##### John Stewart Bell, receiving an honorary degree from Queen’s University, Belfast in July 1988

# Bell

John Stuart Bell (28 June 1928 – 1 October 1990) was a key figure in the revival of interest in the foundations of quantum mechanics in the post-war period. His discovery of the Bell inequality for certain correlation functions relating values of dynamical variables when subject to a locality assumption was one of the most important developments in the philosophy of quantum mechanics since the Einstein, Podolsky, and Rosen paradox of 1935, to which it is closely related. The inequality is violated by measurement outcomes even of spatially-separated systems according to quantum mechanics. This prediction was subsequently been verified, most notably by Aspect’s experiment performed in 1976.

According to Bohr’s interpretation of quantum mechanics, dynamical variables in quantum theory do not have values independent of measurement. Bell’s result was thought to apply only to ‘hidden-variable’ theories, such as the de Broglie-Bohm theory, which demonstrably violates Bell’s locality condition. However, with the possible exception of Everett’s Many Worlds Interpretation (MWI) of quantum theory, it is now widely agreed that the violation of Bell’s inequality is evidence for non-locality independent of theory.

In a series of influential publications in the 1970s, collected in his book *Speakable and Unspeakable in Quantum Mechanics* (1987), Bell argued that the measurement problem of quantum mechanics could not be circumvented by any of the conventional interpretations of the theory, particularly if it was to be applied to cosmology. According to Bell, the measurement problem could only be solved by the introduction of hidden variables, as in pilot-wave theory, or by modification of the unitary dynamics, as in dynamical collapse theories. He regarded the MWI as poorly defined. Bell also showed that the von Neumann ‘impossibility’ proof for the introduction of hidden variables in quantum mechanics was based on unreasonably strong assumptions.

In the light of a certain thought experiment (the ‘spaceship paradox’), Bell also argued for a dynamical interpretation of the length contraction and time-dilation effects in Einstein’s special theory of relativity, referring dynamical models to a single frame of reference. The prevailing view was to appeal to the geometry of Minkowsi space and to a ‘perspectival’ interpretation of contraction and dilation effects. Bell’s approach has subsequently been developed by Harvey Brown and his co-workers. Bell was also co-discoverer of the Adler-Bell-Jackiw anomaly of chiral quantum field theory. Such anomalies were an important clue to the later discovery, by Green and Schwarz in 1985, of the consistency of superstring theory in 10 and 26 dimensions.

Bell was born in Belfast, where he was a graduate of Queen’s University, obtaining his PhD in nuclear and particle physics at the University of Birmingham in 1966. After working at the Harwell Laboratory in Oxfordshire he joined CERN, where he worked as a theorist and in accelerator design.

### Links

Stanford Encyclopedia in Philosophy: Bell’s theorem >

Wikipedia: The Bell Spaceship paradox >

### Multimedia

Oxford Physics. James Binney: Einstein-Rosen-Podolsky experiment and Bell’s inequality >