Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is there any hope at all for Locality?

  1. Aug 3, 2013 #1

    andrewkirk

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    As I understand it, EPR proposed their entanglement thought experiment as a means of demonstrating that Quantum Mechanics was incomplete, and hence that the Copenhagen interpretation (which says that the wave function is a complete description of the state of a system) was wrong. They postulated the existence of hidden variables as a way of 'completing' the theory. Here 'hidden' just means 'not in any way reflected in the wave function'.

    Bell proved that any extension of QM that uses hidden variables will predict correlations for measurements of entangled particles that differ from what QM predicts, if the principle of locality is to be maintained.

    Aspect et al showed, subject to various minor loopholes on which most people seem to place not much reliance, that experimentally observed correlations follow the QM predictions rather than those predicted by a hidden variable theory that preserves locality.

    From this we inductively conclude that there is no valid hidden variable theory that preserves locality.

    Various presentations of this topic suggest that the tests of Bell's theorem have shown that we cannot maintain both locality and something else, where that something else is variously described as realism, counterfactual definiteness, or other similarly vague-seeming terms. This seems consistent with EPR's and Bell's original ideas, which were to challenge or defend the Copenhagen interpretation that a particle does not have a definite position and momentum unless it is in an eigenstate of one of the two operators.

    But I can't see how even accepting that (ie accepting non-realism or non-counterfactual definiteness) allows us to still believe in locality in the face of the Bell theorem and the subsequent experiments. The correlations in Bell's theorem imply that Alice measuring spin along a certain axis has an instantaneous effect on the probability distribution of the results of Bob's measurement. So retreating into the indeterminacy of the Copenhagen interpretation does not appear to have allowed us to preserve locality since an instantaneous effect has occurred across a spacelike interval.

    I realise that this is a hand-wave rather than a mathematical proof, but I find myself unable to imagine what sort of a theory (extension of QM) or interpretation could remain consistent with the Bell results while still preserving locality.

    I would be grateful for any light that contributors are able to shed on my fog of puzzlement.
     
  2. jcsd
  3. Aug 3, 2013 #2

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    It depends on what you mean by locality. If you mean strange correlations can occur instantaneously then yes locality is dethroned. But that is not what is generally meant by locality which is the ability to actually send information. You cant use QM correlations to do that so locality is saved.

    Thanks
    Bill
     
  4. Aug 3, 2013 #3

    Nugatory

    User Avatar

    Staff: Mentor

    Superdeterminism gives us the correlations without the instantaneous effect. However...

    If you feel that accepting superdeterminism is even less palatable than accepting these instantaneous effects, you will have plenty of company.
     
  5. Aug 3, 2013 #4

    Demystifier

    User Avatar
    Science Advisor

    Alternatives to quantum nonlocality:
    https://www.physicsforums.com/blog.php?b=3622 [Broken]
     
    Last edited by a moderator: May 6, 2017
  6. Aug 3, 2013 #5
    I think 'objective reality' requires causality and it seems the MWI has no explanation for the causal connections encountered on a daily basis between macroscopic objects. They are supposed to be emergent in the MWI, right?
     
    Last edited by a moderator: May 6, 2017
  7. Aug 3, 2013 #6
    This post https://www.physicsforums.com/showpost.php?p=4371341&postcount=1 (and the long thread quoted there) may be irrelevant to the questions in your post (as it takes the loopholes seriously), but is clearly relevant to the title of your post.
     
  8. Aug 3, 2013 #7

    meBigGuy

    User Avatar
    Gold Member

  9. Aug 4, 2013 #8

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Errrr - since MWI is completely deterministic and causal why you would say that I have zero idea.

    The issue with MWI is, and always has been, how do you get probabilities from a totally deterministic theory. One can assume the experience of measurement is probabilistic and use Gleason's Theorem or some other means but unless you can derive it from the universal quantum state it assumes the theory is blemished - not incorrect or disproven - but blemished.

