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I New experimental support for pilot wave theory?

  1. May 18, 2016 #1

    ibkev

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    This new research suggesting Debroglie/Bohm pilot wave theory may yet have legs sounded compelling and I would love to hear the thoughts of the PF community:

    https://www.quantamagazine.org/20160517-pilot-wave-theory-gains-experimental-support

    There is some discussion in the comments by Wiseman (one of the authors of the paper) and by the always entertaining Lubos Motl.

    Howard Wiseman says: May 18, 2016 at 7:12 am
    The two comments here (by Pradeep Mutalik and Lubos Motl) nicely illustrate the problem with terminology in this area, a problem I've addressed a number of times, most recently in https://arxiv.org/abs/1503.06413 "Causarum Investigatio and the two Bell's Theorems of John Bell" to be published in http://www.springer.com/us/book/9783319389851

    For Mutalik, nonlocality is demonstrated whenever a Bell's inequality is violated for space-like separated events. I maintain we should call that a violation of *local causality* (a notion defined by Bell in 1976) rather than of locality. The "causality" element here is appropriate because this notion is built on the assumption that correlated events must have a common cause that explains the correlation. This is not the case in a purely operational interpretation of quantum mechanics.

    For Motl, nonlocality means signalling faster than light. I maintain we should call that a violation of *signal locality* rather than of locality.

    The nonlocality we address in this experiment is neither of these. It is the violation of *locality*, in the sense (I maintain) that Bell used it in 1964, and that various philosophers of physics (Jon Jarrett, Don Howard) have used it since. It is also known by the ugly name of "parameter independence". You could think of it as signalling at the hidden-variable level. Because we as experimenters don't have access to the hidden variable level, violation of locality does *not* mean we can signal faster than light.
     
    Last edited: May 18, 2016
  2. jcsd
  3. May 18, 2016 #2
  4. May 18, 2016 #3
    I'm skeptical considering the use of 'weak measurements' in the experiment.
     
  5. May 18, 2016 #4

    Nugatory

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    Is there a pointer to the actual publication (or an arxiv preprint if it's behind a paywall) anywhere?
     
  6. May 18, 2016 #5

    George Jones

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    http://advances.sciencemag.org/content/2/2/e1501466

    I don't see it on the arXiv, but the above article is open-access.
     
  7. May 18, 2016 #6

    N88

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    http://advances.sciencemag.org/content/2/2/e1501466 "Experimental nonlocal and surreal Bohmian trajectories". My emphasis.

    Page 1: "To explain nonlocal phenomena such as Bell nonlocality (10), any realistic interpretation of quantum mechanics must also be nonlocal, and Bohmian mechanics is no exception (2)."

    Page 6: "Indeed, our observation of the change in polarization of a free space photon, as a function of the time of measurement of a distant photon (along one reconstructed trajectory), is an exceptionally compelling visualization of the nonlocality inherent in any realistic interpretation of quantum mechanics."

    But "realism" is (from my readings) the UNrealistic view that each photon had the "measured" polarisation before it was "measured". So, on this point, I find it best to stick to the Copenhagen interpretation and reject such false "realism" (such "quantum classicality") and retain locality in all its forms. So (I wonder):

    Is the claimed "experimental nonlocality" a by-product of their Bohmian unrealism?
    Is my subscription to "locality in all its forms" sustainable?
     
  8. May 18, 2016 #7

    atyy

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  9. May 18, 2016 #8

    bhobba

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    BM was deliberately cooked up to be indistinguishable from the standard formalism.

    Because of that it's highly doubtful there is any experimental way to test it. But we have at least one science adviser here that is an expert in BM and he is the appropriate person to comment IMHO.

    I really want to hear what he thinks.

    Thanks
    Bill
     
  10. May 19, 2016 #9

    Demystifier

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    Exactly!
    There are by now several weak measurements on Bohmian trajectories, and it is well known that such measurements can be explained also by Copenhagen or any other interpretation of QM. So such measurements do not prove that Bohmian trajectories are real, not any more than standard QM measurements prove that wave function is real.

    What these measurement do demonstrate, however, is that Bohmian trajectories are not so meaningless and artificial concept as most physicists used to think. Just as wave function is a very meaningful concept in quantum physics (being real or not), such measurements demonstrate that so are Bohmian trajectories.
     
    Last edited: May 19, 2016
  11. May 19, 2016 #10

    bhobba

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    Oh no. Not this weak measurement stuff again.

    I have lost count of the number of claims that are based on misunderstanding weak measurements.

    Its very annoying.

    Thanks
    Bill
     
  12. May 19, 2016 #11

    Demystifier

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    Conceptually, it can be compared with lines of force associated with a magnetic field. They are a theoretical concept naturally associated with a magnetic field (just as Bohmian trajectories are naturally associated with a wave function) , they can be measured
    https://www.google.hr/search?q=line...qSoSCVBEueZdvZX1EerquIUg3LZ3&q=lines of force
    but it does not necessarily imply that they are real in a fundamental sense.
     
  13. May 19, 2016 #12

    atyy

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    But can this paper be forgiven on the grounds that one bad deed deserves another? ie. ESSW deserved this paper :)
     
  14. May 19, 2016 #13

    Demystifier

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    Not completely forgiven, but it can be taken as a mitigating circumstance.
     
  15. May 19, 2016 #14

    atyy

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    Also, Steinberg is an experimentalist, so he can be forgiven for not knowing Valentini's work (which actually, I did not know about either until reading this forum several years ago).

    But Wiseman has no excuse :P
     
  16. May 19, 2016 #15

    Demystifier

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    How is the Valentini's work related?
     
  17. May 19, 2016 #16

    atyy

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    “The universe seems to like talking to itself faster than the speed of light,” said Steinberg. “I could understand a universe where nothing can go faster than light, but a universe where the internal workings operate faster than light, and yet we’re forbidden from ever making use of that at the macroscopic level — it’s very hard to understand.” (At the end of the Quanta article)

    I think Valentini's work explains that.
     
  18. May 23, 2016 #17

    ibkev

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    Could someone explain what is meant by "weak measurement" please?

    You know, what would be really cool is if every paper published to sites like arxiv.org had it's own discussion forum.
     
  19. May 23, 2016 #18

    stevendaryl

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    I think you need to be a little clearer what you mean by saying that it had the measured polarization beforehand.

    I don't know how Bohmian mechanics handles photons, but in the case of EPR with an electron/positron pair, it's not that the spin is determined ahead of time, it's that the spin measurement result may involve facts about the electron as well as facts about Alice's and Bob's detector settings. So Bob changing his detector setting at the last moment may affect Alice's result.
     
  20. May 23, 2016 #19

    stevendaryl

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    Which work by Valentini are you talking about?
     
  21. May 23, 2016 #20
    I know, right! I am surprised physicist are still using such measurement and then claiming the result conforms to certain statements.
     
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