John Stewart Bell

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John Stewart 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, for 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 "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-Madison and Brandeis University, he wrote a paper (ref 1 p. 14) entitled "On the Einstein-Podolsky-Rosen paradox". In this work, he showed that the carrying forward EPR's analysis (ref 4) permits one to derive the famous inequality. This inequality, derived from some basic philosophical assumptions, conflicts with the predictions of quantum mechanics.

There is some disagreement regarding what Bell's inequality - 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." According to a second camp, the correct conclusion is something different. They would say that it is not all possible local theories in general, but only local hidden variables which have shown incompatibility with quantum.

Despite the fact that hidden variable schemes are often associated with the issue of indeterminism, or uncertainty, Bell was instead concerned with the fact that orthodox quantum mechanics is a subjective theory, and the concept of 'measurement' figures prominently in its formulation. It was not that Bell found 'measurement' unacceptable in itself, but he objected to its appearance at quantum mechanics' most fundamental theoretical level - which he insisted must be concerned only with sharply-defined mathematical quantities and unambiguous physical concepts.

Bell's own words: (ref 1, p. 117) "The concept of 'measurement' becomes so fuzzy on reflection that it is quite surprising to have it appearing in physical theory at the most fundamental level... ...does not any analysis of measurement require concepts more fundamental than measurement? And should not the fundamental theory be about these more fundamental concepts?"

Bell was impressed that within Bohm’s hidden variables theory, reference to this concept was not needed, and it was this which sparked his interest in the field of research.

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' (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 if - as Bohm's does - it includes "superluminal signalling", i.e., nonlocality.

The alternative camp mentioned above would disagree with this conclusion. This group does not assess the EPR/Bell world as having proven the nonlocality of quantum theory. They would claim that by keeping with orthodox quantum and avoiding hidden variables, one retains locality, and they base arguments against hidden variables on this notion.

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.

Blue plaque honouring John Bell at the Queen's University of Belfast

Bell seemed to 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, 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 ..." (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. How can posterity interpret his most famous work - his contribution to the issues raised by EPR?. Some regard him as having demonstrated the failure of local realism (local hidden variables). Bell's own interpretation - as noted above - is that locality itself met its demise.

[edit] 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
Einstein, Podolsky, Rosen Can Quantum Mechanical Description of Physical Reality Be Considered Complete? Phys. Rev. 47, 777 (1935).