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Consensus about Non-Locality & Spacetime

  1. Dec 23, 2011 #1
    What's the present most popular consensus about non-locality and spacetime? Is it since the wave function is not something physical, there is nothing there in spacetime to be non-local about. So let's just extinguish the concept of physicality this means we just treat wave function and spacetime as just equations and don't try to have physical picture of it, and this thinking is enough to put it under the rug?

    To get in the mood. The following is interesting stuff from March 2009 Scientific American article called "Was Einstein Wrong? Quantum Threat to Special Relativity":

     
  2. jcsd
  3. Dec 24, 2011 #2
    I never understood that. I mean, if we take that stance, then what do the equations refer to?
     
  4. Dec 27, 2011 #3

    Demystifier

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    There is no consensus. Some argue that nature is nonlocal, while others argue it is local. For a very brief list of local interpretations see
    https://www.physicsforums.com/blog.php?b=3622 [Broken]
    which you can use for further googling.
     
    Last edited by a moderator: May 5, 2017
  5. Dec 27, 2011 #4

    Demystifier

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    Wave functions are, of course, "physical", but the question is whether they are ontological or merely epistemological. If they are only epistemological, then the ontological may or may not be local, depending on what the ontological, if anything, is.

    For example, ontology may refer to hidden variables which describe only the observer and not the observed objects, in which case ontology may be local:
    http://xxx.lanl.gov/abs/1112.2034
     
  6. Dec 27, 2011 #5

    Ken G

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    I can't at present see any real difference between CI and the solipsistic hidden variables. Nikolic appears to associate the hidden variables as being part of the observer, but that is also what Bohr always said-- physics is what an observer can say about his/her surroundings, so what is unknown about physics is what is unknown about the observer. Perhaps Bohr felt it was unknowable, whereas Nikolic might attribute it to potentially knowable internal "hidden variables" in the observer, but the latter claim is so wholly unsubstantiated I cannot see any real difference. One superficial difference appears to be whether or not the existence of an "objective reality" is postulated, but all Bohr was doing was noting the fact that the word "objective" implies some kind of mutually agreed-on consistencies, which of course depend on the observers to be able to arrive at. Claiming that there's no such thing as an objective reality is thus the same thing as saying that we cannot know how the observer perceptions are altering that reality, which is also the same thing as saying that the variables needed to describe those perceptions are "hidden". Until someone can suggest a way to "unhide" those variables, I see no scientific difference at all.
     
  7. Dec 27, 2011 #6

    Demystifier

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    To a certain extent, you are right. Still, there are some differences between Copenhagen Interpretation (CI) and Solipsistic Hidden Variables (SHV). You may feel that the differences are not important, and I will not insist that they are, but still they exist. Some of them are:

    - While CI is rather vague on the concept of observer and its role, SHV provides an explicit quantitative model of the "observer" in terms of particle trajectories satisfying definite equations of motion.

    - Some people prefer Bohmian interpretation over CI because they find CI rather unintuitive. SHV can be viewed as a variant of CI which will be much more intuitive to adherents of the Bohmian interpretation.

    - Some people dislike Bohmian interpretation because they feel that all these Bohmian particle trajectories are not really needed to explain the quantum phenomena. SHV explains more clearly why all these trajectories are indeed not necessarily needed, but at the same time why at least some radically restricted set of trajectories seems to be needed.

    - SHV provides an explicit counterexample to the wide belief that it is simply impossible to have local hidden variables compatible with QM. Perhaps the price is very big, but it's possible. By having such a counterexample one can better understand what REALLY is impossible.

    - Perhaps the most interesting or most important value of SHV is to demonstrate that Copenhagen interpretation and Bohmian interpretation are not really so different as most physicists think they are. SHV interpolates between them, i.e., they can be viewed as two different limits of one "unifying" interpretation - SHV interpretation.
     
    Last edited: Dec 27, 2011
  8. Dec 27, 2011 #7

    Ken G

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    Yes, I can accept that SHV navigates the differences between CI and deBB, indeed I would say its greatest value is in helping to show why these two interpretations are not necessarily any different either (even though they seem as ontologically different as F=ma and least action). When "many worlds" is similarly unified, we can finally see that all the interpretations are simply barking up the same tree, which is the question "what does the observer do?" To me, the main purpose of CI is to draw attention to this question, without actually answering it, so any approach that also draws attention to that question is a close cousin to CI. Just what the differences between the interpretations actually are, in my opinion, is an issue that must await the answer to that question, if it is even answerable at all.
     
  9. Dec 27, 2011 #8

    But I still don't understand. I mean, typically "physical" meant something spatial/extensible or existing in space-time/field. I don't think there's a problem with treating the wave function as something that defies this explanation (e.g. evolves in configuration space, etc.) and yet still being "physical" but in that case, what would the difference between ontological or epistemological mean? Something is physical and epistemological versus something that is physical (in this novel non-local/holistic way) and ontological? This is confusing. Also, I don't see how a model could be both non-local and local, unless one somehow emerges from the other? It seems weird that there would be 2 different types of "spaces"/ontologies (objects in 3-dimensional space and the wave function in 3-N dimensional configuration space) with both being "physical" and yet they interact, in some way.
     