    Then of course you have this extremely extravagant exponential increase in the number of worlds that simply sounds - well weird.

    However in many other ways it is beautiful, really beautiful.

    Thanks
    Bill
     
    Last edited: Aug 4, 2013
  10. Aug 4, 2013 #9

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Read the link.

    Interesting conclusion:
    'The inevitable conclusion is that if there is a more fundamental truth from which the known laws of quantum physics are emergent, this more fundamental truth must be at least as weird as quantum theory. More in particular, a classical physics theory capable of explaining all of quantum physics - Einstein's hope - can not exist.'

    Errrrr. Not so fast Obi Wan.

    It depends on what you mean by 'weird'. If you mean a theory that conforms to Einsten's view of the world then the jury is out - that may or may not be possible. Einstein was well aware of the problems EPR posed - he was one of the authors after all. GR is weird - but Einstein invented that. It is not mere weirdness that is the issue - it is Einsteins view of the world - namely being real, objective, independent of observation and deterministic. There may be a reality from which QM emerges that is like that - or not - no one knows.

    However if history is any guide there is probably a surprise or two along the way of investigating it.

    Thanks
    Bill
     
    Last edited: Aug 4, 2013
  11. Aug 4, 2013 #10

    meBigGuy

    User Avatar
    Gold Member

    Don't get hung up on weird. if quantum mechanics is quantum-weird, then whatever underlies it must be quantum-weird too. But classical physics can't ever contain the "quantum weirdness", not that it doesn't have classical-weirdness of its own. But, whether or not one agrees with that shouldn't take away from the mental exercise.

    For me it the article was more about the sentence before it:

    "These spooky effects force us to answer the question 'does something exist if we can not know anything about it?' with a resounding 'no'. What can not be observed does not exist. This is not a crazy philosophical thought, but a hard experimental fact."
    That is the source of my "God is Frugal" conclusion.

    But, with respect to the analogy itself, did you see it as a meaningful mental exercise that illustrates a subtlety behind the reality of entanglement?
     
    Last edited: Aug 4, 2013
  12. Aug 4, 2013 #11

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    That does not follow. Eg Primary State Diffusion derives QM from an underlying sub quantum world that is quite classical. Of course there will be departures from QM in the sub quantum world that it applies to, like there is departures from classical physics in the Quantum domain. As Bohr said to Einstein - stop telling God what to do. Same here - stop telling nature how to behave.

    No

    Thanks
    Bill
     
  13. Aug 4, 2013 #12

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    I disagree with that very general looking concept.

    What happened to the matter we see as cosmic microwave background today? It is beyond our event horizon, we cannot observe how it looks today. Do you think the matter just vanished magically? I do not think so. It is way more natural to assume that it still exists, and formed galaxies just as matter around us did.

    Occam's razor: it is easier to assume galaxies outside our observable universe exist, as this gives an easier description of the universe - a universe where galaxies do not vanish magically.
    The same can happen in QM.
     
  14. Aug 4, 2013 #13

    meBigGuy

    User Avatar
    Gold Member

    @mfb, did you read the article?
    BTW, The word "observation" has nothing to do with whether something is observable by you or I, but rather whether it interacts.

    @bhobba
    We're going on about semantics. Whatever ultimately underlies QM must explain, and encompass, quantum-weird, you feel it can do that without being just as weird. OK. I'll accept that.
    Could you elaborate on your "no" answer?
     
  15. Aug 4, 2013 #14

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    No, and as I posted I just think the quoted statement is (or looks) too general.
    Those galaxies do not interact with us any more.
     
  16. Aug 4, 2013 #15

    bhobba

    User Avatar
    Science Advisor
    Gold Member

    Exactly what analogy are you referring to? The socks thought experiment? That's a thought experiment not an analogy. Aside from that all I got from the article was this idea that Einstein was wrong. He wasn't - there may indeed be an underlying reality from which QM emerges that conforms to his intuition.

