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Quantum Decoherence and deBroglie-Bohm theory

  1. Dec 25, 2014 #1

    ShayanJ

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    From time to time I hear that Quantum Decoherence can address the measurement problem only when accompanied by deBroglie-Bohm theory(dBB) (or any hidden variable theory? or MWI?). I wanna know why is that?
    Also, I saw some papers recently(e.g. this) that prove dBB is incompatible with QM. There is also this paper which suggests an experiment for testing dBB against QM and says that the results support QM and contradict dBB's predictions. So it seems at least dBB is in serious danger, or maybe even ruled out.(Ideas on this?)
    Does this mean that Quantum Decoherence is useless now?(Of course if dBB is actually ruled out!)
     
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  3. Dec 25, 2014 #2

    atyy

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    That is not true. The correct statement is that decoherence does not solve the measurement problem, unless accompanied by additional assumptions. One possible set of additional assumptions are those of the de Broglie Bohm theory.
     
  4. Dec 25, 2014 #3

    ShayanJ

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    Is there a set of additional assumptions that can somehow be called Copenhagen? I mean, can we keep Copenhagen interpretation and solve measurement problem using decoherence? Or maybe use some extended or modified Copenhagen interpretation?
     
    Last edited: Dec 25, 2014
  5. Dec 25, 2014 #4

    atyy

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    That would be a bit unusual, but names of interpretations are a matter of convention, so you could call de Broglie-Bohm theory "Copenhagen" if you'd like.

    My own usage is that Copenhagen is the default interpretation of quantum mechanics, and is the interpretation in which the measurement problem is usually stated. The key point of Copenhagen is that there is a classical/quantum cut (or apparatus/system cut), and the wave function is not necessarily real, and a tool to calculate the probabilities of measurement outcome. The measurement problem enters with the classical/quantum cut.

    Decoherence alone cannot solve the measurement problem, because the measurement problem is the lack of definite outcomes when the wave function is extended to the whole universe. Decoherence requires a division of the universe into subsystems, and a restriction of observables to a subsystem. There is no decoherence of the overall wave function of the universe, so if we have a wave function of the universe it seems that nothing happens (or everything does if the many-worlds approach is attempted).
     
    Last edited: Dec 25, 2014
  6. Dec 25, 2014 #5

    ShayanJ

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    I don't mean that. Of course Copenhagen interpretation is not only about that cut. I mean can we keep all things from Copenhagen interpretation and just replace that cut with Decoherence and have a solution of measurement problem?
     
  7. Dec 25, 2014 #6

    atyy

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    I added a bit to the post above to explain why decoherence alone does not solve the measurement problem. The basic problem is that there is no decoherence of the wave function of the universe. So decoherence doesn't allow the cut to be removed.
     
  8. Dec 25, 2014 #7

    bhobba

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    Decoherence only explains apparent collapse, by which is meant how a superposition becomes an improper mixed state. If it was a proper one actual collapse would have occurred. Explaining how an improper mixed state becomes a proper one is the modern version of the measurement problem, also sometimes referred to as the problem of why we actually get any outcomes at all. There are a few other issues such as the preferred basis problem and the factorisation problem but its the biggie. In DBB the problem is trivial - you get outcomes because the particle has an actual position and momentum so the improper mixed state is a proper one.

    I wouldn't say its the only way though.

    I will leave the other questions to those that know DBB better than me.

    Thanks
    Bill
     
  9. Dec 25, 2014 #8
    From reading the summary of that paper on the arXiv website, it appears the experiment was performed and the results reported in the paper you link. If this were the case, then dBB would be refuted. However that paper was published in 2002 and we still see discussions on dBB. So maybe there was some challenges to the experiment and the authors conclusion.
     
  10. Dec 25, 2014 #9

    ShayanJ

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    That always bothers me. How can we know how's the situation now?

    Thanks all
     
  11. Dec 26, 2014 #10

    atyy

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    Ghose's paper has been commented on by:

    http://arxiv.org/abs/quant-ph/0108038
    http://arxiv.org/abs/quant-ph/0101132
    http://arxiv.org/abs/quant-ph/0302085
    http://arxiv.org/abs/quant-ph/0305131

    Edit: There are, however, some holes to be filled to be certain that dBB completely solves the measurement problem, http://arxiv.org/abs/0712.0149: "On the technical side, for DBB to solve the measurement problem we require that the corpuscles track the decohered macroscopic degrees of freedom of large systems. ... Further plausibility arguments have been constructed (e. g. Bell (1981b, section 4), Holland (1993, pp. 336–50), D¨urr, Goldstein, and Zanghi (1996, pp. 39–41), and some simple models have been studied; at present, it seems likely that the corpuscles do track the quasiclassical trajectories sufficiently well for DBB to solve the measurement problem, but there exists no full proof of this."

    I edited this post while Shyan was replying to it.
     
    Last edited: Dec 26, 2014
  12. Dec 26, 2014 #11

    ShayanJ

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    But these are all for over ten years ago!!! Its hard to think nothing has changed since then! But if we're forced to think like that, that only makes me disregard dBB even more than before!
     
  13. Dec 26, 2014 #12

    atyy

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    Why? If the Ghose paper was in error 10 years ago, why would it now become correct?
     
  14. Dec 26, 2014 #13

    vanhees71

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    How can there be a wave function of the entire universe? There are interacting particles in the universe, and it's clearly relativistic. So how can there exist a wave function as a description of the universe's state, if wave functions aren't suitable for two interacting particles?

