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New experimental proof of wave-function collapse?

  1. Apr 8, 2015 #1

    gxu

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    The following experiment claims that it has demonstrated the wave-function collapse:

    http://www.nature.com/ncomms/2015/150324/ncomms7665/full/ncomms7665.html

    I would have no problem if they have claimed that, the experiment demonstrated the "non-local" (or: precisely quantum) steering effect. In my humble opinion, there is no logic to justify that "quantum steering effect is equivalent to the wave-function collapse". Here the wave-function collapse is defined in the strict Von-Neumann's postulate form.

    Do I miss something important here? I posted this question on the physics SE for discussion and got no response. So please let me know your input - thank you so much!
     
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  3. Apr 8, 2015 #2

    bhobba

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    For those interested here is the actual paper:
    http://arxiv.org/pdf/1412.7790v1.pdf

    I am not an experimental person, we have a number of those that post here, and hopefully they will comment on what it actually shows.

    I will simply comment from what QM actually says.

    First QM actually doesn't have collapse - that's an interpretational thing - some like GRW explicitly has it - others like MW and BM don't. Others like Ensemble are ambivalent to it being compatible with interpretations that have it and those that don't - but for simplicity most, including me, would say it doesn't. Others like most versions of Copenhagen have it but it means nothing since its purely subjective.

    So if they have actually observed collapse then they would have thrown out a whole heap of interpretations like BM and MW. That is highly doubtful since they were cooked up to be indistinguishable from standard QM.

    I think when someone who understands experimental stuff better than I do looks at it all it will do is demonstrate something that QM predicts independent of any interpretation and so cant demonstrate wave-function collapse.

    Added Later:
    I gave it a quick squiz. As far as I can see its simply verifying EPR. Interesting - but no big deal.

    Thanks
    Bill
     
  4. Apr 9, 2015 #3

    vanhees71

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    If it's simply demonstrating entanglement for position of two particles as in the EPR paper, there's no necessity to assume a collapse. In the minimal interpretation it's very simply explained without any collapse. It's the preparation in an entangled state, which implies long-range correlations. There's no spooky action at a distance caused by one local measurement on one part (at Alice's place) of the system on the other far-distant part (at Bob's place).

    I've to carefully look into the paper to say something specific about it, of course.

    BTW: I think, the experimental verification of this extremely non-classical predictions of quantum theory is still a very fascinating issue. Usually it's pretty simple to explain the theory, at least when simplified and idealized to the core principles, but still very surprising for our classically trained everyday experience and it delivers (sometimes highliest accurate) verifications of these "weirdest quantum properties" of nature! From an experimental point of view it's usually quite difficult to realize and an art of its own. So the experiments alone have a high esthetic appeal to me.
     
  5. Apr 9, 2015 #4

    Demystifier

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    You are right. They demonstrate non-locality, but not the collapse. For instance, their experiment can be explained by the Bohmian interpretation, which involves non-locality but not the collapse.

    I am sure that at least some of the authors (e.g. Wiseman) of this paper are very well aware of this. But then why do they claim such a non-sense in their paper? Because that sells the paper, especially in high-impact journals such as Nature or Science.
     
  6. Apr 9, 2015 #5

    vanhees71

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    Well, if you want to publish in Nature, you've to find a little "hype argument" [SCNR].
     
  7. Apr 9, 2015 #6

    atyy

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    However, can the minimal interpretation without collapse explain it if the measurements are considered in a frame in which they are not simultaneous?
     
  8. Apr 9, 2015 #7

    atyy

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    As far as I know, there is no known Bohmian interpretation of the standard model, so perhaps there is good reason not to consider the Bohmian interpretation as giving the same predictions as all of quantum mechanics?
     