    Thanks
    Bill
     
  17. Aug 4, 2013 #16

    Nugatory

    User Avatar

    Staff: Mentor

    As with most claims about experimental support for interpretational positions, there's some overreach going on. A weaker but more defensible statement would be:
     
  18. Aug 4, 2013 #17
    Here is a chance for locality. See page 37 of:

    New Insights on Time-Symmetry in Quantum Mechanics, Yakir Aharonov and Jeff Tollaksen



    "...Traditionally, it was believed that “contextuality” was very closely related
    to “kinematic-nonlocality.” Typically, kinematic-nonlocality refers to correlations,
    such as eq. 1.1, that violate Bell’s-inequality with the consequence that
    QM cannot be replaced with a local realistic model. Similarly, contextuality
    refers to the impossibility of replacing QM with a noncontextual realistic theory.
    Applying this now to the relativistic-paradox (§1.1), we see that Lorentz
    covariance in the state-description can be preserved in TSQM [9] because the
    post-selected vector A
    z = +1 propagates all the way back to the initial preparation
    of an EPR state,..."

    I really love this interpretation, perhaps for my ignorance, but it seems to solve apparent "non-locality" and apparent "Time Asymmetry".
     
  19. Aug 4, 2013 #18

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    The story of Albert's socks is pretty long-winded and takes forever to get to the point, but I agree that the situation described is very analogous to weird quantum correlations. In particular, you have correlations that cannot be explained in terms of a locally realistic model, but can easily be explained using nonlocal interactions, or using superdeterminism.

    I'd like to see the details of the argument that the sock drawer can be "implemented" using entangled pairs.
     
  20. Aug 4, 2013 #19

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    It's a nice list except for the inclusion of Joy Christian's approach. That is nonsense, in my opinion.
     
    Last edited by a moderator: May 6, 2017
  21. Aug 4, 2013 #20

    stevendaryl

    User Avatar
    Staff Emeritus
    Science Advisor

    Well, in MWI, I don't think it's really meaningful to talk about the number of worlds increasing. There's only the wave function, and it's a single thing. The many-worlds are just ways of splitting up that single object.

    An objection that I sort of have sympathy for is the business about justifying the Born rule in MWI. On the one hand, that seems pretty crucial, because without the Born rule, quantum mechanics makes basically no predictions at all. (Well, that's not completely true. There are circumstances where QM gives a 0% or 100% probability for something, and we can make sense of those without worrying too much about the meaning of probability.) On the other hand, what could it mean to justify the Born rule?

    I guess going further would be leaving physics and venturing into philosophy, but there is something philosophically puzzling about probabilistic theories. If someone claims that a coin has a 50/50 chance of landing heads-up, you can test that claim by flipping the coin many times and counting how many times it ends up heads. But this kind of test assumes a numerical equivalence: The probability for a single throw = The relative frequency for many throws. But how is that justified? It's certainly possible to flip a coin 1000 times and get "heads" every time. But we assume that that's an unlikely enough occurrence that we can ignore it. But what does "unlikely" mean, here, physically?

    It seems that ultimately what we're doing is defining a "normal world" to be one where relative frequencies are roughly equal to probabilities, and assuming that we live in a normal world. The further piece of information, that the set of "normal worlds" has measure 1, doesn't really add a lot to me, because the set having measure 1 doesn't mean that our world is in it. You can say that it means that it is likely that our world is in it, but once again, what does "likely" mean here?

    What it boils down to, to me, is this: Treating events that have probability 0 as if they were impossible, is not actually justified, but it is a self-consistent way to reason. That's true with classical probabilities. MWI with the Born rule is in no worse shape. We can't justify the assumption that relative frequencies will approximately equal the probabilities given by the Born rule, but it is a self-consistent way to reason, and reasoning about probabilities without some such rule is impossible.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook