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B Speed of interaction between entangled particles

  1. Aug 19, 2015 #1
    When the spin of an entanglement particle is established, it is said that the corresponding spin of its entangled twin is revealed immediately, (via wave collapse?), and that this interaction can occur across a substantial distance. It has also been said that this immediate interaction can occur faster than the speed of light. Is this >speed of light interaction simply a hypothesis, or does anyone know of experimental research into this? Many thanks, TotalNovice
     
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  3. Aug 19, 2015 #2

    ShayanJ

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    Its experimentally verified but its not an interaction between the entangled particles, at least not by the usual meaning of the word "interaction".
    The exact mechanism by which this happens is still an open problem in physics.
     
  4. Aug 19, 2015 #3
    Thanks for the clarification Shyan. I'll stay away from "interaction" from here on. No doubt I'll be able to demonstrate my ignorance in many other ways in future postings! I remain, a TotalNovice
     
  5. Aug 19, 2015 #4

    DrChinese

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    Welcome to PhysicsForums, TotalNovice!

    To add to Shyan's excellent comment: There have been experiments performed to determine the speed of collapse as you describe. While it is expected to be instantaneous, the MINIMUM speed is estimated at over 10,000 times the speed of light (10^4 c).

    http://arxiv.org/abs/0808.3316

    "In science, one observes correlations and invents theoretical models that describe them. In all sciences, besides quantum physics, all correlations are described by either of two mechanisms. Either a first event influences a second one by sending some information encoded in bosons or molecules or other physical carriers, depending on the particular science. Or the correlated events have some common causes in their common past. Interestingly, quantum physics predicts an entirely different kind of cause for some correlations, named entanglement. This new kind of cause reveals itself, e.g., in correlations that violate Bell inequalities (hence cannot be described by common causes) between space-like separated events (hence cannot be described by classical communication). Einstein branded it as spooky action at a distance. A real spooky action at a distance would require a faster than light influence defined in some hypothetical universally privileged reference frame. Here we put stringent experimental bounds on the speed of all such hypothetical influences. We performed a Bell test during more than 24 hours between two villages separated by 18 km and approximately east-west oriented, with the source located precisely in the middle. We continuously observed 2-photon interferences well above the Bell inequality threshold. Taking advantage of the Earth's rotation, the configuration of our experiment allowed us to determine, for any hypothetically privileged frame, a lower bound for the speed of this spooky influence. For instance, if such a privileged reference frame exists and is such that the Earth's speed in this frame is less than 10^-3 that of the speed of light, then the speed of this spooky influence would have to exceed that of light by at least 4 orders of magnitude."
     
  6. Aug 20, 2015 #5
    Blimey, at least 10,000 times the speed of light! That completely explains why Shyan questioned my use of the term "interaction"; this mechanism seems to be a phenomenon outside of the presumed ubiquity of cause and effect...that's done my head in to be honest
     
  7. Aug 20, 2015 #6
    It's fun, eh? :smile:
    I was looking for a clip I've seen (a lecture if I remember correctly) which brought up the "speed experiment" mentioned above by Dr Chinese, but I can't find it at the moment.

    However I found this lecture, which may be interesting to you, TotalNovice (I watched it today and it is about entanglement, "quantum games", long distance experiments, quantum cryptography etc.)

    Anton Zeilinger - Quantum Information and Entanglement
    Excerpt from his lecture "Quantum Games and Quantum Information". World-renowned physicist Professor Anton Zeilinger entertained and informed a UCT (University of Cape Town, South Africa) audience about quantum physics during his Vice-Chancellor's Open Lecture at the University of Cape Town on 25 October 2011.
     
  8. Aug 21, 2015 #7
    He's a fun guy on the evidence of his presentation. Thanks for sending this Dennis. I guess if Quantum computing and/or Quantum cryptography really work, then superpositioning becomes more of a fact than a theory?
     
  9. Aug 21, 2015 #8
    Strictly speaking, nothing "faster-than-light" has been demonstrated experimentally. All violations of the Bell inequalities reported by experimentalists have so-called "loopholes", which make it difficult to confidently accept the relevant radical conclusions. The situation may change in a few years, but right now, 50 years after Bell, we are where we are.
     
