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A 'Spooky' entanglement

  1. Oct 21, 2015 #1

    jim mcnamara

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    Comments, please:

    http://www.nature.com/nature/journal/vaop/ncurrent/full/nature15759.html
    Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres
    B. Hensen,
    H. Bernien,
    A. E. Dréau,
    A. Reiserer,
    N. Kalb,
    M. S. Blok,
    J. Ruitenberg,
    R. F. L. Vermeulen,
    R. N. Schouten,
    C. Abellán,
    W. Amaya,
    V. Pruneri,
    M. W. Mitchell,
    M. Markham,
    D. J. Twitchen,
    D. Elkouss,
    S. Wehner,
    T. H. Taminiau
    & R. Hanson
    Nature (2015) doi:10.1038/nature15759 Received 19 August 2015 Accepted 28 September 2015 Published online 21 October 2015
     
    Last edited by a moderator: May 7, 2017
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  3. Oct 21, 2015 #2

    Doc Al

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  4. Oct 21, 2015 #3

    bhobba

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    It simply confirms QM - no biggie.

    All such entangled correlations are is exactly that - correlations. Nothing spooky about it.

    Thanks
    Bill
     
  5. Oct 21, 2015 #4

    jfizzix

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    I'd say they're definitely just correlations, but that their implications may be what's "spooky"..

    Is it not "spooky" that such correlations can not be explained by a local hidden variable model?
    - i.e.,that there is no way that their correlations could be determined by the (known and unknown) circumstances of the pair's creation?

    It's hard to nail down what the implications would be, but at the very least, it means that if there are underlying deterministic laws beneath quantum mechanics, then they could not have a cosmic speed limit as we know it. On the other hand, it could just as well be that there is no underlying deterministic universe beneath quantum physics. Quantum metaphysics like this is what makes Bell-inequality violations so interesting, I feel.
     
  6. Oct 21, 2015 #5

    bhobba

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    I don't want to get into a semantic discussion of 'spooky' which would be counter productive. Spooky in this context usually is associated with spooky action at a distance. Ordinary QM is based on the Galilean transformations where instantaneous action at a distance is built into it at its foundations - for that not to be the case you need to go to QFT. And in QFT locality is a much more nebulous thing associated with the cluster decomposition property which doesn't apply to correlated systems - which EPR is.

    My view is that locality is not a concept applicable to correlated systems so any 'spookiness' related to that is rather moot. It interesting and important that QM allows a different kind of correlation than classically - but that all it is - a different kind of correlation. I even go as far to as to say entanglement is THE thing that differentiates classical probability theory from QM:
    http://arxiv.org/abs/0911.0695

    It seems to be absolutely foundational to QM. But 'spooky' is not what I would use to describe a theory that's simply a different kind of probability theoy allowing a new kind of correlation.

    Thanks
    Bill
     
  7. Oct 21, 2015 #6
    I just saw this on the front page of the New York Times and am trying to understand it. What exactly does it mean that it rules out hidden variables? And how? I wish I could understand more than like three consecutive words of their paper.
     
  8. Oct 21, 2015 #7

    bhobba

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    Its Bells theorem:
    http://www.drchinese.com/Bells_Theorem.htm

    Bell's original paper on it is also rather good:
    https://cds.cern.ch/record/142461/files/198009299.pdf

    It is a big part of quantum weirdness that you have this strange kind of non-classical correlation.

    My opinion is its simply that - a rather weird correlation. But some read more into it than that. That's totally legit - I simply have a view QM is weird enough without making it weirder than necessary. This however is a view of QM - sort of like an interpretation - it cant be proved right or wrong.

    Thanks
    Bill
     
  9. Oct 21, 2015 #8

    jfizzix

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    The basic idea, is that if there were some local hidden variables (i.e., pieces of information in the shared past of both particles) that could explain away the correlations they see, then the joint measurement probabilities would have to factor in a certain way.

    As a consequence of the joint probability factoring in this way, you can derive Bell inequalities, which the measurement statistics would have to obey.

