Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [178]
A scientific theory that does not agree with experimental facts will either be modified or discarded. Quantum mechanics, however, had passed every test it had been subjected to. There was no conflict between theory and experiment. For the vast majority of Bell's colleagues, young and old alike, the dispute between Einstein and Bohr over the correct interpretation of quantum mechanics was more philosophy than physics. They shared Pauli's view, expressed in a letter to Born in 1954, that 'one should no more rack one's brain about the problem of whether something one cannot know anything about exists all the same, than about the ancient question of how many angels are able to sit on the point of a needle'.43 To Pauli it seemed 'that Einstein's questions are ultimately always of this kind' in his critique of the Copenhagen interpretation.44
Bell's theorem changed that. It allowed the local reality advocated by Einstein, that the quantum world exists independently of observation and that physical effects cannot be transmitted faster than the speed of light, to be tested against Bohr's Copenhagen interpretation. Bell had brought the Einstein-Bohr debate into a new arena, experimental philosophy. If Bell's inequality held, then Einstein's contention that quantum mechanics was incomplete would be right. However, should the inequality be violated, then Bohr would emerge the victor. No more thought experiments; it was Einstein vs. Bohr in the laboratory.
It was Bell who first challenged the experimentalists to put his inequality to the test when he wrote in 1964 that 'it requires little imagination to envisage the measurements involved actually being made'.45 But like Gustav Kirchhoff and his imaginary blackbody a century earlier, it is easier for a theorist to 'envisage' an experiment than for his colleagues to realise it in practice. Five years passed before Bell received a letter in 1969 from a young physicist at Berkeley in California. John Clauser, then 26, explained that he and others had devised an experiment to test the inequality.
Two years earlier, Clauser had been a doctoral student at New York's Columbia University when he first came across Bell's inequality. Convinced that it was worth testing, Clauser went to see his professor and was bluntly told that 'no decent experimentalist would ever go to the effort of actually trying to measure it'.46 It was a reaction in keeping with the near 'universal acceptance of quantum theory and its Copenhagen interpretation as gospel', Clauser wrote later, 'along with a total unwillingness to even mildly question the theory's foundations'.47 Nevertheless, by the summer of 1969 Clauser had devised an experiment with the help of Michael Horne, Abner Shimony and Richard Holt. It required the quartet to fine-tune Bell's inequality so that it could be tested in a real laboratory rather than in the imaginary laboratory of the mind equipped with perfect instruments.
Clauser's search for a postdoctoral position took him to the University of California at Berkeley, where he had to settle for a job doing radio astronomy. Luckily, when Clauser explained to his new boss the experiment he really wanted to perform, he was allowed to devote half of his time to it. Clauser found a willing graduate student, Stuart Freedman, to help. Instead of electrons, Clauser and Freedman used pairs of correlated photons in their experiment. The switch was possible because photons have a property