Monday, February 13, 2023

There is no quantum world

Jeffrey Bub reviews some recent popular books on quantum mechanics.
John Bell’s status in our field has the same [like Isaac Newton, James Watson, and Linus Pauling] mythic quality. Before him there was nothing, only the philosophical disputes between famous old men. He showed that the field contained physics, experimental physics, and nothing has been the same since.
Some do say this, but it is crazy. All Bell did was to show that the predictions of quantum mechanics differ from a classical theory of local hidden variables. As what everyone believed anyway.
In several places Becker invokes the quote, ‘there is no quantum world,’ commonly attributed to Bohr (Becker, p. 14):
What does quantum physics tell us about the world? According to the Copenhagen interpretation this question has a very simple answer: quan- tum mechanics tells us nothing whatsoever about the world. . . . According to Bohr, there isn’t a story about the quantum world because ‘there is no quantum world. There is only an abstract quantum physical description.’
The ‘no quantum world’ comment is actually a quote from Bohr’s assistant Aage Petersen,17 who recounts Bohr saying this sort of thing. Bohr probably did make provocative statements along these lines in discussion, but he certainly did not mean that there is simply nothing there, as Becker seems to suggest.

What could Bohr have meant? Here’s my take on it. Quantum mechanics replaces the commutative algebra of physical quantities of a classical system with a noncommutative algebra of ‘observables.’ This is an extraordinary move, quite unprecedented in the history of physics, and arguably requires us to re-think what counts as an acceptable explanation in physics.

Aage Petersen, ‘The Philosophy of Niels Bohr,’ Bulletin of the Atomic Scientists 19, 8–14 (1963). The quote is on p. 12: ‘When asked whether the algorithm of quantum mechanics could be considered as some- how mirroring an underlying quantum world, Bohr would answer, “There is no quantum world. There is only an abstract quantum physical 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.”
Maybe Bohr meant that quantum mechanics is not a big disguise for a theory of hidden variables, as those pushing "realism" often suggest.
As Bell points out,20 Bohm’s theory involves action at a distance at the level of the hidden variables: ‘an explicit causal mechanism exists whereby the disposition of one piece of apparatus affects the results obtained with a distant piece,’ so that ‘the Einstein-Podolsky-Rosen paradox is resolved in the way which Einstein would have liked least.’ The problem of making sense of probability in an Everettian universe, where everything that can happen does happen in some world, is still a contentious issue.
Yes, these are fatal flaws to the Bohm and Everett theories.
the question of completeness dominated the debates between Bohr and Einstein. What Einstein had in mind was that something was left out of the quantum theory, which, if added to the theory, would restore the sort of ‘Anschaulichkeit’ characteristic of classical theories. ...

The question of ‘Anschaulichkeit’ morphed into a debate about the possibility of a realist interpretation of quantum mechanics, with the dissidents accusing the Copen- hagenists of the sin of positivism or instrumentalism, which by the 1960s had lost much of its appeal among philosophers.

As the review explains, when Einstein said completeness, he really meant commutativity, not determinism.

Yes, philosophers abandoned positivism for silly reasons, but why did physicists? Quantum mechanics is best understood as a positivist theory. So is relativity and other Physics theories. Quantum mechanics was explicitly positivist, before Bohm, Einstein, Everett, Bell, and others ruined it.

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