Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [165]
In early February 1937, Bohr arrived in Princeton, together with his wife and their son Hans, for a week-long stay as part of a six-month world tour. It was the first opportunity that Einstein and Bohr had had to meet face-to-face since the publication of the EPR paper. Could Bohr finally convince Einstein to accept the Copenhagen interpretation? 'The discussion on quantum mechanics was not at all heated', recalled Valentin Bargmann, who later served as one of Einstein's assistants.62 'But to the outside observer, Einstein and Bohr were talking past each other.' Any meaningful discussion, he believed, required 'days and days'. Alas, during the encounter he witnessed, 'So many things were left unsaid'.63
What was left unsaid between them each man already knew. Their debate about the interpretation of quantum mechanics came down to a philosophical belief about the status of reality. Did it exist? Bohr believed that quantum mechanics was a complete fundamental theory of nature, and he built his philosophical worldview on top of it. It led him to declare: 'There is no quantum world. There is only an abstract quantum mechanical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.'64 Einstein, on the other hand, chose the alternative approach. He based his assessment of quantum mechanics on his unshakeable belief in the existence of a causal, observer-independent reality. Consequently he could never accept the Copenhagen interpretation. 'What we call science,' Einstein argued, 'has the sole purpose of determining what is.'65
For Bohr the theory came first, then the philosophical position, the interpretation constructed to make sense of what the theory says about reality. Einstein knew that it was dangerous to build a philosophical worldview on the foundations of any scientific theory. If the theory is found wanting in the light of new experimental evidence, then the philosophical position it supports collapses with it. 'It is basic for physics that one assumes a real world existing independently from any act of perception', said Einstein. 'But this we do not know.'66
Einstein was a philosophical realist and knew that such a position could not be justified. It was a 'belief' concerning reality that was not susceptible to proof. While that may be so, for Einstein 'it is existence and reality that one wishes to comprehend'.67 'I have no better expression than "religious" for confidence in the rational nature of reality insofar as it is accessible to human reason', he wrote to Maurice Solovine. 'Wherever this feeling is absent, science degenerates into uninspired empiricism.'68
Heisenberg understood that Einstein, and Schrödinger, wanted 'to return to the reality concept of classical physics or, to use a more general philosophic term, to the ontology of materialism'.69 The belief in an 'objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them', was for Heisenberg a throw-back to 'simplistic materialist views that prevailed in the natural sciences of the nineteenth century'.70 Heisenberg was only partly right when he identified that Einstein and Schrödinger wanted 'to change the philosophy without changing the physics'.71 Einstein accepted that quantum mechanics was the best theory available, but it was 'an incomplete representation of real things, although it is the only one which can be built out of the fundamental concepts of