Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [171]
Bohm left Brazil in 1955 and spent two years at the Technion Institute in Haifa, Israel before moving to England. After four years at Bristol University, in 1961 Bohm settled once and for all in London after being appointed professor of theoretical physics at Birkbeck College. During his troubled time in Princeton, Bohm had largely devoted himself to studying the structure and interpretation of quantum mechanics. In February 1951 he published Quantum Theory, one of the first textbooks to examine in some detail the interpretation of the theory and the EPR thought experiment.
Einstein, Podolsky and Rosen had conjured up an imaginary experiment that involved a pair of correlated particles, A and B, so far apart that it should be impossible for them to physically interact with one another. EPR argued that a measurement carried out on particle A could not physically disturb particle B. Since any measurement is performed on only one of the particles, EPR believed they could cut off Bohr's counter-attack – an act of measurement causes a 'physical disturbance'. Since the properties of the two particles are correlated, they argued that by measuring a property of particle A, such as its position, it is possible to know the corresponding property of B without disturbing it. EPR's aim was to demonstrate that particle B possessed the property independently of being measured, and since this was something that quantum mechanics failed to describe, it was therefore incomplete. Bohr countered, never so succinctly, that the pair of particles were entangled and formed a single system no matter how far apart they were. Therefore, if you measured one, then you also measured the other.
'If their [EPR] contention could be proved,' wrote Bohm, 'then one would be led to search for a more complete theory, perhaps containing something like hidden variables, in terms of which the present quantum theory would be a limiting case.'6 But he concluded 'that quantum theory is inconsistent with the assumption of hidden causal variables'.7 Bohm looked at quantum theory from the prevailing Copenhagen viewpoint. However, in the process of writing his book he became dissatisfied with Bohr's interpretation, even as he agreed with the dismissal by others of the EPR argument as 'unjustified, and based on assumptions concerning the nature of matter which implicitly contradict the quantum theory at the outset'.8
It was the subtlety of the EPR thought experiment, and what he came to regard as the reasonable assumptions on which it was constructed, that led Bohm to question the Copenhagen interpretation. It was a brave step for a young physicist whose contemporaries were busy using quantum theory to make their reputations rather than risking career suicide by raking over the embers of a dying fire. But Bohm was already a marked man after his appearance before the House Un-American Activities Committee, and, suspended by Princeton, he had little left to lose.
Bohm presented Einstein with a copy of Quantum Theory and discussed his reservations with Princeton's most famous resident. Encouraged to examine the Copenhagen interpretation more closely, Bohm produced two papers that appeared in January 1952. In the first of these he publicly thanked Einstein 'for several interesting and stimulating discussions'.9 By then Bohm was in Brazil, but the papers had been written and sent to the Physical Review in July 1951, just four months after the publication of his book. Bohm appeared to have had a Paul-like conversion on the road not to Damascus, but Copenhagen.
In his papers Bohm outlined an alternative interpretation of quantum theory and argued that 'the mere possibility of such an interpretation proves that it is not necessary for us to give up a precise, rational, and objective description of individual