Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [161]
The EPR paper expressed Einstein's view that the Copenhagen interpretation of quantum theory and the existence of an objective reality were incompatible. He was right and Bohr knew it. 'There is no quantum world. There is only an abstract quantum mechanical description', argued Bohr.40 According to the Copenhagen interpretation, particles do not have an independent reality, they do not possess properties when they are not being observed. It was a view that was later concisely summarised by the American physicist John Archibald Wheeler: no elementary phenomenon is a real phenomenon until it is an observed phenomenon. A year before EPR, Pascual Jordan took the Copenhagen rejection of an observer-independent reality to its logical conclusion: 'We ourselves produce the results of measurement.'41
'Now we have to start all over again,' said Paul Dirac, 'because Einstein proved that it does not work.'42 He initially believed that Einstein had delivered a fatal blow against quantum mechanics. But soon, like most physicists, Dirac accepted that Bohr had once more emerged victorious from a battle with Einstein. Quantum mechanics had long proved its worth, and few were interested in examining Bohr's reply to the EPR argument too closely, for it was obscure even by his own standards.
Shortly after the EPR paper appeared in print, Einstein received a letter from Schrödinger: 'I was very happy that in the paper just published in P.R. you have evidently caught dogmatic q.m. by the coat-tails.'43 After offering an analysis of some of the finer points of the EPR paper, Schrödinger explained his own reservation concerning the theory he had done so much to create: 'My interpretation is that we do not have a q.m. that is consistent with relativity theory, i.e. with a finite transmission speed of all influences. We have only the analogy of the old absolute mechanics … The separation process is not at all encompassed by the orthodox scheme.'44 As Bohr struggled to formulate his response, Schrödinger believed that the central role of separability and locality in the EPR argument meant that quantum mechanics was not a complete description of reality.
In his letter Schrödinger used the term 'verschränkung', later translated into English as 'entanglement', to describe the correlations between two particles that interact and then separate, as in the EPR experiment. He accepted, like Bohr, that having interacted, instead of two one-particle systems, there was just a single two-particle system and therefore any changes to one particle would affect the other, despite the distance that separated them. 'Any "entanglement of predictions" that takes place can obviously only go back to the fact that the two bodies at some earlier time formed in a true sense one system, that is were interacting, and have left behind traces on each other', he wrote in a famous paper published later in the year.45 'If two separated bodies, each by itself known maximally, enter a situation in which they influence each other, and separate again, then there occurs regularly that which I have just called entanglement of our knowledge of the two bodies.'46
Although he did not share Einstein's intellectual and emotional commitment to locality, Schrödinger was not prepared to reject it. He put forward an argument for undoing the entanglement. Any measurement on either separated part A or B of an entangled two-particle state breaks the entanglement and both are once more independent of each other. 'Measurements on separated systems,' he concluded, 'cannot directly influence each other – that would be magic.'
Schrödinger must have been surprised when he read the letter, dated 17 June, that arrived from Einstein. 'From the point of view of principles,' he wrote, 'I absolutely do not believe in a statistical basis