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QM vs SR

  1. Mar 23, 2009 #1
    Greetings! This is my first post so go easy. I read through the thread addressing the Sci Am article claiming that entanglement violates relativity. The article actually addresses a symptom of a larger issue that exists between QM and SR. I would like to get feedback on this issue as illustrated in the following picture...

    3379413463_f6f19c76e5.jpg
    http://farm4.static.flickr.com/3444/3379413463_f6f19c76e5.jpg"

    "If a piece of knowledge is calculated about a system with certainty then such knowledge is intrinsically represented by the system in the form of a physical reality without the need for a verifying measurement." - paraphrasing of the critical assumption made by the authors in the EPR paradox paper.

    The attached picture contains two entangled particles in an EPR experiment, as seen from the perspective of two different observers moving relative to one another. Analyze the picture and reconcile it with the above statement with consideration to "when" each particle's wavefunction has collapsed. The point is that the wavefunction supposedly collapses "simultaneously" between the particles, yet simutaneity is (in absolute terms) meaningless in a Relativisitic world. As can be seen in the picture each particle's wavefunction has collapsed before it is measured.

    I'm not claiming that this violates either theory. It does appear to impose certain restrictions upon which QM interpretations remain viable, however. Before I get into my opinion on that, though, I would welcome comments.

    Thanks
     
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Mar 23, 2009 #2

    DrChinese

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    Welcome to PhysicsForums!

    In QM, the order of these type measurements does not matter to the expected results. So in that sense, special relativity is not an issue.

    On the other hand, entangled particles do seem to be connected in a way which is "non-local" and therefore somehow violating the speed of light c. However, no one has been able to put their finger on any way in which this actually conflicts with any specific part of SR. Certainly, no information can be transmitted in this manner (as we currently understand things).

    Also, you can't technically rule out interpretations of QM on this basis, as you seem to suggest. However, it might affect your own preference. (I certainly *prefer* an interpretation in which c is a fundamental constant. I also prefer an interpretation in which h is a fundamental constant.)
     
  4. Mar 23, 2009 #3
    DrChinese: Hello, and thank-you. This issue is not related to the order of measurements, which I understand would not affect the results. I am speaking specifically about the physicality of each particle, respectively, as described in a manner local to that particle. In other words, a typical QM interpretation might claim that particle A is in a state of superposition at Ta = 10, and possessing a definite spin at Ta=110. What is the description of particle A at Ta=75, however? Is it both in a state of superposition and possessing a definite spin? This would seem to contradict http://en.wikipedia.org/wiki/Lorentz_covariance" [Broken] of space-time.

    Also, once (or rather if) one believes there is a problem here, then it is a small step to see that the many-worlds interpretation suffers from the same problem...
     
    Last edited by a moderator: May 4, 2017
  5. Mar 23, 2009 #4

    DrChinese

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    There are definitely a lot of interpretations that deny there is a local physicality to such particles (since they are in an entangled state - a superposition) and therefore not in a state of definite spin. If you believe the HUP is fundamental - as I do - then you typically would not assert that particles have well-defined attributes outside of a specific measurement anyway. Some folks call that "contextuality".

    I don't think this actually defies the Local Covariance you describe, as the predicted results will be the same in all frames as far as I can see.
     
    Last edited by a moderator: May 4, 2017
  6. Mar 23, 2009 #5
    I think a state of superposition or possessing a definite spin is the same thing for spin.It depends on how you choose the basis vector of hilbert space.But spin mixed state(as sunlight polarization) are different from "definite spin"(or superposition, which is the same thing).
     
    Last edited by a moderator: May 4, 2017
  7. Mar 24, 2009 #6
    Why do you think it contradicts Lorentz Covariance? Do you believe the spins should have an altered 'rate' due to the relative velocities between them - or something along those lines?
     
  8. Mar 24, 2009 #7
    DrChinese: Thanks for your feedback. To deny a physicality to the particles until they are locally measured seems reasonable, but then you must ask yourself what the nature of the wavefunction is. The wavefunction is intended to be a mapping of the possible states of the system which it is describing. Now, for Observer X, particle B has a definite spin at Tb=100 and Ta=70 (his simultaneous space), so I assume we can agree that particle B is not in a state of superposition at this time. If one believes that particle A continues in a state of superposition between Ta=70 and Ta=100, then what wavefunction describes the possible states (i.e. what are the possible spins) of particle A for that period? There is only a single answer (specifically, the complement of particle B), whereas before particle B was measured there were many, which to me suggests that any wavefunction describing particle A as having the same "options of existence" before Ta=70 and after Ta=70 is the wrong one.

