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Local non-realistic theory

  1. Oct 10, 2013 #1

    Bell says there is no local realistic theory that reproduces the (experimentally verified) predictions of QM. That leaves us with three possibilities:

    1. Non-local non-realistic (e.g. Copenhagen interpretation)
    2. Non-local realistic (e.g. Bohmian mechanics)
    3. Local non-realistic

    How would a theory of the 3rd kind look like? I can't see how to possibly explain entanglement in a local way.
    Last edited by a moderator: Oct 11, 2013
  2. jcsd
  3. Oct 10, 2013 #2
    Well, a lot of people don't view Copenhagen as nonlocal. They don't view wave function collapse as a phenomenon that genuinely violates locality, so they'd classify it in category 3.
  4. Oct 11, 2013 #3


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    See e.g.
    http://lanl.arxiv.org/abs/1112.2034 [Int. J. Quantum Inf. 10 (2012) 1241016]
    In this interpretation only the observers are real, while the observed objects are not.
  5. Oct 11, 2013 #4

    Brains are information processors, not particle processors, hence cutting down assumptions leaves us with the only solid knowledge available - that nobody has seen a particle, but always information about particles. If you were able to bypass the brain's ability to represent whatever is 'out there' in terms of particles(where they are applicable), you'd certainly see a very different 'world'. As someone remarked - it must be stranger than we can imagine with our mundane concepts(admittedly mathematical physics has broadened that horizon substantially and some bizarre experiments appear to confirm it in very categorical way). There is no problem with locality and realism as long as you realize that reality is not completely classical and cannot be regarded as such anymore(this has been so for a century but change is slow).
  6. Oct 11, 2013 #5


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    Welcome to PhysicsForums, greypilgrim!

    There are a number of local non-realistic interpretations of quantum mechanics. Not all classify themselves as such, but here are a few:

    1. Time symmetric class includes the retro-causal, transactional, relational blockworld and a few others.

    http://www.npl.washington.edu/npl/int_rep/gat_80/ [Broken]

    In the above, there is never any influence that propagates in excess of c. Each constituent interaction is local. However, because time direction is allowed to go either way, the net effect can appear non-local.

    2. Many Worlds (MWI), although they often consider themselves both local and realistic. But obviously they can't be.
    Last edited by a moderator: May 6, 2017
  7. Oct 11, 2013 #6
    Many Worlds - which is really just the statement that measuring devices should be quantum mechanical too. If you take that view, collapse is impossible (because QM time evolution is linear and unitary, while collapse is neither), and interactions are local.

    What happens in MW (or just in QM applied to detectors too) is that when a detector measures a particle that's in a superposition of eigenstates, the detector ends up in a superposition of states. So there's no unique "reality" - instead, Schrodinger's cat states are allowed.
  8. Nov 2, 2013 #7


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    Then from the Copenhagen interpretation can there be a local - non realistic model ( besides Many worlds and retro causality) that agrees with QM (non-linear) correlation predictions for Bell Inequality violations. Where locality and realism are not conjoined. Where instead of classical EPR/ Bell realism , a less restrictive (QM) definition of realism. And this model applied to modern experiments with entangled photons and emphasis on the cos2 (θ -∅) coincidence rate in the explanation.
  9. May 1, 2015 #8
    Imo a local nonrealistic theory would be a non deterministic one because it does not know the real outcome but only a probability.

    For example Chsh operator is AB+AB'+...

    In a local deterministic theory like Bell the knowledge of the angle and the variable induce the result, hence the above quantity would have as result


    Note that the result for the A operator leads to twice the same whereas a nondeterministic variable would give


    Such a theory can violate the Bell inequality but there are orher arguments that show that such one does not reproduce quantum predictions so that quantum is probably both nonlocal and nonrealistic.
  10. May 1, 2015 #9
    The authors of the relational blockworld in a very recent article considered their model realistic but Ψ-epistemic:

    Relational Blockworld: Providing a Realist Psi-Epistemic Account of Quantum Mechanics
    Last edited by a moderator: May 7, 2017
  11. May 1, 2015 #10


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    That's not many worlds - there are many interpretations that treat measuring devices quantum mechanically eg Consistent Histories.

    Collapse is not part of the QM formalism - its an interpretational thing - some interpretations have it (eg GRW models) and some do not (eg BM). The formalism is however ambivalent.

    That is incorrect. In Many Worlds what happens is, via decoherencre, a superposition is transformed into a mixed state ∑pi |bi><bi| where each |bi><bi| is interpreted as a world. Because its a mixed state it, by definition of what a mixed state is, is NOT in superposition. You may be thinking of what happens to the system as a whole - yes that remains in superposition, but the interpretation specifically states its not in superposition as far as 'world splitting' is concerned.

  12. May 1, 2015 #11


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    The problem with Copenhagen is it comes in quite a few variants. Here is the detail of the usual variant:

    Note since the state in that interpretation represents subjective knowledge, even though it has collapse, it is of no concern since it simply occurs in the head of a theorist. It is local and non realistic.

    Last edited: May 1, 2015
  13. May 1, 2015 #12


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    That's true. It's resolution lies in the subtle difference between realism and counterfactual definiteness:

    Personally I believe such subtle differences aren't really worth much, being more philosophy than physics, and MW rejects realism but is local.

  14. May 1, 2015 #13


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    There cannot be a local non-realistic theory, because realism is a precondition for locality - if we classify Bohmian Mechanics as nonlocal realistic - since that means we are using the notion of "local causalty" or "local explainability" when we say "local".