    Last edited: Dec 27, 2011
  10. Dec 27, 2011 #9

    Ken G

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    One possible meaning for "physical" is simply "of use in physics." That's actually the most defensible meaning for the term, and the wavefunction is certainly that, but I agree that often people imagine a much more highly extrapolated version of the term-- but it's not clear that such extrapolations really mean anything. If one leaves the realm of simply what is useful in physics, then what does one have?
     
  11. Dec 28, 2011 #10

    Demystifier

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    Yes, I can more-or-less agree with that. I only don't agree that we must necessarily await the answer to that question. We should at least try to answer it, because if we don't, the answer will not pop out spontaneously without our assistance.
     
  12. Dec 28, 2011 #11

    Ken G

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    The question is, what does "our assistance" look like? I think it looks like clever insights into the next set of observations that will guide us. Any inquiry into questions like which is more real, configuration space or real space, can help if they focus the question in a way that can lead to that kind of observation. It is certainly a noble effort, and the next big theory might rely on it, I just don't know if we are ever really going to be able to understand what the observer is doing-- the question is just so intimately intertwined with our efforts to answer it.
     
  13. Dec 28, 2011 #12
    Hi Ken, For weeks we have struggled in another thread in uncovering physics of Preferred Foliations courtesy of Maudlin paper on "Non-Local Correlations in Quantum Theory: How the Trick Might Be Done", part of it mentioned:

    "It has been a constant complaint against Bohmian mechanics, from its inception, that it “has no Relativistic version”. The reason that the theory is hard to reconcile with Relativity is clear: it is because of the way the non-locality of theory is implemented. In fact, the easiest way to extend that implementation to a space-time with a Lorentzian metric is to add a foliation, as we have seen. There may be some other way, but no one has discovered it yet."

    http://xxx.lanl.gov/abs/1002.3226

    I guess Maudlin was wrong because he has not known about Demystifier paper on "Making nonlocal reality compatible with relativity" at http://xxx.lanl.gov/abs/1002.3226 which is based on peer reviewed Physical Review Journal paper. Can you please read it to see if it is viable enough or is there possible flaw that we still can't determine in the other thread and it is giving us sleepless nights?

    Part of the paper quote:

    "O: Isn’t it shown that the Bohmian interpretation requires a preferred Lorentz frame?
    R: That is true in the usual formulation of the Bohmian interpretation based on the usual formulation of QM in which time and space are not treated on an equal footing. When QM is generalized as outlined in 2) above, then the corresponding Bohmian interpretation does not longer require a preferred Lorentz frame.
    O: I think I’ve got a general idea now. But I’ll not be convinced until I see the technical
    details."

    Need your valuable comment Ken and others who can grasp the math of the paper. Thanks.
     
  14. Dec 28, 2011 #13

    Ken G

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    That might take a little time, but they do look like interesting questions. Can you link in the Maudlin paper as well? Personally, it doesn't bother me whether the Bohm approach can be made relativistic, because I view the standard version of Bohm as an interpretation of nonrelativistic quantum mechanics. It doesn't matter to me if an interpretation of nonrelativistic quantum mechanics can also be an interpretation of relativistic quantum mechanics, because it would not be unusual for a nonrelativistic theory to call for a different interpretation than a relativistic one. Certainly that is true of ordinary classical physics as it is taught to this day.
     
  15. Dec 29, 2011 #14

    Demystifier

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    The reference is:
    Maudlin, T., “Non-Local Correlations in Quantum Theory: Some Ways the Trick Might
    be Done”, Einstein, Relativity, and Absolute Simultaneity, ed. Quentin Smith and
    William Lane Craig, Routledge 2007, 186-209.

    My paper above did not yet exist in 2007 when the Maudlin's paper was published.
     
  16. Dec 29, 2011 #15
    When I put "http://xxx.lanl.gov/abs/1002.3226" in the search window at Physicsforum. It doesn't exactly give the threads with the url being discussed.. it only returned threads with any of the words gov, lan, 1002... this seems to be a flaw the the web site design.. unless there is really a way to search only exactly for that phase? Anyway, if you remember the thread where this was discussed before. Let me know so I can see the comments of others. Thanks.
     
  17. Dec 29, 2011 #16

    jtbell

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    Use the "Search PF via Google" option and enclose the string in quotes ("").
     
  18. Dec 29, 2011 #17
    Ken, are you a Quantum Mechanic, or a Relativistic Geometry Surveyor? If you are well verse in the latter too.. hope you can comment on at least the following paragraph in Demystifier paper.


    "By 2) I mean that time and space should be treated on an equal footing. Note that in the usual formulation of QM, time and space are not treated on an equal footing. First, for one particle described by the wave function psi(x,t), the infinitesimal probability in the usual formulation is |psi|^2d^3 x, while from a symmetric treatment of time and space one expects |psi|^2 d^3 x dt. Second, for n particles the wave function in the usual formulation takes the form (x1, . . . , xn, t), while from a symmetric treatment of time and space one expects (x1, t1, . . . , xn, tn). I formulate QM such that fundamental axioms involve the expressions above in which time and space are treated symmetrically, and show that the usual formulation corresponds to a special case."

