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Quantum entanglement on a cosmological scale?

  1. Jun 4, 2015 #1
    This is a physics question but since it is on a 'universal' scale, I will ask it here.

    Quantum entanglement. I was watching a panel discussion with Leonard Suskind and others leading theoretical physicists....also watched a Nova program presented by David Green, etc. In both there was a variation of a comment that two entangled particles would act entangled whether they were in the next room or across the Universe.

    A question. Since no particles have been measured at any significant distance apart, how do we know this? Is it just a general assumption? How do we know that on a cosmological scale that the relationship between two entangled particles wouldn't diminish by (just hypothetical example) 1% over a trillion kilometers and eventually reduce to nothing? Or that entanglement just suddenly ceases (like a half life) a billion light years distance?
     
  2. jcsd
  3. Jun 4, 2015 #2
    I'm not nearly qualified to answer your question, or even contribute anything helpful, I'm simply curious as to what you meant by "like a half life." What do you mean there?
     
  4. Jun 4, 2015 #3

    Chalnoth

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    Entanglement becomes masked when one of the particles interacts with something. The further a particle travels, the more likely it is to interact, and thus the more likely that the entanglement will become garbled.

    It may be possible to maintain entanglement over large distances for particles which don't interact very much, but anything that is likely to interact is not going to stay entangled for long.
     
  5. Jun 4, 2015 #4
    This is true . However the entanglement is assumed to be the same at a vast distance 'if' one could keep the two particles in some isolated state so they wouldn't interact with other things.

    Anyways, there seems to be a lot of assumptions about entanglement being independent of distance.
     
  6. Jun 4, 2015 #5

    Chalnoth

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    No, it definitely has nothing to do with distance.

    Entanglement is purely about consistency. Imagine there's a process where two photons are emitted in opposite directions with zero total angular momentum. The fact that the physical process ensures the photons have zero total angular momentum means that the spin of one photon must be opposite the spin of another. So if you measure the spin of one photon, anybody measuring the spin of the other photon in the same basis will necessarily get the opposite result. The only way to change this is if one of the particles interacts with something that can change its spin. Otherwise conservation of angular momentum requires that they have opposite spins.
     
  7. Jun 4, 2015 #6

    Nugatory

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    The alternative to that assumption would be to assume that there is some unknown mechanism (as opposed to the known mechanism that Chalnoth described) that causes the entanglement to break down and (unlike the known mechanism that Chalnoth described) violates conservation of angular momentum.... even though no such violation has ever been observed anywhere under any conditions, which is why we call conservation of angular momentum a "law".

    Yes, that's not impossible, but it's enough of a stretch that no one will take it seriously unless and until we construct an experiment whose results are consistent with this hypothesis but not with the one that Chalnoth described.
     
    Last edited: Jun 4, 2015
  8. Jun 5, 2015 #7
    Some sites say experiments have been done at tens or hundreds of kilometers ...
     
  9. Jun 5, 2015 #8

    phinds

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    Again, as stated above, it really doesn't matter. If you can show absolutely that entanglement works the way it has been stated above to work (and this HAS been shown), then it has to work regardless of whether the separation upon measurement is 1 inch or a zillion light years (absent any interaction prior to the measurement).
     
  10. Jun 5, 2015 #9

    vanhees71

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  11. Jun 5, 2015 #10

    Chalnoth

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    Let me just express my extreme displeasure at the use of the phrase, "quantum teleportation." It's so incredibly misleading. All that they mean is that they maintained coherence of the wavefunction (i.e., managed to prevent it interacting with anything that would disturb the measured variables).
     
  12. Jun 5, 2015 #11

    It doesn't, at most it proves that it is a reasonable assume it will work the same for 1 inch or a zillion light years.
     
  13. Jun 5, 2015 #12

    Chalnoth

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    It's not an assumption. It's a requirement for consistency.

    Of course, in practice entanglement generally won't be maintained for very long unless the particles just don't interact with much of anything.
     
  14. Jun 5, 2015 #13

    You are assuming that theory is consistent, so yes, it could be very reasonable but is still is an assumption on an indirect proof.
     
  15. Jun 5, 2015 #14

    Chalnoth

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    I don't think there's any way to reasonably argue that the universe is inconsistent.
     
  16. Jun 5, 2015 #15

    Nugatory

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    An excellent thread-closing moment.... Done.
     
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