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Why all the clinging to locality?

  1. Oct 10, 2013 #1
    Hi,

    Bell demonstrated that there cannot be a local realistic theory that reproduces the expectation values of QM. I can see that a non-realistic theory is unsatisfactory, we would have to abandon the nice determinism we got so accustomed to in classical physics, from Newton to Maxwell to Einstein.

    But what about non-locality? In non-relativistic QM, there are two different non-local processes:
    1. The Schrödinger propagator is not Lorentz invariant. Allows faster-than-light communication.
    2. Entanglement. Does not allow FTL communication.

    If you fix the first by using the Dirac equation and Lorentz invariant propagators, you get amazingly successful theories as QFT. And you are at peace with SR.

    So why do we even bother about 2.? Signalling is not possible using entanglement, so Einstein won't haunt us. Also with Bohmian mechanics, we already have a nice non-local realistic interpretation.
     
  2. jcsd
  3. Oct 10, 2013 #2

    Vanadium 50

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    A local theory is one that has a finite number of derivatives, and thus a finite number of coefficients of the theory that can be determined from experiment. A non-local theory does not have that guarantee of predictability, and so is not very useful.
     
  4. Oct 10, 2013 #3
    Could you elaborate on this?
    What does this mean? Why doesn't Bohmian mechanics satisfy this "guarantee of predictability?
     
  5. Oct 10, 2013 #4

    DrChinese

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    There are some good non-realistic local interpretations. Why are you clinging to realism? :smile:
     
  6. Oct 11, 2013 #5

    Demystifier

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    One reason is this: Because if I abandon realism then I have to believe that you (DrChinese) and all what I see you write - is not real. That's hard to accept, don't you agree?
     
  7. Oct 11, 2013 #6
    Or maybe our notion of "real" is inadequate.

    Common sense is a very good method of validating physical theories: basically everything we find "rational" proves to be false.
     
  8. Oct 11, 2013 #7
    If one accepts a common-sense view of "non-realism" (there is no physical reality at all), then I don't see what the difference would be between:

    1. local non-realism vs
    2. non-local non-realism

    There doesn't appear to still be something meaningful for "locality" to refer to. As others have argued:
    So , if one argues for this type of non-realism, then the issue of locality vs non-locality seems kind of pointless since there doesn't appear to be any ontological issues. Unless, of course, one means something different when they argue for non-realism and something that is much subtler.
     
  9. Oct 11, 2013 #8

    stevendaryl

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    The general reason that local theories are better:

    The universe is huge (maybe infinite) and for the vast majority of it, the configuration of matter is completely unknown. In order for a theory to be predictive, we have to be able to make predictions based on what we actually know. So a useful theory should have the property that the unknown (things that are far, far away) should have negligible impact on experiments right here.

    Special Relativity has this character. If you want to be able to predict what's going to happen at event e some time T in the future, you only need to know about conditions in the region of space consisting of those points whose distance from e is less than or equal to cT. I don't need to know about what's going on in distant galaxies. In contrast, for a nonlocal theory, I potentially need to know about what's going on in the entire universe to be able to make a prediction about what's happening on Earth 5 seconds from now. General relativity has a slightly different condition, but it's similar: to predict what's going to happen at some future event, it is enough to know what's happening in a region nearby that event.

    This isn't really an insurmountable problem, because you can just make intelligent guesses about conditions in the universe far, far away from here, but conceptually it's annoying. Newtonian physics had this problem, even though I don't think anyone worried too much about it: Although it was a deterministic theory, if your knowledge about the universe was limited to a small region of space and time, then Newtonian physics doesn't allow you to make any predictions at all. In practice, it didn't cause problems, just because people assumed (with no basis other than wishful thinking) that there was nothing relevant happening far far away.
     
  10. Oct 11, 2013 #9

    stevendaryl

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    Well, Dr Chinese will have to explain what he means by non-realistic, but one type of theory that I think might be lumped in as "non-realistic" is something like Many-Worlds, where questions like "Did Alice measure spin-up or spin-down?" don't have unique answers. Both possibilities are "real", although we only experience one or the other. So it's not non-realistic in the sense that it's all a big hallucination.
     
  11. Oct 11, 2013 #10
    Descartes realised long ago that he couldn't prove reality independent of his own mind and accepted that. He didn't even need to trade it off versus locality to come to that conclusion.

    A non-local theory can still have strong locally predictive power, providing the non-local influences are confined in some other way, or slowly varying.

    A notable difference between QM and the other theories that you mention is that we already have a very good description of what happens. The issue of locality or non-locality, is based upon how we interpret QM.