  10. Aug 21, 2015 #9

    Nugatory

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    Although difficulty and confidence are somewhat in the eye of the beholder, enough has been achieved in the past half-century that any conclusions reached by appeal to loophole will be at least as radical and difficult to accept as Bell's conclusion.
     
  11. Aug 21, 2015 #10

    DrChinese

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    Bell tests demonstrate that local realistic theories cannot agree with the predictions of quantum mechanics. The experiment I cited demonstrates that IF there are nonlocal components to QM, they must exceed 10,000 c. Both of these results are generally accepted.
     
  12. Aug 21, 2015 #11
    First, are you saying those tests/experiments were loophole-free?

    Second, I agree that local realistic theories cannot agree with the predictions of standard quantum mechanics. However, standard quantum mechanics' predictions are mutually contradictory, so the disagreement is no more a problem for local realistic theories than the internal inconsistency is a problem for standard quantum mechanics.

    Third, the standard quantum mechanics' prediction of violations of the Bell inequalities, strictly speaking, has not been confirmed experimentally. Not without loopholes (which Zeilinger calls "essential". Whether the results are generally accepted or not).

    I know you've heard it all, and I've heard what you're saying in this thread, but for the benefit of new participants of the forum, such as TotalNovice, we both reiterate our respective views. .
     
  13. Aug 21, 2015 #12

    Nugatory

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    Which contradictions are thinking of here?
     
  14. Aug 21, 2015 #13
    The contradiction between unitary evolution and the projection postulate of standard quantum mechanics. Unlike the projection postulate, unitary evolution cannot produce irreversibility or turn a pure state into a mixed one. This contradiction is fairly well-known as the measurement problem of quantum mechanics (see, e.g., http://plato.stanford.edu/entries/qt-measurement/ )
     
  15. Aug 21, 2015 #14

    Nugatory

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    Thanks, yes, now I understand what you mean.

    That's certainly a blemish (arguably, THE blemish) in QM, but I don't see it as a contradiction. It's ugly and aesthetically displeasing that we have to stick in this extra postulate about projection happening, but it doesn't lead to a contradiction in the formal sense of having valid derivations of both A and not-A.
     
  16. Aug 22, 2015 #15
    I am sincerely grateful to you for admitting this is at least a matter of concern.

    First, a technical note (or you may call it "nit-picking":-) ). This is not about derivations, as postulates are not derived, they are postulated. However, they are typically postulated to achieve agreement with experiments. On the other hand, as far as I know, there is no experimental basis for the projection postulate. I offered the following quote several times here:

    "(i) the universal validity of unitary dynamics and the superposition principle has been confirmed far into the mesoscopic and macroscopic realm in all experiments conducted thus far;

    (ii) all observed ‘‘restrictions’’ can be correctly and completely accounted for by taking into account environmental decoherence effects;

    (iii) no positive experimental evidence exists for physical state-vector collapse;

    (iv) the perception of single ‘‘outcomes’’ is likely to be explainable through decoherence effects in the neuronal apparatus."

    (M. Schlosshauer, Annals of Physics, 321 (2006) 112-149).

    I may have an issue with (iv), but this quote does give a modern summary of experimental results. No positive evidence of collapse so far.

    So you don't see the measurement problem as "a contradiction in the formal sense". Perhaps one can defend such a point of view, for example, saying (following von Neumann) that unitary evolution only takes place between measurements, but this is indeed "ugly and aesthetically displeasing". What exactly do we call "measurement"? How and why is it different from "normal" physical processes?

    Let me also note that in my recent work (http://link.springer.com/content/pdf/10.1140/epjc/s10052-013-2371-4.pdf , Eur. Phys. J. C (2013) 73:2371, open access) I gave examples of local realistic models reproducing unitary evolution of quantum field theory (but not the projection postulate). This is another indication that you need the projection postulate (or something similar) to prove that violations of the Bell inequalities are possible in standard quantum theory (this proof is an important part of the Bell theorem). On the other hand, there is no loophole-free experimental evidence of the violations.
     
  17. Aug 22, 2015 #16

    Nugatory

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    That's probably a good last word for this thread, as we've moved beyond OP's question. We can acknowledge that there is a measurement problem but we aren;t going to resolve it here.
     
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