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Geneva University speed of entanglement results

  1. Aug 14, 2008 #1
    According to the newspaper Students at Geneva
    University have found that "A signal passing
    between entangled photons 18 km apart must
    travel at least 10000 times faster than light"

    Its the same problem about the 'speed' of wave function
    collapse - there is no 'speed' parameter in the wave equation AFIK,
    it is probability rays 'collapsing' not 'things' flying around the Universe FTL.
    Secondly, the fact that one part of the wave packet collapses at the same time
    as another part is not really a signal AFIK?

    I assume 10000 times faster than was what their apparatus could manage.
    Can we be sure of these results?
     
  2. jcsd
  3. Aug 14, 2008 #2

    ZapperZ

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    You need to provide a more complete citation than "according to the newspaper". We request people pay a bit more attention to the sources and how to cite them.

    Furthermore, if you do a search in this forum, there have been several threads already in existence discussing this issue.

    Zz.
     
  4. Aug 14, 2008 #3

    Cthugha

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    I suppose you mean the article published in Nature today (Testing the speed of 'spooky action at a distance' by Daniel Salart, Augustin Baas, Cyril Branciard, Nicolas Gisin & Hugo Zbinden, Nature 454, 7206, p 816), but it seems you did not get the main point.

    The point is: there are some theories, which propose that entanglement is indeed instantaneous and other theories, which propose, there is some "spooky action at a distance", which travels faster than light and needs some preferred frame, which is usually not observable experimentally (like de Broglie-Bohm-like theories). Now the only point of this paper is: if there is such spooky ftl action, there is a very high lower speed limit for it.
     
  5. Aug 14, 2008 #4

    ZapperZ

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    ... or if we go by what Sean Carroll has said, there is no signal going in between these two, and that there's an intrinsic connection instead.

    I have posted the citation to this paper in the Noteworthy Papers thread.

    Zz.
     
  6. Aug 14, 2008 #5
    I don't think there is or has ever been any doubt that if a signal were behind entanglement it would have to travel faster than light and backwards in time, which is why there's most likely no signal.
     
  7. Aug 14, 2008 #6
    I never really got how entanglement was "spooky action at a distance". So you have correlated pieces of information being sent in opposite directions at the speed of light. How does that provide faster than light information from one side to another?
     
  8. Aug 14, 2008 #7
    At an understanding level I think you want to know, so...

    Say we had 100 entangled photons stored here and the 100 entangled partners stored
    5000 miles away in metaphorical boxes, labelled 0 to 100.
    If we measure the polarization of each photon here, then we would know the polarization of each photon at the remote location (its correlated). So the researchers at each end would know the sequence of polarizations of all the photons the 100 boxes. Is that transmission of information?, not really, imo, because we cannot specify that sequence we can just observe it and write down the answers.

    But the exact, unique sequence of polarizations was received by the partner faster than light - so that 'information' was FTL.

    If we could force our entangled photons to collapse at a certain angles, instead of randomly, then we could send information, because the partner would receive the complimentary angles. So 45 degrees followed by 65 followed by 24 (etc to 100) could spell out letters and that is clearly information as we know it.
     
  9. Aug 14, 2008 #8
    In other words, we'd have to bypass the probablistic nature of quantum measurements? Good luck with that.
     
  10. Aug 14, 2008 #9
    Well that truly would raise interesting prospects wouldn't it? Of course, most likely anything you could do to "force" the particle to have a particular spin or polarization would itself be a measurement that would destroy the entanglement.

    MWI at least provides you a potential "out" here - hypothetically, if it were possible to choose "which" universe you wanted to go into, you could thereby ensure that you went into the one where your partner received the message (i.e., had the experimental result) you wanted him to have. This would not be FTL communication, so much as it would be forcing what would otherwise be a highly improbable event.

    Oh geeze, let the Douglas Adams jokes begin.
     
  11. Aug 15, 2008 #10
    We would only have to alter the collapse probability at one end of one of the entangled particle's wave packet by a very slight amount - say the 5th decimal place for it to show up statistically at the remote location instantly -over a large number of samples, of course.

    Since the two particles are remote it is possible to work on one just of them. I find it hard to believe that an extended wave packet (e.g. from two entangled particles) is completely immune to having its probabilities altered at every location with all the high power colliders and fields we have available. Even a minute change would be enough.
     
