Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does quantum entanglement not allow ftl communication

  1. Apr 24, 2008 #1
    quite simply the above question.
    Why does quantum entanglement not allow for faster than light communication?
    Thanks
     
  2. jcsd
  3. Apr 24, 2008 #2

    Ken G

    User Avatar
    Gold Member

    The way I would answer this is that the information encoded in entanglement is only extractable when you look at correlations between measurements on both the entangled systems. So to access that correlation information, you would need communication anyway, and that communication could not be FTL. If you only look at either system, but not the other, then you need no such communication, but you also can extract no information from the entanglement. This is actually a good thing, because much of science is done by ignoring entanglements, and the reason we get away with that is the information we are ignoring cannot interfere with our interpretation of the results of our experiment. In other words, aliens aren't talking to us in our laboratory experiments, because if they were, we could not disentangle their messages from the laws of physics themselves.
     
    Last edited: Apr 24, 2008
  4. Apr 24, 2008 #3
    That is the practical answer and I believe the right one. There are more theoretical answers called "no signaling theorems" and similar names. I don't know how they are derived, but they do enforce the notion that you cannot send a signal using entanglement.

    There are those (myself among them) who believe that there might be a way to use complementarity to send a signal by altering the behavior of entangled quanta depending on how they're measured. There have been experiments showing that an interference pattern in photons is created if and only if the entangled twins are detected with absolutely ambiguous position information. So far, these experiments all require correlations because only a subset of photons is able to be detected at a given time. However, if ALL of the photons can be detected ambiguosly, then, in theory, the entangled twins should create a visible interference pattern and, perhaps, allow for signaling. John Cramer, a physicst at Wash U. is working on such an experiment now.
     
  5. Apr 24, 2008 #4
    ok... so in other words, it's not something that someone has said is impossible because of relativity (specifically the impossibility of ftl travel).

    I must say your aliens analogy is rather confusing, and hence i've ignored it... like any good quantum scientest ignores entanglements;)
     
  6. Apr 24, 2008 #5

    neu

    User Avatar

    These explanations sound over complicated to me.

    Say, Alice and Bob meet together and generate a 2 qubit entangled state, then Alice and Bob each take a qubit, separate and travel to distant locations.

    Assuming each qubit is kept in absolute isolation then the measurement of, say Alice's qubit will be correlated with the corresponding measurement by Bob on his qubit. That is, Alice can tell what state Bob's qubit will be in if he measures in the same basis as her; thus by communicating to him this info classically, Bob can measure his qubit based on Alice's message to obtain a qubit in the state Alice predicted.

    My understanding, therefore, is that the 2 qubits of the entangled state evolve in time such that the state of one implies the state of the other; they have a shared history (and future if isolated).

    In this case the transfer of info is restricted by the speed of classical communications (the speed of light).


    However, I'm also confused about this restriction. If Alice needs to communicate to Bob how to measure his qubit to get the same outcome as her, can't they agree when they generate the state to perform a measurement to get this result at a v.similar time when separated to transfer the info faster than light?

    Showing my ignorance here perhaps.
     
    Last edited: Apr 24, 2008
  7. Apr 24, 2008 #6

    Ken G

    User Avatar
    Gold Member

    Relativity is a set of axioms. We should therefore never say something is impossible "because of relativity", we should say "if it is possible, then something in relativity is wrong". We have many tests of relativity, which gives us some confidence it is not wrong, but it does not mean it could not be wrong, i.e., it does not mean we can use relativity to make statements about how reality must behave.
    It's not meant to be confusing-- the point is, we have to start our experiments somewhere, and we need to assume our particles are coming into our experiments with no hidden FTL entanglements that could mess up our experiment. If we could extract information from them in a acausal way (i.e., FTL), then we have lost control of our experiment, because it would require that two experiments prepared in identical ways could yield statistically different results (due to the acausal information arriving). So my point is simply, one man's "FTL communication" is another's "unreliable experiment". We can't have it both ways.
     
  8. Apr 24, 2008 #7

    Ken G

    User Avatar
    Gold Member

    Yes, they could for example agree on the axis of a spin measurement. Then each would know, as soon as they measure their own qubit, what the other got/will get. So they gain information about distant places instantly, but no information is going FTL, it only travels around in their brains, in a causal way (presumably).
     
  9. Apr 24, 2008 #8

    Hans de Vries

    User Avatar
    Science Advisor


    In the essence it is this :

    Say you have two random number generators, which always produce equal random
    numbers even though they are 100 light years apart. They'll give you random numbers
    but no way to communicate.


    Now, say that the reason that the numbers are the same is because of some super-
    luminal communication between the two. The often heard claim is then this:

    "Special Relativity is not violated because no information is send"

    This however goes straightly against Shannon's information theory which says that pure
    random data has the highest "Information Entropy" This simply means that the number
    of bits you need to represent the data is maximal. You can not compress a file with
    random data.

    http://en.wikipedia.org/wiki/Information_entropy


    Therefor, the amount of information data bits which needs to be send at superluminal
    speed is maximal instead of zero as suggested by the quoted claim above.


    Regards, Hans.
     
    Last edited: Apr 24, 2008
  10. Apr 24, 2008 #9

    Ken G

    User Avatar
    Gold Member

    I'm afraid I don't understand the point you are making there. Are you saying that information is sent and special relativity is violated, that information is sent but special relativity is not violated, or that information is not sent?
     
