Manjit Kumar.
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Here are some excerpts:
‘No more profound intellectual debate has ever been conducted’, claimed the scientist and novelist C.P. Snow. ‘It is a pity that the debate, because of its nature, can’t be common currency.’
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For more than half a century it had been universally accepted that light was a wave phenomenon. In ‘On a Heuristic Point of View Concerning the Production and Transformation of Light’, Einstein put forward the idea that light was not made up of waves, but particle-like quanta.
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In 1924, Einstein was still struggling to accept what he had unearthed: ‘I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will, not only its moment to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming-house, than a physicist.’
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In the two years after their meetings in Berlin and Copenhagen, Einstein and Bohr continued their individual struggles with the quantum. Both were beginning to feel the strain. ‘I suppose it’s a good thing that I have so much to distract me,’ Einstein wrote to Ehrenfest in March 1922, ‘else the quantum problem would have got me into a lunatic asylum.’
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They had to stop making compromises and cease trying to accommodate quantum concepts within the comfortable and familiar framework of classical physics. Physicists had to break free. The first to do so was Heisenberg when he pragmatically adopted the positivist credo that science should be based on observable facts, and attempted to construct a theory based solely on the observable quantities.
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During one discussion Schrödinger called ‘the whole idea of quantum jumps a sheer fantasy’. ‘But it does not prove that there are no quantum jumps’, Bohr countered. All it proved, he continued, was that ‘we cannot imagine them’. Emotions soon ran high.
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In December 1926, Einstein had expressed his growing disquiet at the rejection of causality and determinism in a letter to Born: ‘quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the “old one”. I, at any rate, am convinced that He is not playing at dice.’
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Heisenberg had discovered that quantum mechanics forbids, at any given moment, the precise determination of both the position and the momentum of a particle. It is possible to measure exactly either where an electron is or how fast it is moving, but not both simultaneously. It was nature’s price for knowing one of the two exactly.
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At first even the young quantum magician was uneasy with the Dane’s answers. ‘I remember discussions with Bohr which went through many hours till very late at night and ended almost in despair,’ Heisenberg wrote later, ‘and when at the end of the discussion I went alone for a walk in the neighbouring park I repeated to myself again and again the question: Can nature possibly be as absurd as it seemed to us in these atomic experiments?’86 Bohr’s answer was an unequivocal yes.
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After several days, Heisenberg later recalled, ‘Bohr, Pauli and I – knew that we could now be sure of our ground, and Einstein understood that the new interpretation of quantum mechanics cannot be refuted so simply’.56 But Einstein refused to yield.
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Just then, with a hint of smile, Einstein uttered the deadly words: weigh the box again. In a flash, Bohr realised that he and the Copenhagen interpretation were in deep trouble.
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In his desperation to destroy the Copenhagen view of quantum reality, Einstein had forgotten to take into account his own theory of general relativity. He had ignored the effects of gravity on the measurement of time by the clock inside the light box.
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It had taken the Nazis a matter of weeks to transform Göttingen, the cradle of quantum mechanics, from a great university to a second-rate institution. The Nazi minister of education asked David Hilbert, the most fêted mathematician in Göttingen, whether it was true ‘that your Institute suffered so much from the departure of the Jews and their friends?’ ‘Suffered? No, it didn’t suffer, Herr Minister’, replied Hilbert. ‘It just doesn’t exist any more.’
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In the years following Solvay 1930, there was little direct contact between Bohr and Einstein. A valuable channel of communication ceased with Paul Ehrenfest’s suicide in September 1933. In a moving tribute, Einstein wrote of his friend’s inner struggle to understand quantum mechanics and ‘the increasing difficulty of adaptation to new thoughts which always confronts the man past fifty. I do not know how many readers of these lines will be capable of fully grasping that tragedy.’
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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.
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Yet, in the end, he failed to refute Bohr’s Copenhagen interpretation. ‘About relativity he spoke with detachment, about quantum theory with passion’, recalled Abraham Pais, who had known Einstein in Princeton.4 ‘The quantum was his demon.’
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A poll conducted in July 1999 during a conference on quantum physics held at Cambridge University revealed the answers of a new generation to the vexed question of interpretation.34 Of the 90 physicists polled, only four voted for the Copenhagen interpretation, but 30 favoured the modern version of Everett’s many worlds.
