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Bell's experiment and causality

  1. Jun 17, 2004 #1
    why does Bell's experiment damage causality or cause such problems regarding faster than light information via the instantaneous and corollary collapse of the wave function? photons do not experience time, to them the spin measurement and the emission occur simultaneously...
     
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  3. Jun 17, 2004 #2

    ZapperZ

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    You have faulty information from TWO different areas of physics.

    1. The time dilation, in which you made use of when you said that ".. photons do not experience time.." is NOT the time period of a photon, but rather the time measurement that WE observer OF the photon. Recheck your understanding of Special Relativity. WE STILL measure a time period for photons to travel! This doesn't change! So if something appears to be correlated FASTER than c, we would know!

    2. The whole reason for the existence of Bell-type experiment is to test quantum entanglement. In none of these experiments, and in none of the theory, is there ANY suggestion and indications that there are things being transfered at a speed faster than c! Read any of the papers involved here and this is clearly stated. There's NOTHING moving from one location to another when a local measurement is made.

    Zz.
     
  4. Jun 17, 2004 #3
    1. yes of course WE still measure a time period for a photon but a photon does not - are you saying that a photon experiences time? it is massless and traveling at the speed of light...

    2. doesn't quantum entanglement suggest that either a) information is traveling faster than c or, b) the two photons share a physical sameness or something or other that means the one is affected by the remote collapse of the wave function of the other? Just using the phrase "quantum entanglement" doesn't answer the question of the mechanism or the causality...maybe you can be more specific?
     
  5. Jun 17, 2004 #4

    ZapperZ

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    But we are not measuring time in the photon's reference frame. We're measuring the elasped time in OUR frame. We don't really care what the photon does or does not measure. The violation of causality would be in OUR frame, not the photon's.

    But WHAT information is "travelling"? QM indicates NO info is travelling. The entangled entities are essentially ONE macro particle in which one can only make a joint measurement. The entangled properties are not separable (mathematically). So since no info is travelling faster than c, we have no violation of causality IF we define causality as information travel at c or less.

    http://physicsweb.org/article/world/11/3/9/1

    Zz.
     
  6. Jun 17, 2004 #5
    this is a completely circular definition!


    this is exactly my point! causality is not broken - and we can maintain this view even without considering properties of the wave function or dictating 'quantum entanglement'...all we have to do is consider causality in the photon's frame...
     
  7. Jun 17, 2004 #6

    DrChinese

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    Now you are getting into the semantics of the word "causality". It means something slightly different to different people. Violation of causality is not a strict consequence of Bell. He concluded that any hypothesized hidden variables could not be local. Not exactly the same thing.
     
  8. Jun 17, 2004 #7

    ZapperZ

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    Huh?

    How can something violate causality when there is no "thing" in the first place? A would violate causality if A travels faster than c in ANY reference frame. But "A" doesn't exist in the first place within QM formulation. So what is the problem?

    Secondly, if A travels faster than c in Reference Frame 1, are you saying in another reference frame, it is possible that A isn't travelling faster than c? What kind of transformation are you using? Gallilean?

    Thirdly, read up on the meaning of "proper time" in Special Relativity. Your observation of the clock of the photon has nothing to do with the photon's proper time, the same way the muon really doesn't care what you view its clock to be. All it knows is that it has its own proper time INDEPENDENT of what you measure it. It appears that you have a major flaw in understanding this concept of special relativity.

    Zz.
     
  9. Jun 17, 2004 #8
    DrChinese - you may be right about the semantics of 'causality' - kind of a vague term...but the fact that the proposed 'hidden variables' are non-local and are still corollary implies, to me, a kind of breach of what i think of as 'causality'...it depends on one's interpretation of QM doesn't it? or maybe i'm completely wrong about that...it was my understanding that Bell's experiment created a crisis for the Einstein-Rosen view of 'causality'...


    ZapperZ - i am having a hard time making heads or tails of your last reply...maybe i am not quite clever enough to decode it...

    you are saying that there is no necessary breach of causality if one simply relies on QM formulations of simultaneity of the wave function, yes?...if so, that's fine...but i am saying that you don't even have to resort to that...or better yet, that the corollary collapse of the wave function does not even breach 'classical causality' provided the measurements of spin in Bell's experiment and the simultaneous emissions of the two photons occur at the same time...and in the photon's 'proper time', they do...

    and here it seems you have completely missed my point because you have just restated it for me...i understand the concept of 'proper time' and it, in fact, underlies my argument here...the photon's 'proper time' is one in which the emission and the measurement occur simultaneously...in this light (pardon), 'causality' is completely meaningless...

    i'm still interested in hearing an argument against the idea that the photons 'proper time' is applicable to our measured history of it!
     
  10. Jun 17, 2004 #9

    NateTG

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    If "causation" can occur at faster than the speed of light, then which of two linked events occurs first depends on the observer. So, Abe would see cause -> effect, and Barney would see effect -> cause. This is a type of break in causality.

    However, whether 'spooky action at a distance' represents an FTL transfer of information depends on what notion of information you're using, and your interpretation of QM.

    Finally, Bell's theorem can be applied to experiments with particles that have rest mass, such as electrons - so basing a resolution on the notion that the particle's reference frame does not experience time is not likely to be productive in a resoltion of the EPR 'paradox'.
     
  11. Jun 17, 2004 #10

    ZapperZ

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    Yikes! And you say that MY reply is difficult to decode!

    Can you point out WHERE exactly is the "simultaneous emisson of the two photons occur at the same time" in the EPR-type experiment that made use of the parametric down conversion?

    What are "simultaneity of the wave function" and "corollary collapse of the wave function"?


    Unfortunately, you haven't even attempted to explain why time measured by the photon (if this is even DEFINED) would matter in such a case. If you possess the physics to make such determination, may I invite you to submit that to PRL, Nature, or Science. Unfortunately, you seem to equate the time dilation that one measures of another reference frame as being equal to the proper time of that frame.

    I also would like to see you do a transformation to the photon's reference frame, something which you keep implying to be possible and have physical meaning. If you are unable to do this, but continue to push for scenarios that depends on such a thing, then this belongs in the Theory Development section.

    Zz.
     
  12. Jun 17, 2004 #11

    ZapperZ

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    And in fact, this week's Nature contains not one, but TWO papers back-to-back reporting on quantum entanglement and quantum teleportation of states using ATOMS.

    Zz.
     
  13. Jun 17, 2004 #12
    NateTG - thank you! for addressing the question...that seems to completely discount proper time considerations from the causality issue...

    ZapperZ, this is completely academic but - "simultaneity of the wave function" and "corollary collapse of the wave function" refer to the measurements of corollary spin of spatially separated photons at the same moment in time...i thought that was evident but maybe my language is imprecise, if that is the case then i apologize...

    why it matters? - i thought, evidently, that the photon's proper time could be a way of restoring a kind of classical causality, dependent on time, but NateTg has helpfully negated that argument!
     
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