John Stewart Bell

For other people named John Bell, see John Bell (disambiguation).

John Stewart Bell (28 June 1928 – 1 October 1990) was a physicist, and the originator of Bell's Theorem, one of the most important theorems in quantum physics.

Life and work

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 complete a PhD at the University of Birmingham, specialising in nuclear physics and quantum field theory. His career began with the British Atomic Energy Agency, in Malvern, Britain's, then 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.

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". In this work, he showed that carrying forward EPR's analysis permits one to derive the famous Bell's inequality. This inequality, derived from certain assumptions, conflicts with the predictions of 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." 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". 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'."

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". 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. (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, can Bell's resolution of the EPR paradox be considered a success? According to Bell's interpretation, quantum mechanics itself has been demonstrated to be irreducibly nonlocal. Therefore, one cannot fault a hidden variables scheme if, as in the pilot wave theory of de Broglie and Bohm, it includes a violation of local causality.

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." This, according to Bell, also implied that quantum theory is not locally causal and cannot be embedded into any locally causal theory.

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': "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 the more subjective alternatives such as the Copenhagen interpretation.

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

Bell seemed to be quite comfortable 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 ..."

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, 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. 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.

See also

• Bell's theorem, published in the mid-1960s
• Bell's spaceship paradox
• 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
• Superdeterminism
• Afshar experiment

References

• Aczel, Amir D. (2001) Entanglement: The Greatest Mystery in Physics. New York: Four Walls Eight Windows
• Bell, John S. (1987) Speakable and Unspeakable in Quantum Mechanics. Cambridge Univ. Press, ISBN 0-521-36869-3, 2004 edition with introduction by Alain Aspect and two additional papers: ISBN 0-521-52338-9.
• Albert Einstein, Podolsky, Rosen, (1935) "Can Quantum Mechanical Description of Physical Reality Be Considered Complete?" Phys. Rev. 47: 777.
• Gilder, Louisa (2008) The Age of Entanglement: When Quantum Physics Was Reborn. New York: Alfred A. Knopf.
• Pearle, Philip (1970) "Hidden-Variable Example Based upon Data Rejection," Physical Review D 2: 1418-25.
• John von Neumann (1932) Mathematical Foundations of Quantum Mechanics. Princeton Univ. Press. 1996 ed.: ISBN 0-691-02893-1.

External links

• MacTutor profile (University of St. Andrews)
• John Bell and the most profound discovery of science (December 1998)
• The Most Profound Discovery of Science (September 2006)