Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [159]
Bohr had deliberately chosen the same forum as Einstein, and his six-page response, received on 13 July, was also entitled 'Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?'27 Published on 15 October, Bohr's answer was an emphatic 'Yes'. However, unable to identify any error in the EPR argument, Bohr was reduced to arguing that Einstein's evidence for quantum mechanics being incomplete was not strong enough to bear the weight of such a claim. Using a debating tactic with a long and illustrious history, Bohr began his defence of the Copenhagen interpretation by simply rejecting the major component of Einstein's case for incompleteness: the criterion of physical reality. Bohr believed that he had identified a weakness in the EPR definition: the need to conduct a measurement 'without in any way disturbing a system'.28
Bohr hoped to exploit what he described as an 'essential ambiguity when it is applied to quantum phenomena' of the reality criterion, as he publicly retreated from the position that an act of measurement resulted in an unavoidable physical disturbance. He had relied on disturbance to undermine Einstein's previous thought experiments by demonstrating that it was impossible to know simultaneously the exact momentum and position of a particle because the act of measuring one caused an uncontrollable disturbance that ruled out an exact measurement of the other. Bohr knew perfectly well that EPR did not seek to challenge Heisenberg's uncertainty principle, since their thought experiment was not designed to simultaneously measure the position and momentum of a particle.
Bohr acknowledged as much when he wrote that in the EPR thought experiment 'there is no question of a mechanical disturbance of the system under investigation'.29 It was a significant public concession, one he had made in private a few years earlier as he, Heisenberg, Hendrik Kramers and Oskar Klein sat around the fire at his country cottage in Tisvilde. 'Isn't it odd,' said Klein, 'that Einstein should have such great difficulties in accepting the role of chance in atomic physics?'30 It is because 'we cannot make observations without disturbing the phenomena', said Heisenberg; 'the quantum effects we introduce with our observation automatically introduce a degree of uncertainty into the phenomenon to be observed.'31 'This Einstein refuses to accept, although he knows the facts perfectly well.' 'I don't entirely agree with you', Bohr told Heisenberg.32 'In any case,' he continued, 'I find all such assertions as "observation introduces uncertainty into the phenomenon" inaccurate and misleading. Nature has taught us that the word "phenomenon" cannot be applied to atomic processes unless we also specify what experimental arrangement or what observational instruments are involved. If a particular experimental set up has been defined and a particular observation follows, then we can admittedly speak of a phenomenon, but not of its disturbance by observation.'33 Yet before, during, and after the Solvay conferences, an act of measurement disturbing the observed object peppered Bohr's writings and was central to his dismantling of Einstein's thought experiments.
Feeling the pressure from Einstein's continued probing of the Copenhagen interpretation, Bohr abandoned his previous reliance on 'disturbance' because he knew that it implied that an electron, for example, existed in a state that could be disturbed. Instead, Bohr now emphasised that any microphysical object being measured and the apparatus doing the measuring formed an indivisible whole – the 'phenomenon'. There simply was no room for a physical disturbance due to an act of measurement. This was why Bohr believed the EPR reality criterion was ambiguous.