    What do you make of it where he said time and space should be treated on an equal footing? Don't we treat space and time as equal footing now? Time is in imaginary axis while space is in real axis. Perhaps what he did is make time another space too? (what don't we and if not why do we not do it in the first place?)

    If Demystifier could only make presentation like Brian Greene where laymen can understood everything, much better.. but it looks like Demystifier only speak to physicists.
     
  19. Dec 30, 2011 #18

    Ken G

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    This is how it works in special relativity, but not in nonrelativistic quantum mechanics (and even relativistic quantum mechanics is often a kind of cluge, it doesn't always seem very natural). I am not an expert in relativistic quantum mechanics, but I think what Demystifier is doing in that paragraph is casting regular old nonrelativistic quantum mechanics in a framework that treats time and space symmetrically, so that a later extension to relativistic quantum mechanics will seem more natural. The problem that I see is that the asymmetry between space and time in quantum mechanics goes deeper than just the way we intepret probability measures. It seems to me that the way he is altering the probability measures is pretty natural but by itself isn't really saying anything all that new (I haven't yet gone further to see where he goes with it though), it's pretty straightforward. It's not clear that it yet addresses the deeper issue, which is that time is not an observable in quantum mechanics-- there is no global "time operator", although there can be operators in each situation that function like a time operator in the sense of being instantaneously complementary to the energy operator. I just mean there's an energy/time uncertainty principle, similar to the position/momentum one, but it is actually a bit different because we don't actually have a time operator and we don't talk about time bases, we think of a particle as being in a superposition of different energy states but not as being in a superposition of different "time states." Maybe we should, I don't know.

    What I mean here is that we don't do time measurements on systems, we simply use clock readings as a kind of bookkeeping tool to tell us which predictions apply to the actual measurements that we are making on the system. We can measure the location of a particle, or the momentum of a particle, or the energy of a particle-- but the time is still just what the clock on the wall reads, to tell us when to stop the unitary evolution in our calculation. This is particularly clear in the Heisenberg representation, where there isn't the usual separation between the state of the system and the measurement operators on the system, there is just the fixed measurement basis, and the time-dependent expectation value of a measurement in that basis. A truly time/space symmetric treatment would need to include a concept of destroying the coherences between different times, as if when an observation was made had to be a key element of the docoherences generated by any measurement. We could contrast that with measurements whose time of application was inherently indeterminate, to begin to understanding the meaning of a superposition of events occuring at different times. We're not really used to thinking about quantum mechanics that way, but maybe that's just what is needed to make it relativistic.
    The symmetry of time and space in relativity was always a bit of a shock, because we don't perceive them as symmetric or similar in any way. But that doesn't make time "just another space", because the signature of the metric is different with regard to space and time (like you said, the time axis is in some sense imaginary compared to the spatial axes), so the two will always be juxtaposed as much as opposites as they are cousins. Spacelike and timelike separation are really quite different animals in relativity. But at least they are opposite faces of the same coin-- in quantum mechanics, we tend to think of them as having totally different places, as if they appeared on different coins altogether, since location is an observation that can be performed on a particle, whereas we never perform time measurements on particles-- we just look up at the clock on the wall when we are doing whatever measurement on the particle we are actually doing. This is just yet another difficulty in unifying relativity and quantum mechanics, and I haven't yet gone far enough into Demystifier's paper to see if he is really able to address it.
    There are tradeoffs there. Brian Greene is considered to be highly successful at making complex ideas accessible to nonphysicists, but is he really conveying a true understanding, or just a kind of illusion of understanding? I won't take a position there because it would require more specifics, but as a teacher, I've seen the pitfalls-- students are often happiest when they are allowed to believe that they understand better than they actually do, whereas if they are challenged to really dig into their understanding and find the inconsistencies, it might make them feel dissatisfied and frustrated. It's a very delicate balance to walk, because we only turn people off if we make them feel stupid, but we do them no service if we make them feel like something makes sense when it actually falls apart like a cheap suit when put to the test.
     
    Last edited: Dec 30, 2011
  20. Dec 30, 2011 #19
    Do you agree with the following comment by the author of "Physics Meets Philo at the Planck Scale"?

    Meaning all these can be resolved by a theory of quantum gravity.. which is not just about planck scale physics but the nature of space and time itself and how they are connected to matter. If it were true that both general relativity and quantum theory emerge from a theory very different from both, then we have to rethink about space and time. Therefore Ken, you must be a quantum gravitist to handle both problems. What do you think of the following statement by the same author above:

    What do you think?
     
  21. Dec 30, 2011 #20
    I think Maudlin would probably argue against treating space and time on an equal footing. In fact he does defend the difference between space ant time in this paper:

    Remarks on the passing of Time
    http://www.jstor.org/pss/4545373

    So, I'm guessing he would probably question any attempt (like Demystifier's) to treat space and time as the same? But maybe I'm mistaken?:
    Fundamental Physical Theories: Mathematical Structures Grounded on a Primitive Ontology
    http://www.niu.edu/~vallori/thesis4.pdf [Broken]
     
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