John S. Bell (June 28 1928 – October 1 1990) was a physicist who became well known as the originator of Bell's Theorem, regarded by some in the quantum physics community as one of the most important theorems of the 20th century.

He was born in Belfast, Northern Ireland, and graduated in experimental physics at the Queen's University of Belfast, in 1948. He went on to do a PhD in Birmingham, specialising in nuclear physics and quantum field theory. His working career started with the British Atomic Energy Agency, in Malvern, Britain, then Harwell Laboratory. After several years he moved to the European Center for Nuclear Research (CERN). Here he worked almost exclusively on theoretical particle physics and on accelerator design, but found time to pursue a major "hobby", investigating the fundamentals of quantum theory.

In 1964, after a year's leave from CERN that he spent at Stanford University, the University of Wisconsin and Brandeis University, he wrote a paper entitled "On the Einstein-Podolsky-Rosen paradox". In this work, he derived his famous inequality. There is some disagreement regarding what this result - in conjunction with the EPR paradox - can be said to imply. One camp draws this conclusion: not only local hidden variables, but any and all local theoretical explanations must conflict with quantum theory. Bell himself was in this group: (ref 1 p 196): "It is known that with Bohm's example of EPR correlations, involving particles with spin, there is an irreducible nonlocality." An alternative camp gives a different conclusion to the issue: they say that the post EPR/Bell world requires only that local hidden variables must conflict with the quantum mechanical description.

Bell's interest in the field of research was sparked by Bohm’s hidden variables theory. The importance he saw in this was not so much in its elimination of indeterminism, but that it solved the problem of the subjectivity. Within Bohm's hidden variable scheme, there was no longer a necessity to refer to an 'observer'.

But if Bohm's theory were indeed viable, it would be necessary to answer the challenge posed by the so-called 'impossibility proofs' against hidden variables. Bell addressed these in a paper entitled 'On the problem of hidden variables in quantum mechanics' (ref 1 p.1). Here he showed that von Neumann’s argument does not prove impossibility, as it claims. The argument fails in this regard due to its reliance on a physically unreasonable assumption. 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. (Interestingly, the flaw in von Neumann's proof was previously discovered by Grete Hermann in 1935, but did not become common knowledge until rediscovered by Bell.)

If these attempts to disprove hidden variables failed, perhaps one can instead regard EPR and Bell's theorem as a success? The answer to this question hinges on which group one follows. According to Bell's camp, quantum mechanics itself has been demonstrated to be irreducibly nonlocal. Therefore, one cannot fault a hidden variables scheme such as Bohm's if it includes "superluminal signalling", i.e., nonlocality. The alternative camp mentioned above would disagree, and would not see nonlocality as inevitable. They would claim that one can retain locality by keeping with orthodox quantum and avoiding hidden variables. In this sense, the second camp would regard hidden variables as discredited.

In 1972 the first of many experiments that have shown a violation of Bell's Inequality was conducted. Again, the meaning of this violation differs according to how one sees things. Bell himself concludes (see p. 132 ref 1): "It now seems that the non-locality is deeply rooted in quantum mechanics itself and will persist in any completion." The alternative camp would tell us that the experiment means the elimination of local hidden variable theories.

Bell remained interested in objective 'observer-free' quantum mechanics. He stressed that at the most fundamental level, physical theories ought not to be concerned with observables, but with 'be-ables': (ref 1 p. 174) "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'." He remained impressed with Bohm's hidden variables as an example of such a scheme and he attacked (ref 1 p 92, 133, 181) the more subjective alternatives such as the Copenhagen and Everett "many-worlds" interpretations.

Bell seemed be quite peaceful with the notion that future experiments would continue to agree with quantum mechanics and violate his inequalities. Referring to the Bell test experiments, a remark of his is frequently quoted:

"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 ..." (Ref 1, page 109)

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. This latter loophole, first publicized by Philip Pearle in 1970 (ref below), is such that increases in counter efficiency decrease the measured quantum correlation, eventually destroying the empirical match with quantum mechanics. Most mainstream physicists are highly skeptical about all these "loopholes", admitting their existence but continuing to believe that Bell's inequalities must fail.

Bell died unexpectedly of a cerebral hemorrhage in Belfast in 1990. Interpretations of EPR/Bell's contributions differ. Some regard him as bring about the failure of local realism. Bell's own interpretation - as noted above - is that it was locality itself which met its demise.

References

• Bell, John S, Speakable and Unspeakable in Quantum Mechanics, Cambridge University Press 1987
• Pearle, P, Hidden-Variable Example Based upon Data Rejection, Physical Review D, 2, 1418-25 (1970)
• Aczel, Amir D, Entanglement: The greatest mystery in physics, Four Walls Eight Windows, New York, 2001

See also

• Bell's theorem, published in the mid-1960s
• EPR paradox, a thought experiment by Einstein, Podolsky, and Rosen published in 1935 as an attack on quantum theory
• CHSH Bell test, an application of Bell's theorem
• Quantum mechanical Bell test prediction
• Quantum entanglement
• Local hidden variable theory
• Bell state

External Links

• John Bell and the most profound discovery of science (December 1998)

1928 births | 1990 deaths | Northern Irish physicists | Fellows of the Royal Society | Alumni of Queen's University, Belfast | Natives of Belfast

জন স্টুয়ার্ট বেল | John Stewart Bell | John Stewart BELL | John Stewart Bell | John Stewart Bell | John Stewart Bell | John Stewart Bell | John Stewart Bell