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Featured I Murray Gell-Mann on Entanglement

  1. Sep 9, 2016 #1
    In this video Murray Gell-Mann discuses Quantum Mechanics and at 11:42 he discuses entanglement. At 14:45 he makes the following statement:

    "People say loosely ,crudely,wrongly that when you measure one of the photons it does something to the other one. It doesn't."
    Do most physicists working in this field agree with the above statement ?
     
  2. jcsd
  3. Sep 9, 2016 #2
    I think so. But I don't think this means they necessarily reject non-locality, because non-locality can mean more than one thing.
     
  4. Sep 9, 2016 #3

    DrChinese

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    That's a fair statement. But it really is interpretation dependent. And a lot of physicists don't really get tangled up in the question anyway.
     
  5. Sep 9, 2016 #4
    What would be the definition of "to do something to the other one?" in this example?
     
  6. Sep 9, 2016 #5
    In response to Dr. Chinese, I thought that question that physicists don't want to get tangled up in is the most important question of entanglement, i.e.spooky action at a distance: How can measurement of for example spin of one particle affect instantaneously the spin of a very distant particle?
     
  7. Sep 10, 2016 #6

    atyy

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    In this example, for Murray's statement to be true, he would be talking about the reduced density matrix of an observer who only makes a measurement on the other photon.

    However, it would be equally right to say that measuring one photon does affect the other photon, since a measurement collapses the wave function of both photons.
     
  8. Sep 10, 2016 #7

    vanhees71

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    I don't know about "most physicists", but I couldn't agree more! I don't want to go into the discussion about "collapse" again. I just state once that in the sense used again by atyy, it's inconsistent with relativstic quantum field theory, and that's why Gell-Mann is completely right in his statement. Of course, he knows his QFT better than any of us ;-)).
     
  9. Sep 10, 2016 #8
    The way I see the reason why it's not true that "when you measure one of the photons it does something to the other one", is that it involves the absence of counterfactual definiteness. If "doing something" means "causing an effect", then for me this applies: “We may define a cause to be an object followed by another, and where all the objects, similar to the first, are followed by objects similar to the second. Or, in other words, where, if the first object had not been, the second never had existed.” (Hume, 1748)

    Without CFD there is no "doing" in this case. But it doesn't mean rejecting non-locality, if non-locality for you means that changing the experiment alters the whole situation holistically: there's no action between the two parts of the experimental setup because there are no independent parts, if you change one you just have a different whole experimental setup, not a different part of the same experimental setup.
     
  10. Sep 10, 2016 #9

    atyy

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    But Bell himself knew QFT well too. His lesser accomplishment beyond proving quantum nonlocality was discovering the chiral anomaly.
     
  11. Sep 10, 2016 #10

    vanhees71

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    Bell's accomplishment was to prove that quantum theory enables stronger correlations than any local deterministic hidden-variable theories can explain. He did not disprove local relativistic QFT, and you can well argue whether the discovery of the ABJ anomaly or his inequality were greater or lesser. I think they are pretty equal.
     
  12. Sep 10, 2016 #11

    stevendaryl

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    In all due respect to a physics giant, I think that Gell-Mann's definitive statement that measurement of one particle in EPR has no effect on the other particle is going beyond what we understand about quantum mechanics. He says that

    This explanation of why EPR is not nonlocal is not very satisfying to me. In Alice/Bob terms, he's talking about Alice's measurement of her photon's state of circular polarization revealing Bob's photon's state of circular polarization. But if Alice's measurement is only revealing the state of Bob's photon, that sounds like it's implying that Bob's photon had that state already, before her measurement. That sounds like the "elements of reality" that Einstein, P[whatever] and R[whatever] were talking about, which Gell-Mann says is just wrong. Here's where what Gell-Mann is saying differs from Einstein's hidden variables: Gell-Mann seems to be saying that on this branch of history, Alice measures the circular polarization of her photon, and Bob's photon has a definite circular polarization state (either left-handed or right-handed). On some other branch (one that doesn't actually occur), Alice measured a different property of her photon, and Bob's photon was in some other definite state all along.

    I sort of understand this point of view, but it seems a little mysterious, to me. After all, Alice chooses which branch is actual by choosing which measurement to make. (Actually, I guess her choosing a measurement means picking two possible branches; one in which she has a right-handed photon, and one in which she has a left-handed photon. She can't choose which of those she is in, but she can choose not to be in a possible branch in which her photon is linearly polarized.)
     
  13. Sep 10, 2016 #12

    ShayanJ

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    :DD
    Podolsky and Rosen!
     
  14. Sep 10, 2016 #13

    stevendaryl

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    I knew that.
     
  15. Sep 10, 2016 #14

    ShayanJ

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    Sorry! But that wasn't mockery, it was just fun to read that.
     
  16. Sep 10, 2016 #15

    ShayanJ

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    This was my objection to vanhees71's views on the subject. But this objection is only legitimate if the quantum state is taken to be objective. But if we assume the the quantum state only represents the knowledge of the observer, this objection goes away.
     
  17. Sep 10, 2016 #16
    In line with what Gell-Mann says there, Bell rules out only commutative local hidden variables. See https://arxiv.org/pdf/1106.1453. That does not rule out non-locality though.
     
  18. Sep 10, 2016 #17

    stevendaryl

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    I suppose. But I can't completely make sense of that. In the case of EPR with correlated photons, Alice measures her photon to be vertically polarized along some axis. She then knows that Bob has a 0% chance of measuring horizontal polarization along that axis. If it's just a matter of Alice updating her knowledge of Bob's situation, then I would think that would mean that Bob had 0% chance before Alice's measurement, even if Alice didn't know that. Which to me implies that Bob's result was predetermined, at least for that particular measurement choice, which is sort of a hidden-variables conclusion.
     
  19. Sep 10, 2016 #18
    Is Gell-Mann presenting decoherent histories faithfully here? Kind of an useless question but you never know.
     
  20. Sep 10, 2016 #19
    So we are to assume that this is incorrect because of a pesky factor of 2^1/2?
     
  21. Sep 10, 2016 #20

    DrChinese

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    First, I would object to that reference as generally accepted science. I consider that reference (which I was already familiar with) to be in the "Bell is wrong/non-applicable/etc" camp. Got another from an undisputed source?

    Second, Bell says no such thing as you describe. Bell does NOT rule out commuting local hidden variables. Bell DOES rule out non-commuting local hidden variables. Or more specifically, overlapping (partially non-commuting) observables are ruled out as being local realistic.
     
    Last edited: Sep 10, 2016
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