Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [181]
'God does not play dice', said Einstein memorably and often.9 Just like any modern-day advertising copywriter, he knew the value of an unforgettable tagline. It was his snappy denunciation of the Copenhagen interpretation and not a cornerstone of his scientific worldview. This was not always clear, even to someone like Born who knew him for almost half a century. It was Pauli who eventually explained to Born what really lay at the heart of Einstein's opposition to quantum mechanics.
During Pauli's two-month stay in Princeton in 1954, Einstein gave him a draft of a paper written by Born that touched on determinism. Pauli read it and wrote to his old boss that 'Einstein does not consider the concept of "determinism" to be as fundamental as it is frequently held to be.'10 It was something that Einstein told him 'emphatically many times' over the years.11 'Einstein's point of departure is "realistic" rather than "deterministic",' explained Pauli, 'which means that his philosophical prejudice is a different one.'12 By 'realistic' Pauli meant that Einstein assumed that electrons, for example, have pre-existing properties prior to any act of measurement. He accused Born of having 'erected some dummy Einstein for yourself, which you then knocked down with great pomp'.13 Surprisingly, Born, given their long friendship, had never fully grasped that what really troubled Einstein was not dice-playing, but the Copenhagen interpretation's 'renunciation of the representation of a reality thought of as independent of observation'.14
One possible reason for the misunderstanding may be that Einstein first said that God 'is not playing at dice' in December 1926 when he tried to convey to Born his unease at the role of probability and chance in quantum mechanics and the rejection of causality and determinism.15 Pauli, however, understood that Einstein's objections went far beyond the theory being expressed in the language of probability. 'In particular it seems to me misleading to bring the concept of determinism into the dispute with Einstein', he warned Born.16
At the heart of the problem,' wrote Einstein in 1950 of quantum mechanics, 'is not so much the question of causality but the question of realism.'17 For years he had hoped that he 'may yet work out the quantum puzzle without having to renounce the representation of reality'.18 For the man who discovered relativity, that reality had to be local, with no place for faster-than-light influences. The violation of Bell's inequality meant that if he wanted a quantum world that existed independently of observers, then Einstein would have had to give up locality.
Bell theorem cannot decide whether quantum mechanics is complete or not, but only between it and any local hidden variables theory. If quantum mechanics is correct – and Einstein believed it was, since it had passed every experimental test in his day – then Bell's theorem implied that any hidden variables theory that replicated its results had to be non-local. Bohr would have regarded, as others do, the results of Alain Aspect's experiments as support for the Copenhagen interpretation. Einstein would probably have accepted the validity of the results testing Bell's inequality without attempting to save local reality through one of the loopholes in these experiments that remained to be closed. However, there was another way out that Einstein might have accepted, even though some have said that it violates the spirit of relativity – the no signalling theorem.
It was discovered that it is impossible to exploit non-locality and quantum entanglement to communicate useful information instantaneously from one place to another, since any measurement of one particle of an entangled pair produces a completely random result. After performing such a measurement, an experimenter learns nothing more than the probabilities of the outcome of a possible measurement on the other entangled particle conducted at a distant location by a colleague. Reality may be non-local, allowing faster-than-light