John Stewart Bell
– 1 October
) was a physicist
and the originator of Bell's Theorem
one of the most important theorems in quantum physics
Life and work
born in Belfast, Northern
Ireland, and graduated in experimental physics at the
University of Belfast, in 1948. He went on to complete a PhD at the
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
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
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
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
." 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
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
, 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
.... On this list of bad words from good books, the worst of all is
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
’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
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
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
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
- :"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
, 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
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
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
- 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.
- 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.