## Bells theorem

John Bell investigated the paradoxes of Bohr's quantum mechanics and originally set out to establish Einstein's assertion that the Copenhagen interpretation of quantum mechanics was incomplete. He was able to pinpoint a method for solving the question of whether or not hidden variables exist by invoking the so-called

John Stewart Bell

(1922-1990). Courtesy of Queen's University, Belfast.

Bell's inequality, a mathematical statement which applies to systems with 'realistic' properties. Theories with hidden variables satisfy Bell's inequality; quantum theory does not. Experiments to test Bell's inequality could resolve the question once and for all.

During the period from 1980 onwards many experiments have been performed to test Bell's inequality. The first and perhaps most notable experiments were those with photons of light, by Alain Aspect and his group at Orsay. The overwhelming evidence is that the laws of nature do not obey Bell's inequality. Quantum mechanics and the Copenhagen interpretation agree with experiment. The photon does not have hidden variables which determine how it will behave. We will see further examples of the strange behaviour of light when we discuss experiments on individual photons one at a time in the next chapter.

John Stewart Bell

(1922-1990). Courtesy of Queen's University, Belfast.

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