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I How "spooky action..." may work?

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  1. Jun 5, 2016 #1

    Suppaman

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    If we have two entangled particles that are separated by physical distance and show by test the "spooky action..." results is it possible they are still connected in time at the point when they were entangled and therefor are really still connected?
     
  2. jcsd
  3. Jun 5, 2016 #2
    They are still "connected", in the sense that the two particles - even though spatially separated - form one entity described by one common wave function.
     
  4. Jun 5, 2016 #3

    Suppaman

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    So, as I suggested, they are common in time?
     
  5. Jun 5, 2016 #4

    haushofer

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    What does that mean?
     
  6. Jun 5, 2016 #5

    Suppaman

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    Because I see there is some doubt about the concept of time that perhaps when the particles are entangled that they are linked to a specific instant in time and when separated and tested they are still connected to that point in time. An alternate way to say this is that they are not communicating at a distance faster than light but rather they communicate back to/from the instant in time they were entangled.
     
  7. Jun 5, 2016 #6
    They don't communicate at all, nor do they need to - entanglement is just a statistical correlation.
     
  8. Jun 5, 2016 #7

    vanhees71

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    Yes, they are connected from the very beginning by being prepared in the entangled state. To be more precise you have to say, in which observables they are entangled.

    The most famous example is the EPR example going back to a famous debate between Einstein and Bohr. Einstein, to his later regret with his coauthors Podolsky and Rosen, wrote a paper, asking the question, whether quantum mechanics can be considered complete, and Bohr answered with another article with the same title.

    There the example was a particle, initially at rest, decaying into two other particles running with opposite momenta away. Then these particles are entangled with respect to their momenta. As often here the entanglement is due to a conservation law (here conservation of momentum).

    Another example is the decay of a scalar particle into two particles of spin 1/2. Due to angular-momentum conservation the two particles' spins are entangled.

    The most accurate Bell experiments are made with photons, because nowadays it's very easy to prepare two-photon states whose polarizations are entangled by a mechanism called "parametric down conversion". You shoot with a laser at a certain type of birefringent crystal and then a photon, picked up out of the laser field, becomes split into two photons that have opposite polarization ("horizontal" and "vertical" with respect to any arbitrarily chosen direction).

    In all these examples the entanglement between the observables is due to the "preparation" of the corresponding quantum state. Another important point is that, as long as the so prepared quantum systems are not disturbed by any external influence, the entanglement in all these cases persists due to conservation laws! This means the particles or photons in all these examples can get very far apart of each other if you wait long enough, but the entanglement still persists, and thus, although being very far apart, the particles/photons still are one system.

    Now, if Alice observes the entangled observable on one of the particles/photons, nothing happens instantaneously to the other particle according to our present understanding of this question, since all interactions of the particles with A's measurement apparatus obey themselves the rules of local relativistic quantum field theory, and this theory has built into its foundations the principle of microcausality and locality, i.e., there are only local interactions, and observables (like the energy density of the electromagnetic field, which describes the probability to find a photon in a detector put at a given place) which are separated by a space-like distance commute, i.e., the measurement at A's place cannot do anything at Bob's (perhaps far distant place).

    Nevertheless, although the polarizations of each of the photons in the example with the two-photon polarization state, are maximally unknown, i.e., when Alice and Bob measure very many so prepared photons, they cannot predict in any way what they will find. Both A and B just have a stream of unpolarized photons. Nevertheless, if they keep the time of their measurement events carefully enough, so that after the measurement they can check on correlations between the polarizations of photons belonging always to a polarization-entangled two-photon state, they will find 100% correlation, i.e., when A has found a H-polarized photon B will have found a V-polarized one and vice versa.

    Now taken these two statements, which both are fully consistent with relativistic local quantum field theory (here particularly Quandum Electrodynamics, describing the electromagnetic field and its interactions with matter), together can only lead to the conclusion that the 100% correlation must be due to the preparation in the entangled state, and that it is not caused on B's photon by A's measurement (or vice versa). This has also been verified with many very accurate measurements, where the choice of what was measured at A's and B's place was decided so short before the photon's registration that there cannot be any influence from the measurements at the other place.