  9. Apr 9, 2015 #8

    vanhees71

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    Sure, it simply doesn't matter in which order the meausrements are, as long as the local measurement at A doesn't influence the measurement at B. Particularly if the measurement events are space-like separated, there shouldn't be such an influence according to relativistic causality constraints. This is the core of the EPR paradox, if you postulate that the 100% correlation is due to the "collapse" due to A's measurement, which is before B's measurement in some frame and if A's and B's measurements are space-like separated events. This is the main reason, why I don't think that collapse is a very helpful notion in the interpretation of quantum theory but only causes trouble. Since it's fortunately not needed at all, I just don't use it in my thinking about quantum theory and its interpretation.

    The long-range correlations are due to the preparation procedure which is before each of these measurements are made, and this is a Poincare invariant notion, because the preparation procedure is in the past lightcone of both measurement events.
     
  10. Apr 9, 2015 #9

    Demystifier

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    Actually there is such a version of Bohmian interpretation, but Bohmians are not very interested in writing it down explicitly because it looks very ugly. Bohmians (like many other theorists) are motivated by theoretical beauty and elegance. (Which, of course, are subjective notions, so not all theorists agree that Bohmian theory is beautiful and string theory elegant).
     
  11. Apr 9, 2015 #10

    atyy

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    How can one see that a Bohmian standard model exists?
     
  12. Apr 9, 2015 #11

    vanhees71

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    Well, the Bohmian interpretation looks ugly to me already for non-relativistic quantum theory. I never understood, why one should be appealed by it from an esthetic point of view, but that's hardly an argument against any theory. My subjective fealing about a theory being ugly or beautiful doesn't matter. If nature likes to behave in a way which I consider ugly, that's my personal problem with nature but that's then how it is.

    My argument against Bohm's interpretation is rather that it introduces unobservable "trajectories", which are superfluous to explain the observable facts described by quantum theory, and these observable facts are probabilistic (statistical) and given by Born's rule. There's nothing more to QT than the particular realization of a probability theory defined by it.
     
  13. Apr 9, 2015 #12

    Demystifier

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    E.g. by showing that Bohmian model for any quantum theory exists. And how one can show that? See
    http://lanl.arxiv.org/abs/quant-ph/0302152 [Found.Phys.Lett.18:123-138,2005]
    Sec. 5.
     
  14. Apr 9, 2015 #13

    atyy

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    Yes, that's among the papers I know about. I have tried to read almost all your papers with great interest! I guess I'm not enough of an expert to evaluate its correctness by myself, and I don't know if there is consensus about whether it really works, at least not the way Bohmian Mechanics for non-relativistic quantum mechanics has been examined for all sorts of tricky situations, and really does seem to work. Would it be fair to say that this is still pretty much at the frontier of research, rather than textbook knowledge? I have the same reservations about MWI - is it really an alternative interpretation to Copenhagen - or is it still an approach that it is unclear whether all the problems have really been worked out?

    So would it be fair to say that at the consensus level - eg., what one can teach to undergraduates - Copenhagen is still the only interpretation of quantum mechanics?

    (Consistent histories, maybe - but it essentially has collapse and all the same problems as Copenhagen, just declared not to be problems)
     
  15. Apr 9, 2015 #14
    Note that EPR, Bell's inequality and entanglement don't demonstrate "nonlocality" (though this is the common word for it) so much as it confirms the initial "superposition of states" as predicted by quantum mechanics. In other words, the initial state of the photons are not polarized in a particular direction, the initial spin of the fermions are not in some specific x-y-z direction. The "nonlocalitiy" has to do with those states being 100% correlated antisymmetrically, as required by standard quantum mechanics.

    Like others in this thread, I'm not seeing anything that looks like "proof of wave function collapse". It's called "proof of existing quantum theory." There is an unfortunate tendency in physics to conceive of the math as being the reality. The math is the description of the reality, the quantitative language we use to communicate about the reality, subject to experimental verification.

    Or to use an analogy from the Matrix, the quote of "There is no spoon." There is no wavefunction. There are phenomena that we measure that are described by math we call "wavefunctions", which aptly predict our measurements. The notion that you can "prove" that a mathematical construct has objective material behavior (collapsing or otherwise) is absurd.
     
  16. Apr 9, 2015 #15

    JK423

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    Let me try to defend the title and the authors a bit.