    I appreciate your reference to HUP, and I'm not suggesting it isn't fundamental, but a fundamental limitation upon knowledge available to us does not necessitate that the physicality of the system in question simply doesn't exist.

    Aristurtle and Debra: Unfortunately, I labeled the particles as photons in the diagram while in my text I am referring to measuring "spin" (rather than angle of polarization), which may be confusing. You may think of the particles as electrons whose spin must be complementary in order for the conservation of angular momentum to hold. Local Lorentz Covariance would be violated if particle A possessed a spin at a given point in space-time yet did not possess that same spin at the same point in space-time after a Lorentz transformation.
     
  9. Mar 24, 2009 #8

    DrChinese

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    We know there are issues when you make deductions about measurements that weren't made. :smile: Obviously, you can end up with a paradox. As to the HUP: believing in it does not, as you point out, require you to deny physicality of elements of the system under study (HUP implying limits to our knowledge). But now, after numerous studies of entangled pairs, you might agree there is the distinct possibility that the HUP actually reflects the underlying reality itself.
     
  10. Mar 24, 2009 #9
    DrChinese: Forgive me but I took the liberty of checking out your website, and specifically your resume, because I've discussed this issue with a handful of Physicists and these words would never pass their lips, except behind closed doors if they believed it. Actually it appears you and I have quite a bit in common (I have a degree in Comp Sci but a passionate hobby in studying QM). It appears we may differ on politics...but I digress :wink:

    Anyway, I'm having a problem understanding the relationship you are referring to between HUP and entangled particles. I agree that HUP will apply to the results of any measurement made upon a system, quantum or not, but I am not personally familiar with lab results which suggest that the HUP limitation is due to a lack of physicality of the particles themselves. Or maybe you are suggesting a parallel in that if we assume that HUP is a consequence of the indefiniteness of reality then it makes accepting wavefunctions as a physical representation of reality seem a more natural thing to do?
     
  11. Mar 24, 2009 #10
    in entanglement, the probability for the two photons is treated as a system, rather than as two individual photons. they share a probability function, such that when you determine polarity for one, the wave function collapses for the system, determining the state for both photons at the same time. SR is not violated.

    another way to look at this is to recall that "communication" between photons (unbound by c) is not the same as information transfer via photons to the physical world (bound by c). photons exist at c, and hence their wavefunctions occupy all locations within the universe simultaneously, overlapping - i.e., there is no distance between photons.
     
  12. Mar 24, 2009 #11
    jnorman: thanks for joining the discussion; I understand and do not contest what you say. It is your opinion, then, that particle A possesses a distinct spin at Ta=70 without the need for a verifying measurement due to the A+B system's wavefunction collapsing upon particle B's measurement at Tb=100?
     
    Last edited: Mar 24, 2009
  13. Mar 24, 2009 #12

    DrChinese

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    :smile:

    If there is particle property realism (or hidden variables or whatever you want to call it) depper than the HUP allows us to see: then why do entangled particle pairs "conspire" to hide that from us? A reasonable expectation - certainly that of Einstein - was that you would be able to "beat" the HUP using entanglement. That was the EPR Paradox. But that was later demostrated to be false, refuting the EPR statement to the effect (last paragraph of the paper) that "no reasonable viewwould allow reality to be shaped by a measurement at a distant location."

    As to the paradox I was referring to about making assumptions about mwasurements not made: Hardy's Paradox - which has been experimentally verified recently - specifically involves making assumptions about the history of a particle.

    Direct observation of Hardy's paradox by joint weak measurement with an entangled photon pair

    I'll see if I can post some additional related to this too.
     
  14. Mar 24, 2009 #13
    DrChinese: Measuring each particle along a different axis produces results which speak to the very nature of quantum phenomena - the particle will produce a spin along any axis measured. I don't believe that this is an attack on HUP, nor a validation of it; rather I believe Einstein was using HUP in his example to prove that QM was incomplete because it was a virtually incontrovertible tool for him to use at the time, as was the notion that the propagation of information at speeds FTL was impossible. He was wrong, of course, but not because his weapons were faulty; only his interpretation of the QM results.

    I look forward to discussing my opinion of when the wavefunction collapses, including Hardy's paradox, but first I would like to tally your "vote". Do you believe that the wavefunction collapses upon measurement? If so, does measurement of any part of the system suffice? Could you refer to the picture when giving your answer? Yes, I'm probing a bit, but it's because I'm searching for new perspectives which I feel are consistent, and in my experience most perspectives are simply not completely thought out because the math behind QM and its predictive accuracy is so powerful that a full explanation of what is going on is simply not needed.:bugeye:
     
  15. Mar 24, 2009 #14

    DrChinese

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    Another experimental version of the Hardy Paradox

    "For nearly a century, the widespread interpretation of quantum mechanics suggests that everything is uncertain until it is observed, and that observation inevitably alters reality," says Professor Steinberg. "However, in the 1990s, a technique known as 'interaction-free measurement' seemed to promise the ability to 'see without looking,' as a Scientific American article put it at the time. But when Lucien Hardy proposed that one could never reliably make inferences about past events which hadn't been directly observed, a paradox emerged which suggested that whenever one attempted to reason about the past in this way they would be led into error."

    Over the course of nearly two years of work, Steinberg and then-student Jeff Lundeen, now a research associate at the National Research Council of Canada, built a complicated quantum optical experiment and developed new theoretical tools. In essence, they combined Hardy's Paradox with a new theory known as weak measurement proposed by Tel Aviv University physicist Yakir Aharonov, showing that in one sense, one can indeed talk about the past, resolving the paradox. Weak measurement is a tool whereby the presence of a detector is less than the level of uncertainty around what is being measured, so that there is an imperceptible impact on the experiment. "We found that all of the seemingly paradoxical conclusions in Hardy's Paradox can, in fact, be experimentally verified," says Steinberg, "but that the use of weak measurement removes the contradiction."


    The point, as it relates to your post, is that discussing timing of these measurements similarly is using classical reasoning to make inferences. Those inferences, with entangled particles, are already known to be paradoxical. And yet QM keeps on going, and going... :smile: I am a strong believer in the fundamental nature of c, but I don't think there is an actual discrepancy between these theories. Somehow, they just manage to brush up against one another without being in absolute conflict.
     
  16. Mar 25, 2009 #15
    Well my personal explanation is a version of Absorber Theory which involves retro-causality. I'm a bit tired to go into it right now but accepting block time returns much (almost all!) of our classical intuitions that QM heretofore has required us to throw out the window (e.g. locality, reality, determinism, et al). It also has the benefit of being a FULL explanation, rather than something like MWI which, in my experience, reduces to a bit of arm waiving once its proponents are questioned. I'll explain more tomorrow if you care to hear it.
     
  17. Mar 25, 2009 #16
    Yes, there's a problem, wrt the assumption that is the point of departure for MWI and the basis of your OP. It's exemplified in the following statements/questions (and some others in the thread):
    These statements seem to assume that quantum wavefunctions are descriptions of real physical states in some underlying reality between emitters and detectors.

    Why should we make that assumption? I can't think of any good reason for it. On the other hand, there are some good reasons to assume that quantum wavefunctions are not descriptions of an underlying physical reality -- not the least of which is that it obviates all the problems, paradoxes, nonsensical interpretations, and conflicts with SR that come with assuming that they are.

    Whether this was the point of your post or not, you have, imho, in effect, provided yet another variation on that theme.

    QM doesn't require us to throw locality, reality, or determinism out the window.

    The acceptance of 'block time' (a la GR?) entails some inconsistencies with some very sound, imho, inferences from observations.
     
  18. Mar 25, 2009 #17
    The electrons exist in information space as data - all next to each other - and only communicate with 3D space through the HUD / wavefunction when 'observed'. Otherwise they are not in 3D space - they send information to co-ordinates in 3D space that are being 'observed' (3D space is also algorithmic data in information space).

    Entangled particles share the same piece of data about state correlation because they are in reality, not 'living' in space-time they 'live' in information space where there is no spacial separation.
     
  19. Mar 25, 2009 #18
    Alright, I'm not very familiar with information space (so please correct me if I misinterpret something), but I don't see how this resolves the paradox.
    Is the proposition that the information on the result of the measurement has always existed, and the two measurements independently retrieve it?
    Or is the information in some way altered by the measurement? In this case we would still have the "which event happened first" paradox.
     
  20. Mar 25, 2009 #19

    DrChinese

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    Like Cramer's? http://www.npl.washington.edu/npl/int_rep/gat_80/ [Broken]

    Along the same line, there is more recently (Aharonov & Tollaksen, 2007): New Insights on Time-Symmetry in Quantum Mechanics

    Both of these preserve the fundamental status of c and h, and explain the reason for the apparent non-locality we observe with wave function collapse. I think these type approaches deserve more attention.
     
    Last edited by a moderator: May 4, 2017
  21. Mar 25, 2009 #20
    Am I correct in that this paradox is in CI, but not, for example in MWI?
     
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