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Entanglement: Speed more than light

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  1. Feb 17, 2015 #1
    We can have speed more than light in Quantum entanglement...
    but i wanna know can we improve it with some tests?
    Are there any tests that can improve that?

    [mentor's note: this post has been edited to clean up the formatting and remove some unnecessary text]
     
    Last edited by a moderator: Feb 17, 2015
  2. jcsd
  3. Feb 17, 2015 #2
    Really? What makes you think so?
     
  4. Feb 18, 2015 #3
    there is no speed faster then light. I think you can deduce it from maxwell's ecuations from E&M.
     
  5. Feb 18, 2015 #4
    Hi shakila, Welcome to Physicsforums. :smile:

    Probably you are referring to "nonlocality", which relates to claims of "spooky action at a distance" at apparently infinite speed, as based on QM and Bell's theorem.

    The situation is less clear-cut than many popular science articles may make you think! Other interpretations are possible and it has to my knowledge never been demonstrated beyond doubt that something spooky goes on. For a typical discussion on this forum see for example https://www.physicsforums.com/threa...nstration-of-nonlocality.600976/#post-3924297
     
  6. Feb 18, 2015 #5

    Nugatory

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    That is almost (but not quite) correct, but I think you are missing the point of shakila's question.

    First, why is that just "almost" correct? Special relativity prohibits faster than light travel, but it takes more than just Maxwell's equations to get to special relativity. You also need Einstein's additional postulates, and especially the principle of relativity. Furthermore, even after you've derived special relativity, the precisely stated conclusion is not that "there is no speed faster than light". The more precisely stated result is that "no causal influence can propagate faster than light" (which implies no faster-than-light travel, as beginning your journey is obviously a necessary condition for arriving at the destination).

    This distinction matters a lot in the context of Shakila's question, because....

    Second, the effects of quantum entanglement do appear at first to involve faster-than-light travel. I create a pair of entangled particles, allow them to be separated by some enormous distance, and then measure one of them - that measurement appears to instantaneously affect the results of a measurement of the other particle. This effect has been observed, perhaps most convincingly in this paper. However, a more careful analysis shows that it is impossible to send any information in this fashion, and therefore there is no violation of the relativistic prohibition on faster-than-light information.

    This would be a really good time for everyone not already familiar with the topic to take a look at our own DrChinese's web page and search this forum for some of the previous discussion: HarryLin's link and much more.

    (And I would ask the other specialists here to jump in slowly, please.... I know that I have run roughshod over many subtleties here, but I don't want to start a debate that goes over the heads of the people who are here to learn).
     
    Last edited: Feb 18, 2015
  7. Feb 18, 2015 #6
    Consider: I put some the same data on two different flash drives. I separate them by some enormous distance, then read one of them. That measurement appears to instantaneously affect the results of the measurement of the other flash drive.

    Why isn't the result predetermined but unknown at the moment of entanglement?

    (Note: edited for politesse)
     
    Last edited: Feb 18, 2015
  8. Feb 18, 2015 #7

    phinds

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    What do flash drives have to do with entanglement? Measuring a flash drive has zero effect on the other flash drive. You KNOW what's on the flash drive. The drives are not entangled.
     
  9. Feb 18, 2015 #8
    That was a metaphor, obviously flash drives have nothing to do with quantum entanglement. I would appreciate it if you would please consider the main idea behind my argument, rather than trivial details:

    Why isn't the result predetermined but unknown at the moment of entanglement?

    (Note: edited for politesse)
     
    Last edited: Feb 18, 2015
  10. Feb 18, 2015 #9

    phinds

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    Entanglement is about a pair of objects that act like a single system with two parts. The two parts each have a characteristic that has one of two values but the system only allows one of each value. So the two parts can't have the same value.

    When you make a measurement on one of the parts there is a 50:50 chance that it will have one of the two possible values for the characteristic. There is absolutely nothing predetermined about what that result will be. It is random.

    However, once you have determined the characteristic of one of the parts you don't have to measure the characteristic of the other part because it has to be the opposite of whatever you got when you measured the first part.
     
  11. Feb 18, 2015 #10
    How do we empirically know the random result is generated on measurement, rather than at the moment of entanglement? We are talking over each other, neither of us truly understanding what the other is saying. I will attempt to alleviate this on my end by restating your statement. Tell me if I am not correctly regurgitating your statements:

    You are claiming that there is nothing predetermined about the result, and that measuring an entangled particle directly causes the other, distant particle to have the opposite value on measurement. You are also claiming this effect is symmetric, i.e. if the other particle is read, that random result has a direct effect on the value of the current particle.

    Both of these cannot be true without a third cause, as that is a logical loop whose result is dependent on itself. I posit that the third cause is the random result generated at the time of entanglement.

    If you wish, we can discuss this on a chat board somewhere. (Or Skype)
     
    Last edited: Feb 18, 2015
  12. Feb 18, 2015 #11

    phinds

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    Why? Since they ARE both true, there's obviously a flaw in whatever reasoning you used to arrive at that conclusion.

    Clearly you don't think I have this right, so I suggest that you find some other source that you trust as a reliable explanation of entanglement. You will find that they say the same thing.
     
  13. Feb 18, 2015 #12

    Nugatory

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    You'd think so, wouldn't you? And you'd be in good company, as that was also Einstein's first reaction when presented with this problem. But it turns out that it cannot be that way, which is why I also said:
    There you will find a discussion of Bell's Theorem, which basically says that no theory that predetermines the results at entanglement time can yield the same results as quantum mechanics, and the experiments, including the Wiehs paper that I referenced above, that confirms that the quantum mechanical results are what actually happens. Google for "Bell's Theorem" and "Alain Aspect for more information.
     
    Last edited: Feb 18, 2015
  14. Feb 18, 2015 #13
    Oh. Amazing. If unmeasured variables don't exist, I do wonder what other model can be made to explain this.
     
    Last edited: Feb 18, 2015
  15. Feb 18, 2015 #14

    bhobba

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    That's not quite the case.

    The key thing is being able to send information FTL which entanglement can't do.

    Also Bells theorem shows naive reality is wrong - but it may not be locality that needs to be abandoned.

    Thanks
    Bill
     
  16. Feb 18, 2015 #15

    bhobba

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    You have the cart before the horse.

    QM explains it perfectly.

    Its what it means that's up for grabs.

    It contradicts naive reality:
    http://en.wikipedia.org/wiki/Naïve_realism

    But that has two main parts either of which may or may not be false - or both may be false (that's my take) - locality is just one of them.

    Thanks
    Bill
     
  17. Feb 18, 2015 #16

    Nugatory

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    We've drifted far from the original question, and Shakila who started this thread hasn't been back in a while. It's time to close it, I think.
     
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