Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [158]
EPR were aware of the possible counter-argument that 'two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted'.16 This, however, made the reality of the momentum and position of particle B dependent upon the process of measurement carried out on particle A, which could be light years away and which does not disturb particle B in any way. 'No reasonable definition of reality could be expected to permit this', said EPR.17
Central to the EPR argument was Einstein's assumption of locality – that some mysterious, instantaneous action-at-a-distance does not exist. Locality ruled out the possibility of an event in a certain region of space instantaneously, faster-than-light, influencing another event elsewhere. For Einstein, the speed of light was nature's unbreakable limit on how fast anything could travel from one place to another. For the discoverer of relativity it was inconceivable for a measurement on particle A to affect instantaneously, at a distance, the independent elements of physical reality possessed by particle B.
As soon as the EPR paper appeared, the alarm was raised among the leading quantum pioneers throughout Europe. 'Einstein has once again made a public statement about quantum mechanics, and even in the issue of Physical Review of May 15 (together with Podolsky and Rosen, not good company by the way)', wrote a furious Pauli in Zurich to Heisenberg in Leipzig.18 'As is well known,' he continued, 'that is a disaster whenever it happens.' Pauli nevertheless conceded, as only he could, 'that if a student in one of his earlier semesters had raised such objections, I would have considered him quite intelligent and promising'.19
With the zeal of a quantum missionary, Pauli urged Heisenberg to publish an immediate rebuttal to prevent any confusion or wavering among fellow physicists in the wake of Einstein's latest challenge. Pauli admitted that he had considered, for 'educational' reasons, 'squandering paper and ink in order to formulate those facts demanded by quantum theory which cause Einstein particular intellectual difficulties'.20 In the end it was Heisenberg who drafted a reply to the EPR paper and sent Pauli a copy. But Heisenberg withheld the publication of his paper, as Bohr had already taken up arms in defence of the Copenhagen interpretation.
The EPR 'onslaught came down upon us as a bolt from the blue', recalled Léon Rosenfeld, who was in Copenhagen at the time.21 'Its effect on Bohr was remarkable.' Immediately abandoning everything else, Bohr was convinced that a thorough examination of the EPR thought experiment would reveal where Einstein had gone wrong. He would show them 'the right way to speak about it'.22 Excitedly, Bohr started dictating to Rosenfeld the draft of a reply. But soon he began to hesitate. 'No, this won't do, we must try all over again', Bohr mumbled to himself. 'So it went on for a while, with growing wonder at the unexpected subtlety of the [EPR] argument', recalled Rosenfeld. 'Now and then, he would turn to me and ask: "What can they mean? Do you understand it?"'23 After a while, an increasingly agitated Bohr realised that the argument Einstein had deployed was both ingenious and subtle. A refutation of the EPR paper would be harder than he first thought, and he announced that he 'must sleep on it'.24 The next day he was calmer. 'They do it smartly,' he told Rosenfeld, 'but what counts is to do it right.'25 For the next six weeks, day and night, Bohr worked on nothing else.
Even before he had finished his reply to EPR, Bohr wrote a letter on 29June for publication in the journal Nature. Entitled 'Quantum Mechanics and Physical Reality', it briefly spelled out his counter-attack.26 Once again, the New York Times smelt a story. 'Bohr and Einstein at Odds/ They Begin a Controversy Concerning the Fundamental Nature of Reality' were the headlines of the article that appeared on 28 July. 'The Einstein-Bohr controversy has just begun this week in