  12. Aug 15, 2008 #11

    ZapperZ

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    You seem to forget that you are dealing with the quantum superposition here. Unless you have something concrete to base this on, I suggest you re-read the PF Guidelines regarding speculative posts.

    Zz.
     
  13. Aug 15, 2008 #12
    Its officially recognised that the theory of entanglement needs new ideas.

    There is nothing to prevent actions on individual superposed paths and we need a forum to discuss openly issues. I suggest you contribute in a proactive manner rather than being reactive. Also, I can get research funding pointing in that direction.
     
  14. Aug 15, 2008 #13

    ZapperZ

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    I never said anything about research into quantum entanglement. The whole area of studying the Schrodinger Cat-states is doing just that. I have made plenty of "contribution" towards such discussion here if you simply look. However, per the guidelines that you have agreed to, if you are making guesswork without any valid peer-reviewed citation as the basis, then you are making unfounded speculation. If what you've said is based on published work, then you should cite them clearly.

    If you wish to work out your own personal theory, then that's what we have the IR forum for.

    Zz.
     
    Last edited: Aug 15, 2008
  15. Aug 15, 2008 #14
    LaserMind,
    I think you're not quite understanding entanglement. The probability of a single unpolarized photon passing a polarizer is always 50% no matter what you do to it. If you "polarize" it - which really means throw out all the photons that wouldn't pass a particular polarizer - you do indeed alter the odds of passing particular polarizers down the line, but you also destroy any entanglement that existed between that photon and any other. In other words, what happens to photon A after the initial measurement will have no relationship with what happens to A'. The link has been broken, so to speak. So what you're suggesting is conceptually not possible.
     
  16. Aug 15, 2008 #15
    Zapper, I know you're just quoting Sean Carroll when you say that "there's an intrinsic connection", but isn't this term ("intrinsic connnection") a little too ill defined for professional physicists to be using to explain/describe something? I ask because when I search google for that term, the search results were... well, of questional material. And I couldn't find an explanation on google scholar either. So basically, I'm wondering if you have a peer reviewed link to an article discussing what "intrinsic connection" means.
     
  17. Aug 15, 2008 #16
    I am suggesting the wave packet is acted on before a polarization filter is reached (say, by very strong gravity, electric field etc), - and this action does not collapse the wave function immediately - then the entangled, remote, twin particle should be affected as its part of the same wave packet and the combination is still entangled.

    It may be that nothing at all will change an entangled wave packet's state probabilities without collapsing it, but there again there is a chance.


    I also read the 'intrinsic connection' of entangled particles in the reference above telling me they had not much idea as to a physical model and many admit it honestly.
     
  18. Aug 15, 2008 #17
    "Collapsing" and "change the probabilities" is the same thing. Therefore what you are suggesting is, by definition, impossible. *Anything* that changes the probabilities of a given variable of one will cause the two to no longer be entangled as to that variable. So even if you could artificially change the polarization of a photon in mid-flight (something conceptually impossible for other reasons, namely because it has no well-defined polarization in mid-flight) then the results of the subsequent measurements on both will no longer show entanglement.
     
  19. Aug 15, 2008 #18

    ZapperZ

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    It's one of those things that we don't have an explanation for as of yet, like "intrinsic charge" and "intrinsic spin". These properties come with the whole show and we currently have no explanation for the origin. So quantum entanglement would be an intrinsic property (i.e. it is just there) of the system, in this case, a bipartite photon state. This appears to be what Sean Carroll is saying, and within the framework of QM as it is (i.e. with no signal of any kind, be it hidden variables or not), is also what QM is implying.

    Not sure if this is a satisfactory explanation. I doubt that anyone can go beyond that at the moment with any degree of certainty.

    Zz.
     
  20. Jun 1, 2009 #19

    trv

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    Firstly apologies for raising this thread from the dead. However I was trying to understand the article in question and have a quick question. Question is....

    Could someone please explain what is the need for this reference frame? And also what is special about it? Is it that it is a non-inertial frame, which then does not place the speed of light limit for information transer?
     
  21. Jun 1, 2009 #20

    DrChinese

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    Generally, Bohmian type theories (also called dBB or pilot wave theories) have a preferred rest frame as a requirement. These are the most common theories that have instantaneous or FTL influences. The concept of the experiment includes trying to place limits on what these - or other potential theories - could look like.

    The experiment itself does not rule out Bohmian theories, although there is some disagreement as to the implications. Clearly, instantaneous propagation is faster than the observed lower limit.
     
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