Realists on the counter attack

Martin Daumer, Detlef Duerr, Sheldon Goldstein, Tim Maudlin, Roderich Tumulka and Nino Zanghi, a collection of scholars noted for their advocacy or realist interpretations of quantum mechanics, and Bohmian mechanics in particular, have posted an article on quant-ph that attacks the idea that quantum theory is “fundamentally about information”. The article is a response to a recent essay in Nature by Anton Zeilinger, and is mainly a criticism of his particular viewpoint.

Most of their argument is based on the fact that interpretations like Bohmian mechanics offer a clear counterexample to various claims, such as that QM shows nature is fundamentally indeterministic and that the Bell and Kochen-Specker no-go theorems rule out realism. I think this is all fair enough, and I agree that it is well worth taking the time to become familiar with the Bohm interpretation if one is at all interested in foundations. It is quite amazing how often it can be used as an example to clear up confusion and misunderstandings about what we can infer from QM. On the other hand, this is a far cry from saying that Bohmian mechanics should be taken seriously as a description of reality. There are several arguments against doing so, which would take too long to go into right now. Perhaps I will do so in another post when I have more free time.

In any case, Zeilinger’s Nature essay seems a rather easy target to me. It was a short article, and there was clearly not enough space for any detailed arguments. Whether or not you think that Zeilinger in fact has any compelling arguments, there are many other contemporary approaches that also claim QM is about “information” in some sense, and it would be good to see a more in depth response to all of these from the realist camp. Examples include the quantum Bayesianism of Caves, Fuchs and Schack; the axiomatic approach of Bub, Clifton and Halvorson; and Hardy’s axiomatics.

Those of you who are waiting for Rovellifest 2 – fear not, for it is coming within the next week or so. For now, I feel like I need to write something on a topic I feel positive about, to aviod this blog descending into a sea of negative criticisms.

7 responses to “Realists on the counter attack

  1. … this is a far cry from saying that Bohmian mechanics should be taken seriously as a description of reality. There are several arguments against doing so, which would take too long to go into right now.

    I am very much interested in the foundations of QM. Could you point me to some relevant books/papers regarding the thing you quoted? For I thought that Bohmian mechanics was consistent with (most) experimental observations?

  2. Well, I was talking about my own personal reasons for not believing the theory, which I’ll go into in another post sometime. They are more philosophical than physical, in that I don’t think that there is an experimental test to distinguish BM from QM. I just have a different take on what we should expect to get out of a realist interpretation, and BM doesn’t meet those criteria at present.

    In fact, there have been a number of attempts to find a testable contradiction with QM. These mostly concern “empty waves” and usually rely on unfounded assuptions about Bohmian mechanics.

    To be fair, I do think it is one of the more sane interpretations on the table. It also now has a couple of different reasonable extensions to QFT, which have been developed in the last few years. This was one of the biggest outstanding problems for the theory.

  3. David Booth

    A brief 2 questions. I was under the impression that where BM agreed with QM they both agreed with experiment and that where BM and QM disagree only QM agrees with experiment.
    1. Did I remember this correctly?
    2. If so, why should we consider BM at all?
    Thank you.

  4. I think it’s safe to say that BM and QM agree with all known experimental predictions. In fact, it should not be possible to distinguish BM and QM via any experiment (with one small caveat described below). The point of BM is that it is designed to agree with the predictions of QM, whenever those are unambiguous.

    Why consider BM at all? Well, I’d argue that it gives us insight into what a hidden variable theory has to look like to reporduce QM and it is a good source of counter-examples for a lot of gumph that people claim about quantum theory. Advocates would argue that it gives one a clear picture of what is going on in the world, and avoids the ubiquitous measurement-talk of standard interpretations. On the other hand, the postulation of degrees of freedom that can’t necessarily be directly observed is a good reason for being skeptical in my opinion, although that’s only a cartoon sketch of my reasoning.

    For BM to agree with QM something called the “equilibrium hypothesis” has to be true. If we drop this hypothesis then BM is more general than QM, and makes different predictions. This is very much like the difference between equilibrium and non-equilibrium statistical mechanics. However, not observing these discrepancies is no reason for rejecting BM, since we may be in an analog of the “heat death” in which only equilibrium states exist. I don’t have time to go into this now – maybe in another post.

  5. David Booth

    Thank you for the discussion. I traced my original source and found that I had remembered it incorrectly.
    D. B.

  6. Bub Jeffery argues that quantum mechanics is fundamentally a theory about the representation and manipulation of information, not a theory about the mechanics of nonclassical waves or particles.

  7. That should be Jeffrey Bub. It is true that his arguments, and those of others such as Alexei Grinbaum and Chris Fuchs, are rather more sophisticated than Zeilinger’s. Still, I don’t think the Bohmian camp would find much to agree with in what they say.

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