Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [175]
'I think he was looking for a much more profound rediscovery of quantum phenomena', Bell said as he tried to understand Einstein's reaction.29 'The idea that you could just add a few variables and the whole thing would remain unchanged apart from the interpretation, which was a kind of trivial addition to ordinary quantum mechanics, must have been a disappointment to him.' Bell was convinced that Einstein wanted to see some grand new principle emerge on a par with the conservation of energy. Instead, what Bohm offered Einstein was an interpretation that was 'non-local', requiring the instantaneous transmission of so-called 'quantum mechanical forces'. There were other horrors lurking in Bohm's alternative. 'For example,' clarified Bell, 'the trajectories that were assigned to the elementary particles were instantaneously changed when anyone moved a magnet anywhere in the universe.'30
It was in 1964, during a year-long sabbatical from CERN and his day job designing particle accelerators, that Bell found the time to enter the Einstein-Bohr debate. Bell decided to find out if non-locality was a peculiar feature of Bohm's model or if it was a characteristic of any hidden variable theory that aimed to reproduce the results of quantum mechanics. 'I knew, of course, that the Einstein-Podolsky-Rosen setup was the critical one, because it led to distant correlations', he explained. 'They ended their paper by stating that if you somehow completed the quantum mechanical description, non-locality would only be apparent. The underlying theory would be local.'31
Bell started out trying to preserve locality by attempting to construct a 'local' hidden variable theory in which if one event caused another, then there had to be enough time between the two to allow a signal travelling at the speed of light to pass between them. 'Everything I tried didn't work', he said later.32 'I began to feel that it very likely couldn't be done.' In his attempt to eliminate what Einstein decried as 'spooky actions at a distance', non-local influences that were transmitted instantly between one place and another, Bell derived his celebrated theorem.33
He began by looking at a version of the EPR thought experiment first devised by Bohm in 1951 that was simpler than the original. Whereas Einstein, Podolsky and Rosen had used two properties of a particle, position and momentum, Bohm used only one, quantum spin. First proposed in 1925 by the young Dutch physicists George Uhlenbeck and Samuel Goudsmit, the quantum spin of a particle had no analogue in classical physics. An electron had just two possible spin states, 'spin-up' and 'spin-down'. Bohm's adaptation of EPR involved a spin-zero particle that disintegrates and in the process produces two electrons, A and B. Since their combined spin must remain zero, one electron must have spin-up and the other spin-down.34 Flying off in opposite directions until they are far enough apart to rule out any physical interaction between them, the quantum spin of each electron is measured at exactly the same time by a spin detector. Bell was interested in the correlations that could exist between the results of these simultaneous measurements carried out on pairs of such electrons.
The quantum spin of an electron can be measured independently in any one of three directions at right angles to each other, labelled x, y, and z.35 These directions are just the normal three dimensions of the everyday world in which everything moves – left and right (x-direction), up and down (y-direction), and back and forth (z-direction). When the spin of electron A is measured along the x-direction by