    It's also interesting that QM has elements that are not predicable. I'm not going to argue that these are due to non-local influences. Just that it's interesting, in the context of your description of the appeal of local theories.
     
    Last edited: Oct 11, 2013
  12. Oct 11, 2013 #11
    "Non-realistic" is a very vague term. What you give up is the notion that there is a unique reality. Instead, in the Many Worlds interpretation (or simply in QM applied to measuring devices) measurements can be thought of as branch points on a tree, because the wavefunction ends up in a (weighted) superposition of states where all possible results were obtained. Personally, I see nothing wrong with that.

    As for locality, local quantum field theories are the most successful theories in the history of science, at least when it comes to making precise, quantifiable, and experimentally verified predictions. Why give that up on the basis of some vague philosophical unease?
     
  13. Oct 11, 2013 #12

    DrChinese

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    I know everything you write is very real! :smile: Seriously, you are the most prolific writer and I love your work.

    The answer is that realism can be any of a variety of things. The answer to the question of "where are the hidden variables?" is "not here and now".

    We both know that the Bohmian (BM) view involves non-local influences. However, there is no non-local signalling mechanism in BM. Ie it is not as if a measurement on Alice results in a signal propagating to Bob. As I understand it, more like Alice and Bob are both part of the same system at all times.

    You could call that non-realistic as easily as you call it non-local. Bohm referred to the holographic paradigm. From a blog entry at DailyGalaxy: "University of London physicist David Bohm, for example, believes Aspect's findings imply that objective reality does not exist..." I would say that is as perfect a definition of non-realism as anything: there is no objective reality independent of measurement context.
     
  14. Oct 11, 2013 #13
    I think the terminology is confusing me. That author also writes:
    I think one would still consider such a view consistent with "realism", only that reality is "veiled", in some sense. I understand that "mind-independent reality" cannot be known directly as it is filtered through our mental structures with their particular cognitive limitations but it seems hard to dispute the claim that there is some objective reality independent of us, at least for me.
     
  15. Oct 11, 2013 #14

    stevendaryl

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    Well, this shows a problem with the notion of "realism". No matter how cockamamie a theory's ontology is, you can always say that that's the reality, and so it's a realistic theory. But I think that people use the word realistic to mean the relationship between the ontology and our observations and measurements, that our observations are actually revealing something about the world that we didn't know before we made the observation. So in this sense, a Many-Worlds interpretation isn't realistic, because measuring spin-up for an electronic that was initially in a superposition of spin-up and spin-down doesn't tell us anything that we didn't know already.
     
  16. Oct 11, 2013 #15
    We should just read the word realism as shorthand for objective realism, as opposed to subjective realism where reality is observer dependent.
     
  17. Oct 11, 2013 #16

    DrChinese

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    That was exactly how EPR treated it. So I follow that.
     
  18. Oct 14, 2013 #17

    Demystifier

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    Thanks, I like your writing too! Is there some of my writing which you particularly like? (I am not asking this to raise my ego, but to see which kind of writing I should keep doing.)

    It seems to me that what you call non-realistic others call contextual. In other words, your notion of non-reality does not avoid non-locality. Just uses a different name for it.
     
  19. Oct 14, 2013 #18

    Demystifier

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    I know that. In fact, I was quite influenced by Descartes when I wrote
    http://lanl.arxiv.org/abs/1112.2034 [Int. J. Quantum Inf. 10 (2012) 1241016]

    But according to Descartes, as well as according to the paper above, at least the mind is real. By contrast, according to the non-realistic variant of the Copenhagen interpretation, not even mind is real.

    See also my blog entry:
    https://www.physicsforums.com/blog.php?b=4657 [Broken]
     
    Last edited by a moderator: May 6, 2017
  20. Oct 15, 2013 #19
    I've never come across anyone interpreting the Copenhagen interpretation (whether the realistic or non-realistic version) as suggesting that mind is not "real". But maybe I'm mistaken? Moreover, a non-realistic interpretation to both mind and matter would seem self-contradictory, I think. Even solipsism does not question the reality of mind (e.g. a solipsist accepts the reality of one's own mind, at least, but nothing else). I take anti-realism to be the denial that there is a way the world really is as distinct from our perceptions or conceptions of it. Perhaps one means something different when some of these authors use the term "realism"? Perhaps replacing the term "realism" with "pre-existing properties" or "counterfactual definiteness" would make it easier to try to understand the different interpretations as suggested by Maccone:
    A simple proof of Bell’s inequality
    http://arxiv.org/pdf/1212.5214.pdf
     
  21. Oct 15, 2013 #20

    fully concur.



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