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Quantum Locality

  1. Aug 21, 2009 #1


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    A couple of articles showed up in the Arxiv today regarding aspects of Quantum Locality (as opposed to quantum non-locality). In essence, both theoretically and experimentally, these attempt to show that QM and Special Relativity are compatible and there is no quantum non-locality. Neither of these papers are close to the last word on the subject.

    1. Quantum Locality by Robert Griffiths, well known scientist and author of an oft-referenced textbook on QM. The abstract:

    "It is argued that while quantum mechanics contains nonlocal or entangled states, the instantaneous or nonlocal influences sometimes thought to be present due to violations of Bell inequalities in fact arise from mistaken attempts to apply classical concepts and introduce probabilities in a manner inconsistent with the Hilbert space structure of standard quantum mechanics. Instead, Einstein locality is a valid quantum principle: objective properties of individual quantum systems do not change when something is done to another noninteracting system. There is no reason to suspect any conflict between quantum theory and special relativity."

    Reference is made to the positions of Bohmians Goldstein and Norsen (who is an advocate of quantum non-locality and is against the relevance of realism in Bell's Theorem).

    2. Testing Non-local Realism with Entangled Coherent States by Paternostro and Jeong. Using Leggett as a foundation, they present experiment evidence in contradiction to non-local theories. (If you reject Leggett as relevant, then you will not see this as evidence against non-local realism.)

    "We investigate the violation of non-local realism using entangled coherent states (ECS) under nonlinear operations and homodyne measurements. We address recently proposed Leggett-type inequalities, including a class of optimized incompatibility ones and thoroughly assess the effects of detection inefficiency."

    This is an expected result, as similar experiments with discrete variables have provided similar results.

    As we have had several threads recently about QM vs. SR, I thought this might be of interest.

    Last edited: Aug 21, 2009
  2. jcsd
  3. Aug 21, 2009 #2
    Thanks for the links. I notice that the first article makes two assumptions that I have not seen together before:
    real reality is not classical reality.​
    The present discussion is also based on the assumption that quantum mechanics is fundamentally stochastic or probabilistic, and this stochasticity is the way the world is, not something limited to the special times or places at which measurements occur.​
    The argument has always been that we only need to claim "real reality" is inherently random if classical reality is real reality. The argument for supposing that there is a real reality that is not classical has been an assumption of determinism.

    This seems to add to the list of choices that we can pick from. If we assume locality above all else, we lose both classical reality and determinism. I'm not sure why we should care about locality if a) causation is denied and b) classical entities are denied. We can't say that particles or colors or sounds or desks or chairs are real. And we can't say that everything must have a cause rooted in underlying natural law. Locality for the sake of locality doesn't seem very metaphysically satisfying if we are giving up both causation and classical entity realism, but the argument looks sound to me given their assumptions.

    Actually, I guess I'm still not satisfied that SR is compatible with a denial of classical properties, but I haven't studied that question with the possibility in mind that someone might deny both classical properties and determinism.

    Edit: Okay, I've convinced myself. Einstein himself was for determinism above all else. SR is based on an assumption of basic symmetry that is violated if nature is inherently random. The assumptions above seem to be incompatible in that they deny the only reason we had to believe in SR to begin with. They may not be logically contradictory, but they are surely untenable. (However, that is not to say that presenting novel and arguably consistent interpretations is a wasted effort.)
    Last edited: Aug 21, 2009
  4. Aug 21, 2009 #3

    Doc Al

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    Robert Griffiths, a long-time advocate/developer of the "consistent histories" approach, is the author of "Consistent Quantum Theory". (Not to be confused with David Griffiths, author of the popular textbook, "Introduction to Quantum Mechanics".)
  5. Aug 21, 2009 #4


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    Oops. I thought they were the same person. :eek:
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