    Then, if you can violate those Bell inequalities, the joint probabilities cannot be factorable this way, and as a result, all the pieces of information in the shared past of both particles cannot be enough the explain the correlations we see.

    What counts for locality in local hidden variables, is that these pieces of information have to be in the shared past of both particles, or rather that those pieces of information could conceivably affect both particles through actions travelling at or below the speed of light.
     
  10. Oct 22, 2015 #9

    Nugatory

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    We scooped the New York Times by many weeks :)
    There are several threads from a few months back when earlier reports of this experiment were making the rounds.
     
  11. Oct 22, 2015 #10
    Can you explain why (if) is it that if we don't require determinism then we can get locality again? Is that what consistent histories does?
     
  12. Oct 22, 2015 #11

    bhobba

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    That's not what Bells theorem says.

    It says cant have both properties existing independent of measurement (called Counterfactual Definiteness - although strictly speaking its a bit subtler than that - but no need to go into that) and locality. Its really got nothing, per se, to do with determinism.

    Thanks
    Bill
     
  13. Oct 22, 2015 #12

    stevendaryl

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    Determinism is a red herring in understanding Bell's theorem. No local realistic theory can reproduce the predictions of quantum mechanics, whether or not you allow nondeterminism.

    Determinism comes into play in the EPR reasoning in the following way: In a twin pair experiment, the measurement of the spin of one particle along a particular axis by Alice allows us to predict, with 100% certainty, the result of a second measurement by Bob of the twin particle along that same axis. So after the measurement of Alice's particle, a property of Bob's particle becomes definite. So there are two possibilities, according to a realistic theory: Either Bob's particle had a definite spin along that axis BEFORE Alice's measurement, or Alice's measurement changed something about Bob's particle. So the EPR experiment, together with realism, implies either determinism or nonlocality. Then Bell goes on to show that determinism is out, as well.

    But determinism isn't an assumption in this argument, it's a CONCLUSION from realism and the facts of twin-particle experiments.

    In the above, by "realism" I just mean, vaguely, that individual particles have properties that exist independently of whether those properties have been observed yet.
     
  14. Oct 22, 2015 #13

    haushofer

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    Why is determinism out then? The confirmation of EPR still allows for the Bohmian interpretation which obeys realism and is non-local, right?
     
  15. Oct 22, 2015 #14

    haushofer

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    Btw, i never understood the problems people are having with the Bohmian interpretation being non-local explicitly. Should 't this be regarded as a merit since experiments like the one in the OP show that nature is non-local (according to the definition in the original EPR-paper)?

    I guess i'm missing something because the Bohmian interpretation is notoriously hard to reconcile with relativity, whereas standard QFT is not.
     
  16. Oct 22, 2015 #15

    bhobba

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    That's correct.

    Determinism isn't really an issue - you can have it if you want.

    I personally have no issues with BM. The reason I don't hold to it is, as you correctly point out, it has a preferred frame which IMHO is at odds with the central lesson of SR and the symmetries of the POR. It doesn't disprove it or anything like that, it simply doesn't gel with my world view of physics where symmetry is the fundamental thing.

    Thanks
    Bill
     
  17. Oct 22, 2015 #16
    Thought there were advances in making dBB Lorentz-invariant?
     
  18. Oct 22, 2015 #17

    bhobba

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    I thought it already can be - but Dymystifyer is our expert not me. Obeying the POR and not having a preferred frame and Lorentz invariance are different things eg LET is Lorentz invariant.

    Thanks
    Bill
     
  19. Oct 22, 2015 #18

    stevendaryl

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    I meant that EPR shows that any realistic theory must be either deterministic or nonlocal, and Bell shows that deterministic doesn't work. So any realistic theory is nonlocal (whether or not it is deterministic).
     
  20. Oct 22, 2015 #19

    haushofer

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    How does one reconcile EPR then with standard QFT if it is a nonlocal effect? I'm missing something important, apparently.
     
  21. Oct 22, 2015 #20

    bhobba

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    Locality in QFT is based on the cluster decomposition property which excludes correlated systems.

    Thanks
    Bill
     
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