    This shows that local relativistic quantum field theory combines both the principles of locality of interactions and microcausality with the possibility of 100% correlations between separate parts of quantum systems, which are described by entanglement. As Einstein made clear later with another paper, his main concern was this inseparability of far-distant parts of quantum systems, not so much the "spooky action at a distance", which is an issue only if one assumes the socalled "collapse of the quantum state" due to measurement processes, but that's an unnecessary additional assumption on top of the quantum theoretical formalism. As the above example shows, you do not need to assume it anywhere to fully describe what's measured on such entangled systems! The demonstration of entanglement, however, shows that Nature is very different from a naive worldview based on classical (i.e., non-quantum) physics in the sense that the quantum states describe very strong correlations between far-distant parts of a quantum system, which cannot be explained with local deterministic hidden-variable models, i.e., they cannot be described with classical statistics within a deterministic local classical theory.
     
  9. Jun 5, 2016 #8
    I thought it was always due to a conservation law, not just often, are there exceptions?
     
  10. Jun 5, 2016 #9

    Suppaman

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    An article on this site "
    NIST team proves 'spooky action at a distance' is really real"

    So, reading vanhees71 and the above article I am convinced it is both real and not real. Did someone let a cat into the discussion?
     
  11. Jun 5, 2016 #10

    vanhees71

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    I'm not sure. I've no counterexample in mind.
     
  12. Jun 5, 2016 #11
    It's a language issue. "Spooky action at a distance" in the article (and I think in the OP as well) is a violation of the Bell inequality. Vanhee didn't say that this is not real.

     
  13. Jun 5, 2016 #12

    Suppaman

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    So, if it is actually something observed and unexplained and my initial question/idea "Because I see there is some doubt about the concept of time that perhaps when the particles are entangled that they are linked to a specific instant in time and when separated and tested they are still connected to that point in time. An alternate way to say this is that they are not communicating at a distance faster than light but rather they communicate back to/from the instant in time they were entangled." is what I am looking for feedback on. Now if there are any explanations of how spooky action at a distance (since it exists?) actually works I would be very interested. My idea makes sense to me as I believe I read some place other phenomena could only be explained by sending something into the past. Not my idea, just an application for the concept.
     
  14. Jun 5, 2016 #13
    There are interpretations of quantum theory that incorporates backward-in-time influences such as John Cramer's transactional interpretation and the two-state vector formalism by Yakir Aharonov and others. I'm not familiar with them so I can't say more about them. Perhaps others in this forum can. But IMO these "explanations" aren't any more palatable than having faster-than-light communication between particles.
     
    Last edited: Jun 5, 2016
  15. Jun 5, 2016 #14

    Suppaman

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    Faster then light seems to have a lot going that prohibits it. Sending something back in time is not prohibited like faster then light so is it not a better explanation?
     
  16. Jun 5, 2016 #15
    I have to admit that I have always thought of faster-than-light signaling and going backward in time as more or less the same because the former leads to the latter in special relativity. But how does nonlocal hidden variable theory like Bohmian mechanics differ from retrocausal theories? (that is, how I might favor one over the other) You got me beat.
     
  17. Jun 5, 2016 #16

    Suppaman

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    An alternate thought, If we have two entangled particles that when entangled are also connected to the fabric to each other and a point in the fabric of space which does not move when they are separated by physical distance. So, no FTL or SAAAD required. It would really be nice to have a textbook from the distant future so I could understand the universe better. xD
     
  18. Jun 5, 2016 #17
  19. Jun 5, 2016 #18

    bhobba

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    :smile::smile::smile::smile::smile::smile::smile:

    Indeed. Nothing mysterious at all. Its just different to classical correlations.

    Thanks
    Bill
     
  20. Jun 5, 2016 #19

    bhobba

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    These and similar claims are simply misunderstandings of so called weak measurents as has been discussed here many times.

    Thanks
    Bill
     
  21. Jun 5, 2016 #20

    morrobay

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    See this paper on conservation laws and entanglement : http://arxiv.org/pdf/quant-ph/0407041.pdf
     
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