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Bounding the speed of `spooky action at a distance'

  1. Mar 7, 2013 #1
    These guys claim "Spooky Action at a Distance" is 10,000 times c.

    This doesn't make sense to me. Since they're trying to measure a time difference that's faster than light, don't they're measurements depend on their inertial frame of reference? I mean wouldn't they come up with a different answer just by changing they're speed relative to the experiment? I would think they could even pick the speed they want, even backwards in time, just by moving the right way.

    Quote from paper:
    In any inertial frame, c is the same. So I don't understand that.

    (I admit I haven't read the whole thing.)

    http://arxiv.org/abs/1303.0614

    http://www.extremetech.com/extreme/...stance-at-least-10000-times-faster-than-light
     
  2. jcsd
  3. Mar 7, 2013 #2
    This is interesting, all of the experiments I've read about this had various problems ("locality loops"). Its very interesting that they confirmed its faster than light. Most likely, this just reflects that it is instantaneous as quanum mechanics predicts.

    Their actual measurement is just a lower bound on the actual speed, so it doesn't really matter what reference frame you're in, especially if the true speed is infinite. An "infinite" speed would just suggest that entangled particles are connected to each other in such a way that measuring one instantly chooses the other.
     
  4. Mar 7, 2013 #3

    DrChinese

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    The Earth's reference frame IS a factor. We can't be certain as to our overall velocity through space. But by considering a period in which the earth rotates, you should be able to locate spots in which there is no component of motion between the two measurement points. That would be when those two points define a line perpendicular to Earth's motion. If the speed of spooky action were, say, 2000c then the correlations could not occur because the measurements were taken too closely together (even considering the very high speed of the action). Obviously that never happened, so the speed of spooky action at a distance must be higher.
     
  5. Mar 7, 2013 #4

    PeterDonis

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    I think what they mean is this: in the set of inertial frames in which the Earth's speed is less than 10^-3 c, the "speed of spooky action at a distance" measured in their experiment is greater than 10^4 c. They're not trying to say that c itself varies. The "speed of Earth" is a way of picking out a particular set of inertial frames, which as you note, you have to do in order to place limits on the "speed of action at a distance" between spacelike separated events.

    I can't find anything wrong with the paper itself, but the extremetech article, as I've come to expect, makes grandiose and unwarranted claims like "now we can have an internet that transmits information much faster than light". Entanglement can't be used to transmit unknown bits of information faster than light.
     
  6. Mar 7, 2013 #5
    Got it. Makes sense now. Thanks.

    The word "instantaneous" keeps making me think there's a preferred frame. Instantaneous for one frame is backward in time for some other. I just can't get around to thinking that QM makes anything happen instantaneously. To me "instantaneous" just means at the same time or simultaneously, which leaves you with the simultaneity problem.
     
  7. Mar 7, 2013 #6

    PeterDonis

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    All they are really "confirming" is that the measurements are spacelike separated; in other words, the error bars on the time and space measurements are small enough to be able to definitely say that the time interval between the events is much smaller than the space interval.

    QM doesn't "predict" that anything "happens instantaneously"; it just predicts that the correlations between the spacelike separated measurements are such that the Bell Inequality is violated. Saying that that reflects an "instantaneous collapse of the wavefunction" or something like that is an interpretation of the theory, not a prediction of it.
     
  8. Mar 7, 2013 #7
    You are right, kindof. If you imagine the measurement of particle 1 being event A, and the measurement of particle 2 being event B, the relative time between these two events is free to vary. If they are simultaneous in 1 frame they will occur at different times in another, just like you said. The thing is, it doesn't matter which occurs first. QM predicts that the two particles are unavoidably linked, and the measurements will be correlated no matter when they occur!

    When I said "instantaneous" I just meant that you could measure them at exactly the same time and you would still find the same correlation as if you measured them an hour apart. So if you make the measurements simultaneously, you will find that the "spooky action at a distance" is instantaneous. But as Peter mentioned, no information is transmitted between the particles, so nothing is actually travelling faster than light.
     
  9. Mar 7, 2013 #8
    idk I just read the extremetech article, I've been to busy to checkout the arxiv paper (bookmarked to read later). I just meant if they did their experiment properly their results would confirm that the measurements are spacelike separated.


    I never said collapse of a wavefunction, personally I prefer MWI :P But QM does predict that the QM correlations are preserved even under spacelike separation. Speaking in lay terms, I would call this predicting that the spooky action at a distance is instantaneous
     
  10. Mar 8, 2013 #9

    DrChinese

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    The Bohmians do believe that there is truly instantaneous cause/effect. And there is a body of work that says that the Bohmian view does lead to a preferred reference frame. Although that is denied by some.

    Also: what is wrong with backwards causation? (Or more properly, a context that includes future components?) This seems perfectly in keeping with many of the sophisticated new experiments on correlations between particles that have never interacted.
     
  11. Mar 8, 2013 #10

    PeterDonis

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    I'm not sure that equating "spacelike separated" with "instantaneous" is standard "lay person" terminology, but I can see how the concepts are connected, yes.
     
  12. Mar 8, 2013 #11

    PeterDonis

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    I'm not familiar with these, can you give any links?
     
  13. Mar 8, 2013 #12

    DrChinese

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    Here is a great example, and it is difficult (at least for me) to imagine a mechanism for this which does not span time in a manner we are not accustomed to.

    http://arxiv.org/abs/1209.4191

    Entanglement Between Photons that have Never Coexisted
    E. Megidish, A. Halevy, T. Shacham, T. Dvir, L. Dovrat, H. S. Eisenberg
    (Submitted on 19 Sep 2012)

    "The role of the timing and order of quantum measurements is not just a fundamental question of quantum mechanics, but also a puzzling one. Any part of a quantum system that has finished evolving, can be measured immediately or saved for later, without affecting the final results, regardless of the continued evolution of the rest of the system. In addition, the non-locality of quantum mechanics, as manifested by entanglement, does not apply only to particles with spatial separation, but also with temporal separation. Here we demonstrate these principles by generating and fully characterizing an entangled pair of photons that never coexisted. Using entanglement swapping between two temporally separated photon pairs we entangle one photon from the first pair with another photon from the second pair. The first photon was detected even before the other was created. The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime."
     
  14. Mar 8, 2013 #13
    Bohmain: Is that the pilot wave theory with the hidden variable going back in time along the path of the particle? I'm kinda partial to that interpretation. I didn't know it could have a preferred frame. I like it because I think it has a nice solution to that problem. I seems to be nicely compatible with special relativity. I always assumed the wave mozies backward through time at the same pace the particle moves forward through time. That way the measurements match all along the path and regardless of when they're done. And nothing instantaneous is needed.

    When I hear "instantaneous", I assume it's an interpretation that intentionally excludes backwards through time. Interpretations seam to have one or the other. I prefer back in time only because I don't like thinking about things moving outside the light cone.
     
  15. Mar 8, 2013 #14
    Most Bohmian models need to have a preferred reference frame so there's no conflict with relativity but there is a Bohmian model that avoids having a preferred reference frame by accepting "backward causation":
    Causally Symmetric Bohm model
    http://arxiv.org/ftp/quant-ph/papers/0601/0601095.pdf

    Demystifier's Bohmian model also avoids a preferred frame by treating time like space:

    Making nonlocal reality compatible with relativity
    http://arxiv.org/pdf/1002.3226.pdf
     
    Last edited: Mar 8, 2013
  16. Mar 8, 2013 #15
    Cool! I did not know that. It needs a better name. What other interpretations are "causally symmetric"?
     
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