Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [162]
Their letters had crossed in the post. Two days after having written his, Einstein received Schrödinger's on the EPR paper and replied immediately. 'What I really intended has not come across very well,' Einstein explained, 'on the contrary the main point was, so to speak, buried by erudition.'48 The EPR paper written by Podolsky lacked the clarity and style that characterised Einstein's published work in German. He was unhappy that the fundamental role of separability, that the state of one object cannot depend upon the kind of measurement made on another spatially separated object, had been obscured in the paper. Einstein wanted the separation principle to be an explicit feature of the EPR argument and not as it appeared, on the last page, as some sort of afterthought. He wanted to draw out the incompatibility of separability and the completeness of quantum mechanics. Both could not be true.
'The actual difficulty lies in the fact that physics is a kind of metaphysics', he told Schrödinger; 'physics describes reality; we know it only through its physical description.'49 Physics was nothing less than a 'description of reality', but that description, Einstein wrote, 'can be "complete" or "incomplete"'. He attempted to illustrate what he meant by asking Schrödinger to imagine two closed boxes, one of which contains a ball. Opening the lid of a box and looking inside is 'making an observation'. Prior to looking inside the first box, the probability that it contains the ball is ½, in other words there is a 50 per cent chance that the ball is inside the box. After the box is opened, there is either a probability of 1 (the ball is in the box) or 0 (the ball is not in the box). But, says Einstein, in reality the ball was always in one of the two boxes. So, he asks, is the statement 'The probability is ½ that the ball is in the first box' a complete description of reality? If no, then a complete description would be 'The ball is (or is not) in the first box'. If before the box is open is deemed to be a complete description, then such a description would be 'The ball is not in one of the two boxes'. The ball's existence in a definite box occurs only when one of the boxes is opened. 'In this way arises the statistical character of the world of experience or its empirical systems of laws', concluded Einstein. So he poses the question, is the state before the box is opened completely described by the pro ability ½?
To decide, Einstein brought in the 'separation principle' – the second box and its contents is independent of anything that happens to the first box. Therefore, according to him, the answer is no. Assigning the probability of ½ that the first box contains the ball is an incomplete description of reality. It was Bohr's violation of Einstein's separation principle that resulted in the 'spooky action at a distance' in the EPR thought experiment.
On 8 August 1935, Einstein followed up his ball-in-the-box with a more explosive scenario to demonstrate to Schrödinger the incompleteness of quantum mechanics because the theory could only offer probabilities where there was certainty. He asked Schrödinger to consider a keg of unstable gunpowder that spontaneously combusts at some time during the next year. At the beginning the wave function describes a well-defined state – a keg of unexploded gunpowder. But after a year the wave function 'describes a sort of blend of not-yet and of already-exploded systems'.50 'Through no art of interpretation can this wave-function be turned into an adequate description of a real state of affairs,' Einstein told Schrödinger, '[for] in reality