Copenhagen - Michael Frayn [35]
Questions of translation apart, Heisenberg’s choice of word suggests that, at the time he wrote his paper, he had not fully grasped the metaphysical implications of what he was saying. Indeed, he concludes that the experiments concerned are affected by Unbestimmtheit ‘purely empirically.’ He was not, as Bohr complained, at that time greatly interested in the philosophical fallout from physics and mathematics (though he became much more so later on in life), and he was publishing in a hurry, as Bohr also complained, before he had had a chance to discuss the work with either Bohr or anyone else. His paper seems to imply that electrons have definite orbits, even if these are unknowable; he talks about a quantum of light completely throwing the electron out of its ‘orbit’, even though he puts the word into inverted commas, and says that it has no rational sense here. The tide of the paper itself reinforces this impression: Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik. Again there are translation problems. ‘Anschaulich’ means graphic, concrete, ‘look-at-able’; the tide is usually translated as referring to the ‘perceptual’ content of the disciplines concerned, which again seems to suggest a contrast with their unperceived aspects—as if Heisenberg were concerned merely about our difficulties in visualising abstractions, not about the physical implications of this.
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The Copenhagen Interpretation of quantum mechanics has been scientific orthodoxy for most of this century, and is the theoretical basis (for better or worse) on which the century’s dramatic physical demonstrations of nuclear forces have been constructed. But it has not gone unchallenged. Einstein never accepted it, though he could never find a way round it. The mathematician Roger Penrose regards the present state of quantum theory as ‘provisional’, and quotes Schrödinger, de Broglie, and Dirac as forerunners in this view.
An alternative to the Copenhagen Interpretation, explaining the apparent superimposition of different states that appears at the quantum level in terms of a multiplicity of parallel worlds, was developed after the Second World War by Hugh Everett III, who had been a graduate student of John Wheeler, Bohr’s associate in the famous paper which opened the way to an understanding of uranium fission. David Deutsch, who proposes an extreme version of Everett’s ideas in his book The Fabric of Reality, claims that ‘hardly anyone’ still believes in the Copenhagen Interpretation. I have put this view to a number of physicists. They all seemed greatly surprised by it; but maybe I have hit upon precisely the supposed handful who remain in the faith.
Another follower of Everett (though he seems to differ quite sharply from Deutsch) is Murray Gell-Mann, who with Yuval Ne’eman revolutionised elementary particle theory in the sixties with the introduction of the quark, in its three different ‘colours’ and six different ‘flavours’, as the fundamental unit of the material world. Gell-Mann believes that quantum mechanics is the fundamental tool for understanding the universe, but he sees the Copenhagen Interpretation, with its dependence upon an observer and the human act of measurement, as anthropocentric, and as characterizing merely a special case that he calls ‘the approximate quantum mechanics of measured systems.’ I hesitate to express any reservations about something I understand so little, particularly when it comes from such an authority, but it seems to me that the view which Gell-Mann favours, and which involves what he calls alternative ‘histories’ or ‘narratives’, is precisely as anthropocentric as Bohr’s, since histories and narratives are not freestanding elements of the universe, but human constructs as subjective and as restricted in their viewpoint as the act of observation.
The relevance of indeterminacy to quantum