    The words "nonlocal wavefunction collapse" describe the words and position of EPR and Schrodinger on the subject. They thought exactly that,
    Alice can collapse the state of Bob into totally different states depending on her measurement choice: If she measures position, then Bob's state will be
    an eigenstate of position. If she measures momentum Bob's state will be an eigenstate of momentum. Right after her measurement, she knows instantly (i.e. "nonlocally")
    whether Bob's state is |x> (with a random x) or |p> (with a random p). Yes, of course, we know that there is no collapse (probably), and Wiseman also knows that pretty well as Demystifier noticed. But the authors don't talk about the measurement problem, and the problem of collapse. They use the word 'collapse' to describe the fact that Bob's local state changes instantly ('collapsing') upon Alice's measurement.

    So yes they use these words cleverly to have a catchy title, but what they say is not wrong scientifically if you translate the words the way they mean it (by reading the main text).

    The above effect that these authors demonstrate, where Alice can steer Bob's state, that may seem so impressive, also has a classical counterpart. It is possible classicaly for Alice to 'steer' the classical (probabilistic) state of Bob into different ensembles. The quantum steering effect that the authors above demonstrated of course cannot be performed classically, since entanglement allows Alice to 'steer' much more than what is classicaly possible. However, that doesn't change the fact that this type of "nonlocality" and "collapse" has a classical analogue.
    It's good to demystify some quantum effects so that we won't be more impressed than we should, right demystifier? :smile:

    (check this paper for the classical analogue: http://arxiv.org/ftp/quant-ph/papers/0310/0310017.pdf )
     
    Last edited: Apr 9, 2015
  17. Apr 9, 2015 #16

    atyy

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    Maybe to defend the title even more one can say that even if Bohmian Mechanics or Many-Worlds were true, one can derive collapse as an effective theory. So the only problem is that the observed phenomenon may be consistent with more theories than quantum mechanics. So very strictly, they should say the do not falsify quantum mechanics, but of course everyone knows this, so that is acceptable sloppiness, the same way that general relativity has been "proven".
     
  18. Apr 10, 2015 #17

    Demystifier

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    Absolutely right! :smile:
     
  19. Apr 10, 2015 #18
    Speaking of Matrix. Guys. Why is it valid to consider Many Worlds or Bohmian Mechanics or even Transactional Interpretation.. while physicists want to ignore the Matrix Interpretations? Did you notice many worlds or bohmians are Newtonian like in which you want to attribute the wave functions have trajectories or the branches real? Why not just consider the wave function as just part of the software or algorithm used by reality. Then it doesn't have to be Bohmians or Many worlds. It is just a program.. obsevations or measurements are just interactions in the program.. problem solved. If you consider this philosophy and avoided thinking of it, isn't Many worlds or Bohmian mechanics philosophy too since it has same prediction as QM and no new predictions? Note i'm not trying to discuss philosophy.. just want to know why Bohmians or Many worlds and even Consistent Histories are valid interpretations while the Matrix (reality created by program) Interpretation is automatically shutted out. Could anyone explain so I know? Knowing how the algorithm or the program work in the Matrix Intepretations can even solve key physics problems like quantum gravity, the hierarchy problems, etc.
     
  20. Apr 10, 2015 #19

    Demystifier

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    Actually, I've noticed that you read u huge number of serious physics papers with great interest, and you do all this while actually being a biologist. I am impressed and I always wonder how do you manage to do that. :wideeyed:

    I think it would.

    The latter.

    Depends on what exactly do you mean by Copenhagen interpretation:
    https://www.physicsforums.com/threads/there-is-no-copenhagen-interpretation-of-qm.332269/

    What is mostly taught to undergraduates is the shut-up-and-calculate interpretation.
     
  21. Apr 10, 2015 #20
    I've never heard about this as it sounds straight out from new age conspiracy sites, but aren't you essentially talking about superdeterminism? If I understood it correctly, it's uninteresting simply because it's not an explanation that is scientifically appealing, since you're not making a model that can be used for predictions, but are just saying "the law is that what happens had to happen".
     
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