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 Queen's University, 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. 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 persue a major "hobby", investigating the fundamentals of quantum theory.

In 1964, after a year's leave from CERN that he spent at Stanford, the University of Wisconsin and Brandeis University, he derived his famous inequality. This must be satisfied by any local hidden variable theory but can be violated under quantum mechanics. He had first shown that von Neumann’s proof against deterministic quantum mechanics lacked rigor and that Bohm’s work got around von Neumann’s objections through the use of superluminal signalling. (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.)

In 1972 the first of many experiments that have shown a violation of Bell's Inequality was conducted. The fate of local hidden variable theories is currently generally regarded as sealed. Bell's Theorem has come to be regarded a proof that the underlying theory of the universe must be a quantum-mechanical one, although Bell's original goal was just the opposite.

Bell became a supporter of the Bohm interpretation, a nonlocal hidden variable theory involving superluminal signaling, calling its suppression a "scandal" within physics, and took to defending his work against those whom he perceived as distorting its meaning to favor indeterministic quantum mechanics such as the Copenhagen and Everett "many-worlds" interpretations.

The outcomes of the experiments, showing as they did a violation of his inequalities, did not make John Bell particularly happy: he had hoped that they would always be satisfied in Nature, and that experiments would eventually falsify quantum mechanics. Despite the observed violations, he kept some belief that future experiments would change the situation. Nevertheless he was ready to accept the validity of the orthodox quantum mechanics. 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 the probabilistic quantum mechanics must be incorrect. They argue that 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 publicised 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 quantum mechanics is correct. It is claimed, after all, never to have failed any other test.

Bell died unexpectedly of a cerebral haemorrhage in Belfast in 1990. Believers in local realism and quantum theory alike continue to search for ways of making sense of the apparent failure of local realism in the Bell test experiments.

References:

• Bell, John S, The 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
• EPR paradox
• CHSH Bell test
• Quantum mechanical Bell test prediction
• Quantum entanglement
• Local hidden variable theory
• Bell state