    If anything as a (pure?) quantum state of the entire universe exists it cannot be described as a single wave function. Despite this technical problem I've also a principle problem with the idea of a quantum state of the entire universe: How can you ever observe the predictions made by associating such a state to the whole universe? You cannot prepare many universes and make experiments with them. So how do you figure out whether the probabilistic physics content of the wave function describes the statistics of an ensemble of universes correctly if there's no possiblity to prepare many universes in this state. I think the notion of a quantum state of the entire universe is a pretty empty idea without any testable consequences and thus unscientific.
     
  15. Dec 26, 2014 #14

    ShayanJ

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    No, that's not what I mean. I mean why can't we find recently published papers on dBB? I searched but all papers I found are old, not newer than these papers. Of course dBB has many problems to be solved and the fact than there is no(or maybe little) recently published papers on it, means people aren't working on it. It seems that they just wait for someone to criticize it and they answer!!! It seems like a dead theory.
    Also, I should confess I'm looking forward to see dBB ruled out. Of course not because of physical reasons because I'm in no way a physicist. Its just that I like QM as it is. de-Broglie and Bohm just spoil the fun!:D
     
  16. Dec 26, 2014 #15

    atyy

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    In a sense it is good that dBB is dead, since that means most of its problems are solved. The remaining problems for dBB applied to non-relativistic QM are as difficult as the remaining problems of the foundations of classical statistical mechanics. I confess I did not believe dBB was viable until I found out about Valentini's work. I don't think you can like QM the way it is as a final theory, since there truly is a measurement problem. And it is important to understand that the point of dBB is not that it is the correct theory of nature, but rather that it shows that the measurement problem can be solved. And yes, I too prefer an epistemic understanding of the wave function. Unfortunately, nature doesn't care about what I like. I think there is still hope for an epistemic understanding of the wave function even within dBB.

    Anyway, the major problem in dBB is the formulation of the theory for chiral fermions interacting with non-abelian gauge fields. So you can think of all papers about chiral fermions interacting with non-abelian gauge fields as secretly dBB papers (ok, maybe I went too far there, but it is my interpretation of the literature :D). Anyway, that means that these are very recent dBB papers:
    http://arxiv.org/abs/0908.0591
    http://arxiv.org/abs/0912.2560
    http://arxiv.org/abs/1401.6655
     
  17. Dec 26, 2014 #16

    vanhees71

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    Objection! Approximate chiral symmetry of the light-quark sector in QCD is not secretly on de Broglie Bohm. In my field, I guess there's nobody believing in or even thinking about dBB at all. Why should we? It's just a very important tool to build effective theories of hadrons with some foundation in the standard model. (Untarized) chiral perturbation theory is thus imply a tool to get good phenomenological models for strongly interacting particles/matter at low energies, where (perturbative) QCD is not applicable, and this approximate symmetry pretty much rules the dynamics of hadrons. That's why it's also important for lattice-QCD people to find a good description of chiral fermions. They also don't seem to be thinking about dBB at all.
     
  18. Dec 26, 2014 #17

    ShayanJ

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    Of course I didn't mean I like QM as a final theory. The point is, people mostly like dBB because it preserves(at least partly) the classical nature of physical laws. In fact because of Bell's theorem, it seems that most classical-like interpretation of QM should be something like dBB because non-locality is inescapable. But I just don't see the reason why so many people should be so insistent that nature is as people thought before QM. I think its really more like it that nature behaves as standard QM says, its somehow more like it, that nature doesn't waste "piles and piles of paper" to store information that she actually can do without them. It more beautiful that she actually doesn't. Of course nature doesn't have to look as I prefer, so no one has the right to stop people working on dBB. Its just that I don't like it.
    And about those papers. I can't see how those can be related to dBB. Because they can do everything in the context of standard QM and don't bother with dBB. So even if they succeed, they may do it somehow that doesn't do a favuor to dBB.
    Also dBB doesn't seem to be able to make a good marriage with SR too. I know people had some attempts but things don't seem to fit nicely.
    And quantum fluctuations. How can dBB account for particles coming from nothing when it doesn't accept uncertainty principles as they are in standard QM?
     
  19. Dec 26, 2014 #18

    atyy

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    Ha, ha! Yes, I was pretty sure I went too far there. :oldlaugh: Anyway, the good thing is that if you guys actually solve your problem, you will have also solved the major problem of dBB. (Hmm, do you work on the lattice? I thought you mainly used non-lattice methods?)
     
  20. Dec 26, 2014 #19

    atyy

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    The point of dBB is not the classical nature of physical laws. The point is why do we have a difficulty with the notion of the wave function of the universe, unless hidden variables are introduced? Without hidden variables, it seems we cannot have a wave function of the universe, and we always need an observer who is not in the wave function.

    Also, dBB is not against the idea that QM is beautiful because it does not waster piles and piles of paper. It is really the possibility of hidden variables (motivated by the measurement problem) that can make the notion of QM as a superb effective theory precise. For example, http://arxiv.org/abs/0711.4770 shows how much simpler QM is compared to a hidden variable theory. The situation is like string theory (let's assume it is correct for the sake of argument) and general relativity - GR is incomplete compared to string theory, but GR doesn't waste the piles and piles of paper that string theory does.

    Those papers try to formulate the relativistic standard model of particle physics as non-relativistic quantum mechanics, for which dBB is probably able to solve the measurement problem.
     
  21. Dec 26, 2014 #20

    vanhees71

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    No, I'm not a lattice guy. I'm in the real-time QFT and transport business as well as phenomenological models in heavy-ion physics. Of course, lattice QCD (particularly finite temperature) delivers important input into these models too (e.g., the equation of state of strongly interacting matter).
     
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