Monday, April 17, 2017

There is no quantum world

Physicist N. David Mermin
If you ask Google to search for “no quantum world,” you will get nearly 300 hits. They all give the following quotation (or recognizable corruptions of it):
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.
Over 90% of them attribute the statement to Niels Bohr, with phrases like “Bohr’s dictum ...,” “Bohr insisted that ...,” “Bohr proclaimed ...,” “Niels Bohr said, in a frequently quoted passage ...,” “Niels Bohr wrote [my emphasis] ...,” and even “Explain and evaluate Bohr’s philosophy of quantum theory with reference to his assertion ... .”
Mermin is unable to confirm that Bohr said exactly those words, and gets conflicting opinions about whether it correctly reflects what Bohr did say.

The respected philosophy professor Harvey R. Brown writes in a new paper:
Of course, for the post-1927 Einstein the wavefunction is, as we have seen, essentially a probability distribution over hidden ontic states; it is (at least) these ontic states that correspond to a "concept" that is "independent of experience", if we are "thinking physically". According to Einstein, orthodox quantum mechanics is incomplete precisely because it does not specify what such ontic states are. The idea that quantum physics can do without them altogether seems to me to be antithetical to Einstein’s program, metaphysically shy though it is.
Brown goes on to criticize those who claim to have some interpretation of quantum mechanics consistent with the above Einsteinian realism, such as the version of QBism by Fuchs.

Lubos Motl has a new rant against John Preskill largely for having some of those same Einsteinian views about quantum reality.

Bohr's quote that there is no quantum world means that there are no Einsteinian hidden ontic states. It is a firmly positivist view. The XX century has proved the Bohr view to be correct (whether he said it exactly that way or not).

Update: A comment says of Bohr:
He invariably made ontologically neutral statements, such as "The formalism is to be regarded as a tool for deriving predictions, of definite or statistical character...".
That is how I interpreted "there is no quantum world", but I guess two views are possible: (1) there is no underlying ontology; or (2) quantum mechanics does not rely on an underlying ontology. The latter is the more positivist view, and presumably Bohr's view as well.

The possibility of these two interpretations may explain why Mermin had trouble confirming the quote.

Positivism means talking about what the theory can do for you. Positivists avoid metaphysical speculations. So a positivist would not say that there can be no other description of the world. I take Bohr's statement as a statement about quantum theory, and not a denial that other theories are possible. Quantum mechanics does not have a world of hidden variables with values for unobserved physical quantities.

1 comment:

  1. The Bohr "quote" about "no quantum world" actually came from Aage Peterson, who recalled that Bohr said that to him in private conversation. So it is a second-hand quote, and we don't know how accurate it is. In Bohr's direct quotes, he was actually very careful to avoid making any definite assertions about whether there is an objective reality underlying our (or his) observations. He invariably made ontologically neutral statements, such as "The formalism is to be regarded as a tool for deriving predictions, of definite or statistical character...". His philosophy of complementarity is unclear on whether there is some external objective reality underlying the observations. We do know, however, that Bohr did not espouse solipsism, i.e., he was not a pure positivist.

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