  11. Apr 24, 2008 #10

    Hans de Vries

    User Avatar
    Science Advisor



    What I (or rather Shannon's information theory) says is that:

    Random-data-send may not be confused with No-information-send

    Information = number of bits, and random data actually needs the maximum
    amount of data bits to transmit from one place from another. It can not
    be compressed.



    If the random outcome of an entanglement experiment is communicated at
    superluminal speed then the claim is:

    "Special Relativity is not violated because no information is send"

    It is erroneously assumed that random data does not contain information and
    therefor nothing physically relevant is transmitted at superluminal speed and
    so special relativity is not violated.


    For example: One of the outputs of the detector may be configured to trigger the
    "mother of all bombs" which "blows up earth". One can hardly say that the random
    bit transmitted at superluminal speed has "no information", is not physically relevant,
    and therefor doesn't violate special relativity......


    Regards, Hans
     
    Last edited: Apr 24, 2008
  12. Apr 24, 2008 #11

    JesseM

    User Avatar
    Science Advisor

    I think it's been definitively proven that if the accepted equations of quantum theory are correct, then no possible experiment can be used for FTL communication--this source says it's ruled out by "Eberhard's theorem" in section 2.3, and this article by Cramer says:
    So, it seems Cramer is pinning his hopes on a nonlinear modification to the accepted equations of QM. I think this is the paper by Eberhard they're referring to, by the way.
     
  13. Apr 24, 2008 #12

    Ken G

    User Avatar
    Gold Member

    But the reason no information is sent is not because the information looks random, it is because the information is not sent.
    But you still haven't explained why you think that information is ever sent in an entanglement experiment. I don't see it going anywhere.
     
  14. Apr 24, 2008 #13

    Ken G

    User Avatar
    Gold Member

    Yes, it seems that. My "horse sense" tells me that if quantum mechanics with Weinberg's nonlinearity worked in the real world, there would also appear some other aspect of the correction that still makes FTL communication impossible. In other words, there does not seem to be any principle of a corrected quantum mechanics that would be more fundamental than the principle of causality. Of course, experiment may prove me wrong, I'm just saying where'd I'd put my money if anyone gave me the chance.
     
  15. Apr 25, 2008 #14

    Hans de Vries

    User Avatar
    Science Advisor


    The process goes by the name "quantum teleportation".

    This literally says that something (the quantum state) is being transported over far.


    Regards, Hans
     
  16. Apr 25, 2008 #15
    The "teleportation" process requires waiting for information to be transported across classical channels. (Are you unfamiliar with this?)
     
    Last edited: Apr 25, 2008
  17. Apr 25, 2008 #16

    Hans de Vries

    User Avatar
    Science Advisor

    Read more carefully.

    You seem to have the idea that I said that superluminal communication is
    possible. I never did.

    I am pointing out the distinct difference between "random information" and
    "no information" according to Shannon's information theory. I was criticizing
    the idea that superluminal propagation of random data (random quantum states)
    can be reconciled with SR.


    Regards, Hans
     
  18. Apr 25, 2008 #17

    Ken G

    User Avatar
    Gold Member

    Quantum teleportation is not FTL. As I said, I agree that transporting "random" information FTL would still violate SR, the point is, that's the incorrect reason that entanglement doesn't produce FTL communication. The correct reason is simply that it doesn't "transport" anything, random or otherwise, FTL. Perhaps you are not disagreeing with that.
     
  19. Apr 25, 2008 #18

    Hans de Vries

    User Avatar
    Science Advisor

    No, This is the reasoning of the EPR crowd:

    1) The quantum state is teleported instantaneous.
    2) We can not control the collapse of the wave-function.
    3) Therefor we can not use it to communicate data.
    4) Therefor no information is send (FTL)

    Claim 4) violates Shannon's information theory. that's my point

    Of coarse, the majority of the EPR crowd doesn't believe this to be fundamental.
    I get the impression that many of them are really chasing their Science Fiction
    dreams and that statements like: Special Relativity is not really violated are
    more to appease peer reviewers than that they themself believe in it.


    Regards, Hans
     
    Last edited: Apr 25, 2008
  20. Apr 25, 2008 #19

    JesseM

    User Avatar
    Science Advisor

    Are you sure about the claim that the "quantum state is teleported instantaneously"? Do you have a reference? It seems to me that if that were the case, one could still gain probabilistic information about the original, distant state that was teleported by looking at the outcome when the teleported state was measured.
     
  21. Apr 25, 2008 #20

    Ken G

    User Avatar
    Gold Member

    There is no meaning to "instantaneous" except for the person doing the original experiment. Nothing is "teleported" unless there is classical slower-than-light communication, so it is not instantaneous.
    That's not the correct reason why we can't communicate instantaneously, the correct reason is that nothing is transported instantaneously in the first place. I cannot speak for whoever you mean by "the EPR crowd"-- I agree that argument would be spurious, but it's not the right argument anyway.
    But it's all a strawman, that's my point. If the "EPR crowd" think they require that explanation, they don't understand information theory, but since a lot of people do, I don't see that as likely. There may be a difficulty in finding people interested in philosophy who are also versed in physics.
    I don't know what they believe, but I don't think personal beliefs are terribly relevant either.
     
    Last edited: Apr 25, 2008
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why does quantum entanglement not allow ftl communication
Loading...