Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [156]
On Saturday, 4 May 1935, the New York Times carried an article on page eleven under the attention-grabbing headline 'Einstein Attacks Quantum Theory': 'Professor Einstein will attack science's important theory of quantum mechanics, a theory of which he was a sort of grandfather. He concluded that while it is "correct" it is not "complete".' Three days later, the New York Times carried a statement from a clearly disgruntled Einstein. Although no stranger to talking to the press, he pointed out that: 'It is my invariable practice to discuss scientific matters only in the appropriate forum and I deprecate advance publication of any announcement in regard to such matters in the secular press.'11
In the published paper, Einstein, Podolsky and Rosen started by differentiating between reality as it is and the physicist's understanding of it: 'Any serious consideration of a physical theory must take into account the distinction between the objective reality, which is independent of any theory, and the physical concepts with which the theory operates. These concepts are intended to correspond with the objective reality, and by means of these concepts we picture this reality to ourselves.'12 In gauging the success of any particular physical theory, EPR argued that two questions had to be answered with an unequivocal 'Yes': Is the theory correct? Is the description given by the theory complete?
'The correctness of the theory is judged by the degree of agreement between the conclusions of the theory and human experience', said EPR. It was a statement that every physicist would accept when 'experience' in physics takes the form of experiment and measurement. To date there had been no conflict between the experiments performed in the laboratory and the theoretical predictions of quantum mechanics. It appeared to be a correct theory. Yet for Einstein it was not enough for a theory to be correct, in agreement with experiments; it also had to be complete.
Whatever the meaning of the term 'complete', EPR imposed a necessary condition for the completeness of a physical theory: 'every element of the physical reality must have a counterpart in the physical theory.'13 This completeness criterion required EPR to define a so-called 'element of reality' if they were to carry through their argument.
Einstein did not want to get stuck in the philosophical quicksand, which had swallowed so many, of trying to define 'reality'. In the past, none had emerged unscathed from an attempt to pinpoint what constituted reality. Astutely avoiding a 'comprehensive definition of reality' as 'unnecessary' for their purpose, EPR adopted what they deemed to be a 'sufficient' and 'reasonable' criterion for designating an 'element of reality': 'If, without in any way disturbing a system, we can predict with certainty (i.e. with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.'14
Einstein wanted to disprove Bohr's claim that quantum mechanics was a complete, fundamental theory of nature by demonstrating that there existed objective 'elements of reality' which the theory did not capture. Einstein had shifted the focus of the debate with Bohr and his supporters away from the internal consistency of quantum mechanics to the nature of reality and the role of theory.
EPR asserted that for a theory to be complete there had to be one-to-one correspondence between an element of the theory and an element of reality. A sufficient condition for the reality of a physical quantity, such as momentum, is the possibility