    Also, although it is ok to say Copenhagen is nonlocal nonrealistic, it is better to say that it is nonrealistic, and that is why it fails to be local, since realism is a precondition for locality.

    I'm not sure MWI is local, but it is not necessarily nonlocal by Bell's theorem, since by assuming all outcomes occur, a condition for the theorem to apply is violated.

    As for Demystifier's soplisistic hidden variables, which I like very much, I wouldn't classify it local nonrealistic in the sense implied in the OP, where it seems that the options are those allowed by Bell's theorem after accepting that a Bell inequality has been violated at spacelike separation. I would say that in Demystifier's scenario, the Bell inequalities are not violated at spacelike separation, so the question of nonlocality due to a Bell inequality violation does not arise. A similar situation can arise in Copenhagen, because Copenhagen allows us to consider the other observer "not real", in which case there is also no violation of a Bell inequality at spacelike separation, and the issue of nonlocality simply does not arise.

    One more point about Copenhagen. Although the wave function is not real, it is real FAPP, and collapse is real FAPP. So althouh Copenhagen is nonrealistic (without FAPP), it is also nonlocal realistic FAPP.
    Last edited: May 2, 2015
  15. May 2, 2015 #14


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    I think the same - that there can't be local non-realistic theory, but I would like to add my opinion that there can't be non-local realistic theory just as well.
    People imagine non-locality as FTL communication and that of course does not conflict with realism (while it conflicts with Special relativity). But the problem is that the way non-locality is described and how it is viewed in QM is not FTL communication but fundamental absence of distance in case of entangled particles. But such non-locality is completely new philosophical concept that is mutually exclusive with locality i.e. two concepts can't be fundamental at the same time. And without concept of distance (if non-locality is fundamental) there is no realism.
  16. May 2, 2015 #15


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    Why is realism a precondition for locality ? Why cant they be independent ?
    And in general why no Bell inequality violations at spacelike separation ? I thought that was an example of non locality ?
  17. May 2, 2015 #16


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    Locality is described in terms of physical things that exist and interact independently from our ideas.
  18. May 2, 2015 #17


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    I think both the words "local" and "realism" are ambiguous in their connotations. (Even if they are given unambiguous definitions, there are cases that are intuitively at odds with those definitions) There are at least two senses of "local": (1) Signal locality: No effects can propagate faster than the speed of light. (2) I don't know what the technical word is, but something like "local factorizability". Roughly speaking, the "state" of the universe is completely described by the states of little neighborhoods, together with information about how the neighborhoods fit together. The first is amenable to direct falsification through experiment, and so far seems completely solid. The second is more difficult to test experimentally (because it's much less specific in its observable consequences), but Bell's analysis shows that it can have testable consequences, nevertheless.

    I think "realism" means several different things as well. I don't quite understand Dr. Chinese' claim that retrocausality (back-in-time influences) violate realism. But the two meanings that occur to me are: (1) The mathematical objects in the theory assumed to describe the state of an objective, observer-independent world. This is in contrast to a subjectivist theory, where the mathematical objects might represent an observer's information about the world (subjective probabilities, for instance), but don't directly represent anything objective. (2) Observations or experiments are understood as revealing something objective about the world. A many-worlds type theory can be understood as non-realistic in this sense, because an observation such as looking to see the result of a coin flip is not actually revealing anything objective about the world. You know before you look at the coin that the result will be heads in some branches of the multiverse and tails in some other branches. So observation is not revealing anything about the world (or collection of worlds), it's only "self-locating"; it tells which of the branches you find yourself in.

    QM, in which the wave function is viewed purely epistemologically, as a description of an observer's knowledge, then that's not a realistic model according to sense (1), but it can be realistic according to sense (2). QM is local according to sense (1), but not according to sense (2) (because of entangled states).

    MWI is realistic, according to sense (1), because it views the wave function as real, but it is not realistic according to sense (2). It is local according to sense (1), but not according to sense (2).
  19. May 2, 2015 #18


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    By "locality", I meant "local causality" or "local explainability". A cause or an explanation must be real, by definition.

    By using "local explainability" as a synonym for "local causality", I am using similar terminology as

    Yes, if the Bell inequalities are violated at spacelike separation, then local causality is ruled out.

    However, if one denies that the Bell inequalities are violated at spacelike separation, then Bell's theorem does not apply, and local causality is not ruled out.

    So nonrealism can be used to prevent the violation of the Bell inequalities and thus prevent ruling out local causality by Bell's theorem. But if the Bell inequalities are violated at spacelike separation, then nonrealism cannot save local causality, since realism is a prerequisite for local causality.

    Norsen makes this point in http://arxiv.org/abs/quant-ph/0607057, which is basically in line with the references from Hall, Cavalcanti and Lal, and Wiseman and Cavalcanti given above. (Norsen is entertaining, maybe too entertaining, and for "serious" physics perhaps I should only have given the other references, but Norsen is where I first read the point, and his argument is acknowledged by others, eg. http://arxiv.org/abs/0706.2661.)
    Last edited: May 2, 2015
  20. May 3, 2015 #19
    Norsen sure has no time for MWI, does he?
    "The bottom line is the impossibility of any scientific basis for any (allegedly scientific) theory requiring the rejection of Perceptual Realism. MWI requires such a rejection, and hence cannot be taken seriously as a scientific theory"
    Pulls no punches there!
  21. May 3, 2015 #20


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    He does like some aspects of MWI: "Despite the harsh criticisms of MWI in the previous section, there is one aspect of the theory which I fully support: it accepts the existence of a single, objective, external world “out there” whose existence and identity is independent of anyone’s awareness (or, in the case of MWI, non-awareness) of it."
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