- John Stewart Bell
John Stewart Bell (
June 28 1928– October 1 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 physicsand 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 physicsand on accelerator design, but found time to pursue a major avocation, 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-Madisonand Brandeis University, he wrote a paper entitled "On the Einstein-Podolsky-Rosen Paradox" [John Bell, "Speakable and Unspeakable in Quantum Mechanics", p. 14] . In this work, he showed that the carrying forward EPR's analysis [Einstein, "et al.", "Can Quantum Mechanical Description of Physical Reality Be Considered Complete?"] permits one to derive the famous Bell's 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. Bell held that not only local hidden variables, but any and all local theoretical explanations must conflict with quantum theory: "It is known that with Bohm's example of EPR correlations, involving particles with spin, there is an irreducible
nonlocality." [Bell, p. 196 ] According to an alternate interpretation, not all local theories in general, but only local hidden variables have shown incompatibility with quantum theory.
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. 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.
In Bell's words: "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, p. 117 ]
Bell was impressed that within Bohm’s
nonlocal hidden variable 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". [ Bell, p.1 ] Here he showed that von Neumann’s argument [John von Neumann, "Mathematical Foundations of Quantum Mechanics"] 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 Hermannin 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 "
superluminal signalling", i.e., nonlocality.
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." [Bell, p. 132] This, according to Bell, also implied that quantum mechanics cannot be embedded into a locally causal hidden variables 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'." [Bell, p. 174] 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 and Everett "many-worlds" interpretations. [Bell, p. 92, 133, 181]
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 ..." [Bell, p. 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 [Philip Pearle, "Hidden-Variable Example Based upon Data Rejection"] , 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 hemorrhagein 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.
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
Local hidden variable theory
*Aczel, Amir D, "Entanglement: The Greatest Mystery in Physics" (2001), New York: Four Walls Eight Windows
*Bell, John S, "Speakable and Unspeakable in Quantum Mechanics" (1987), Cambridge University Press, ISBN 0-521-36869-3, 2004 edition with introduction by
Alain Aspectand two additional papers: ISBN 0-521-52338-9
*Einstein, Podolsky, Rosen, "Can Quantum Mechanical Description of Physical Reality Be Considered Complete?", "Phys. Rev." 47, 777 (1935).
*von Neumann, John, "Mathematical Foundations of Quantum Mechanics" (1932), Princeton University Press 1996 edition: ISBN 0-691-02893-1
*Pearle, Philip, "Hidden-Variable Example Based upon Data Rejection", Physical Review D, 2, 1418-25 (1970)
* [http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Bell_John.html MacTutor profile (University of St. Andrews)]
* [http://physicsweb.org/articles/world/11/12/8 John Bell and the most profound discovery of science (December 1998)]
* [http://www.rds.ie/home/index.aspx?id=1755 The Most Profound Discovery of Science (September 2006)]
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