument fails in this regard due to its reliance on a physical assumption that is not valid for quantum mechanics—namely, that the probability-weighted average of the sum of observable quantities equals the sum of the average values of each of the separate observable quantities.[12] In this same work, Bell showed that a stronger effort at such a proof (based upon Gleason's theorem) also fails to eliminate the hidden variables program. The flaw in von Neumann's proof had been previously discovered by Grete Hermann in 1935, but did not become common knowledge until rediscovered by Bell.
In 1972 the first of many experiments that have shown (under the extrapolation to ideal detector efficiencies) a violation of Bell's inequality was conducted. Bell himself concludes from these experiments that "It now seems that the non-locality is deeply rooted in quantum mechanics itself and will persist in any completion."[13] This, according to Bell, also implied that quantum theory is not locally causal and cannot be embedded into any locally causal theory. Bell regretted that results of the tests did not agree with the concept of local hidden variables:
For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational that I think that when Einstein saw that, and the others refused to see it, he was the rational man. The other people, although history has justified them, were burying their heads in the sand. ... So for me, it is a pity that Einstein's idea doesn't work. The reasonable thing just doesn't work."[14]
Bell seemed to have become resolved to the notion that future experiments would continue to agree with quantum mechanics and violate his inequality. Referring to the Bell test experiments, he remarked:
It is difficult for me to believe that quantum mechanics, working very well for currently practical set-ups, will nevertheless fail badly with improvements in counter efficiency ..."[15]
Some people continue to believe that agreement with Bell's inequalities might yet be saved. They argue that in the future much more precise experiments could reveal that one of the known loopholes, for example the so-called "fair sampling loophole", had been biasing the interpretations. Most mainstream physicists are highly skeptical about all these "loopholes", admitting their existence but continuing to believe that Bell's inequalities must fail.
Bell remained interested in objective 'observer-free' quantum mechanics. He felt that at the most fundamental level, physical theories ought not to be concerned with observables, but with 'be-ables': "The beables of the theory are those elements which might correspond to elements of reality, to things which exist. Their existence does not depend on 'observation'."[16] He remained impressed with Bohm's hidden variables as an example of such a scheme and he attacked the more subjective alternatives such as the Copenhagen interpretation.[17]
Bell died unexpectedly of a cerebral hemorrhage in Belfast in 1990. Unbeknownst to Bell, that year he had been nominated for a Nobel prize (which is never awarded posthumously). His contribution to the issues raised by EPR was significant. Some regard him as having demonstrated the failure of local realism (local hidden variables). Bell's own interpretation is that locality itself met its demise.
In 2008 a prize named in honor of John Bell was created by the Centre for Quantum Information and Quantum Control[18] at the University of Toronto. Officially called "The John Stewart Bell Prize for Research on Fundamental Issues in Quantum Mechanics and their Applications," the prize is awarded every other year for significant contributions first published during the six preceding years. The award recognizes major advances relating to the foundations of quantum mechanics and to the applications of these principles. In 2009 the first award was presented by Alain Aspect to Professor Nicolas Gisin, University of Geneva for his theoretical and experimental work on foundations and applications of quantum physics - notably quantum nonlocality, quantum cryptography, and quantum teleportation.[19]
Other work by Bell: