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Advanced Question about EPR / Bell / Apsect

  1. Mar 21, 2005 #1
    [I invite people to post links as responses in case the idea I'm putting forward
    is already covered someplace that I missed.]

    Something has been bothering me about this type of experiment for
    a long time, but this is clearly the place to bring it up.


    My problem is this: that in an EPR-type experiment, one simply cannot say
    that one of the measurements took place before the other. Ever. So it
    is simply NOT true that one measurement forces the other into a known
    state.

    Of course we *can* wait at B for the A-B order to become unambiguous
    but this is exactly the uninteresting case in EPR. It's always the interesting
    case when the two measurements are not in one another's light cone.


    I advance the idea that the correlation must take place and therefore that
    the macroscopic A measurement outcome is actually entangled with the
    corresponding macroscopic B measurement on the other side. In other
    words, widely seperated quantum engangled measurements are in fact
    macroscopically entangled.

    Comments?
     
  2. jcsd
  3. Mar 21, 2005 #2
    Well, what are you meaning by macroscopic? An event due to a lot of quantum particles interacting at once, or a single particle interaction outside the light-cone of the corresponding particle?
     
  4. Mar 22, 2005 #3

    DrChinese

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    Assuming that you accept the idea that you can deduce the order of space-like separated events in the first place...

    It is meaningful to say that a measurement of one particle places the other particle into a known state. If I measure A "before" I measure B, then I can say that I now know B's state. Any subsequent measurement on B will be consistent with that knowledge, and so my statement is experimentally verifiable.

    In fact, I can even bring the B photon back to where I measured A at a later time (using fiber optics, for example) and prove to myself that one measurement occurred before the other. What is your problem with that?
     
  5. Mar 22, 2005 #4

    I have no problem with that. Neither did the EPR authors because in that
    case, it is plausible that a causitive influence has traveled (at no faster
    than c) from A to B. This is the uninteresting case which I'll call Situation 1.

    The EPR situation was posed precisely to examine Situation 2, the case
    where the influence fom measurement A cannot have reached B at luminal
    speeds, yet the correlation is there. And in this case, special relativity
    says it is meaningless to assert that A ocurred before B or vice versa.

    What I am saying is this: It is not meaningful to speak of one measurement
    happening before the other so long as you are in Situation 2 because the
    order of measurement cannot be established- it is frame dependent.


    Well, I mean macroscopic like looking at a bucket of superfluid with a
    quantized vortex in plain sight. Not a small system, just a quantized one.

    If the correlation must occur (as is experimentally found to be the case)
    but the question of order is meaningless than you have 2 widely seperated
    macroscopic results that will come out unknown but correlated.

    To re-clarify macroscopic, it's no longer a quantum question of a small
    system. It's the result as seen by the "large" instruments in
    two laboratories.
     
    Last edited: Mar 22, 2005
  6. Mar 22, 2005 #5

    DrChinese

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    1. I don't think most physicists would agree that SR states that measurement A cannot unambiguously occur before B in both A and B's reference frames. But maybe I'm wrong on this point.

    2. And what you call the "uninteresting" case is interesting to a lot of people. Aspect already showed that locality has nothing to do with EPR results. So that separation is no longer required in subsequent experiments to demonstrate violation of Bell Inequalities. It is a misunderstanding of what Aspect's time varying analyzers were doing in the first place to suggest otherwise. It just demonstrated that the detection systems themselves were not active participants in the results using some previously unknown deterministic communication mechanism. That was ruled out once and for all. (You also don't need to test for results varying by day of week, as this was ruled out once and for all a long time ago .)
     
  7. Mar 22, 2005 #6

    1. - Yes, this is a well-known result in special relativity. Some of the fun
    "parlor tricks" involve a 100 foot rocket going through a 90 foot garage
    with both front and back doors closed at once, yada yada. But the
    serious element of it here is that simultanaity and the order of events
    seperated in space is not an absolute.

    2. - It was uninteresting to E, P and R. It may have expeimental interest
    now for sure, but it was not the case meant to bring about a contradiction
    between relativity and QM.

    2a. No, Aspect's analyzers may or may not have been set up that way,
    I don't know and I'm not suggesting they were or not. What I'm saying
    is that there are two cases and only in one is there an apparent
    problem with superluminal causation/correlation.

    2b.
    No, there is MUCH more to it than that. The problem as I've aluded to,
    and the whole reason for thinking up the thought experiment in the
    first place was to assert that if you took quantum mechanics seriously,
    you may imply superluminal communication through spacetime. Either that,
    or the results are determined prior to the measurement. That's the real
    meaning of the nonlocality that you referred to.
     
  8. Mar 22, 2005 #7

    Hans de Vries

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    A while ago here I proposed an EPR experiment using a Parametric Down
    Converter which emits the second photon long after the first one. (ca. 5 ns)
    For instance the one used in the historic experiment performed by Grangier,
    Roger and Aspect.

    http://chaos.swarthmore.edu/courses/phys6_2004/QM/17_EPR_Bell_Details.pdf

    In this experiment there are always reference frames in which the state of
    the A photon is detected before the B photon is even emitted! The proposed
    experiment should assure that this is the case in all reference frames.

    I personally have not seen any investigation in the SR consequences of EPR
    experiments. Like an attempt to determine in which particular reference frame
    the "instantaneous" action occurs. There is generally very little physics
    discussed.


    Regards, Hans.
     
  9. Mar 22, 2005 #8
    Hans,

    That's an outstanding paper. I will study it to gain a better understanding of the issues.
     
  10. Mar 22, 2005 #9

    DrChinese

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    Nope. The results are NOT determined prior to the measurement, that is specifically ruled out. There can be superluminal action. And why wouldn't you take QM seriously?

    And your comments about SR really have absolutely nothing whatsoever to do with EPR, and they did not need to consider the semantics of the word "before" in their paper either. They specifically assumed the model I described, because they mentioned that the results of an experiment on a space-like separated particle could be predicted in advance. Einstein did not know about Aspect, so obviously locality was of more interest to him then. Who knows what he would say today?
     
  11. Mar 23, 2005 #10
    I do take QM seriously. If you go back and read what I wrote, you'll see
    that it was Albert Einstein who was first troubled by it, not me. And there
    is NOT superluminal action, merely correlation. Which is at the heart of
    the question and why I bring up the order of observers.

    I think you're missing the point Dr. I know that SR is not usually associated
    with EPR and that it wasn't part of the original discussion way back when.

    That's the whole point of my post- I'm the one bringing it up here and now.
     
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