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B Here is a good video that explains non-locality simply

  1. Feb 16, 2017 #1
    [Mentor's note: YouTube videos are not generally an acceptable reference under the PhysicsForums rules. Many are oversimplified, seriously misleading, or just plain wrong, and no one who is serious about understanding quantum mechanics should be trying to learn from them. We've left this thread up because the misconceptions spawned by the video are common enough that it's worth discussing them]
     
    Last edited by a moderator: Feb 16, 2017
  2. jcsd
  3. Feb 16, 2017 #2
    Why couldn't entangled states be used to transmit information, instanteously over large distances. For instance lets say I have one of the two entangled particles and someone else anywhere in the universe has the other. If he modulates his state ( say from 0 to 1 to create a an 8 bit byte) I will receive the complement of that 8 bit byte instantaneously. Is that right? All I have to do is negate the byte I received and I can retrieve exactly what he sent?
     
  4. Feb 16, 2017 #3

    Vanadium 50

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    I just did this as a test. Here's what you received: 110010011010010011101010.

    What did I send?
     
  5. Feb 16, 2017 #4

    Nugatory

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    No.
    We have many threads about this already. The basic problem is that if you and I each measure our particles, one of us will get spin-up and one of us will get spin-down. But we have absolutely no way of controlling which one it will be - the entangled state is constructed to be one in which the results "you up, me down" and "you down, me up" are equally likely. So if I get a spin-up measurement, I know that when and if you measure your particle you will get spin-down, but there's no message there.
     
  6. Feb 16, 2017 #5

    berkeman

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    42

    :smile:
     
  7. Feb 16, 2017 #6
    The way I understand it all I need to know is what it was initially. If I know what it was initially I know what state your's was in initially.
     
  8. Feb 16, 2017 #7
    you sent 001101100101101100010101
     
  9. Feb 16, 2017 #8

    berkeman

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    Have you read the threads that Nugatory mentions? If not, please do so. If so, can you link to the parts of the threads that you don't understand? Thanks.
     
  10. Feb 16, 2017 #9
    There are too many posts using the word entanglement. Its like looking for a particular strand of hay in a haystack. Nugatory's response was too ambiguous to be of any help.

    Is it true that once I know what is the state of my entangled particle, I know the state of your entangled particle(assuming you and I have the entangled pair) and henceforth any change you make to your particle is immediately seen as a change in my particle?
     
    Last edited: Feb 16, 2017
  11. Feb 16, 2017 #10

    Nugatory

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    No. You only get one measurement and after that the entanglement is broken.

    So let's say you measure your particle and get spin-up. It might be that you made the first measurement on the pair, randomly got spin-up, and now you know that when and if I measure I will get spin-down. Or it might be that I made the first measurement and randomly got spin-down, and that's why you got spin-up. Neither of us have any way of distinguishing the two cases (and in fact they cannot be distinguished if the hypothetical communication would be faster than light - in this case relativity of simultaneity means that both measurements can be said to have come "first"). So we can't even say that one of us transmitted a signal that the other received - our measurements are the same whether we're the sender or the receiver.

    After we've made that first measurement, any subsequent measurements on either side will be uncorrelated with one another. So...

    No, you just sent 1100100110.... in the other direction.
     
    Last edited: Feb 16, 2017
  12. Feb 16, 2017 #11
    Thank you very much for that.
     
  13. Feb 17, 2017 #12

    Demystifier

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    No, that's not right. If he modulates his state, you will receive nothing instantaneously. It takes time to receive information about his modulation.
     
  14. Feb 17, 2017 #13

    DrChinese

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    In addition to the correct comments already in response: Entangled particles don't have a specific individual state while entangled. When you measure one, you do not "do" anything to one/both any more than the other one does something to one/both. The direction of causation (if there is one) cannot be determined by any objective test.

    So logically no message can be sent: you wouldn't know who is sending to who, and you couldn't control whether you sent a 1 or a 0 anyway.
     
  15. Feb 17, 2017 #14

    Vanadium 50

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    But what was the message that I sent?
     
  16. Feb 17, 2017 #15
    This I don't quite understand yet: what is "instantaneous" in this context, and how does one measure it? It seems to me that spacelike-separated events (if we could speak of that) have no way of being "simultaneous". Information has to travel to compare aspects of the events (like "happening at time ...").

    Also, is it even possible to influence one's own measurement outcome in an entanglement experiment like this? If not, how to put in information to send?

    I hope I'm not too simplistic. Please let me know if I am. (I didn't want to intrude on other threads)
     
  17. Feb 17, 2017 #16
    Entanglement in no way demonstrates non-locality. Local interpretations of entanglement require multiverse models (which many physicists find aesthetically unappealing). The reason we can know that action-at-a-distance (AAAD) is an incorrect interpretation of entanglement is that it requires simultaneity and more than a century of testing has proven "non-simultaneity". AAAD requires the first observation of a system to, in that instant for all observers in the universe, change state ("collapse" from a multiplicity of allowed states into one definite state).

    Because non-simultaneity is a proven fact we know AAAD cannot be a correct interpretation and that physicists need to practice saying, "I don't know."
     
  18. Feb 18, 2017 #17
    Is it possible that the entangled particles are doing nothing more than spinning and tumbling 180° out of phase? And we do not know the phase until we make the first observation, but once that first observation is made the complete behavior is known although not predictable? And if this were true, would the phase be considered a hidden variable?

    Is it true, that until we make the first measurement we do not even know they are entangled particles. How many measurements do you have to make to determine that the two particles are entangled?
     
    Last edited: Feb 18, 2017
  19. Feb 18, 2017 #18

    Nugatory

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    An explanation of that sort is known as a "local hidden variable theory" - the idea is that there is some property of the particles that had opposite values of all along, and if we knew what it was we could calculate the results at both detectors without invoking any quantum weirdness. A common analogy is: I can put one glove from a pair in a box and mail it to you, keep the other; when you open the box and find a left-handed glove you know that I have a right-handed glove and vice versa.

    However, it turns out that there are statistical differences between the predictions of quantum mechanics and any possible local hidden variable theory when you do multiple tests with the detectors at different angles (not just opposite). We can and have done experiments that show this difference, and the results are consistent with quantum mechanics and inconsistent with any local hidden variable theory.

    For more information, google for "Bell's theorem" and pay particular attention to the web page maintained by our own DrChinese.
     
  20. Feb 20, 2017 #19
    The correlation between the measurements on respective particles doesn't so much depend on the angles of the spin of the particles, as on the angle of the basis they are measured in. If particle A is detected, the angle of that detector correlates with the angle of the other detector in the form of incident respective detections (collapse to a certain state).

    Besides that, I understand that the entangled state has no angle to associate.
     
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