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Thought Experiment Involving Quantum Entanglement

  1. Mar 20, 2009 #1
    My friend just gave me a good thought experiment involving quantum entanglement violating special relativity that I can't seem to refute.
    There are two immortal people A and B. A and B synchronize their clocks far away from any gravitational influence (say spot Y). B departs for spot(at an infinitely slow speed...therefore they do not lose synchronization) X a 1000 light years away from the place they synchronized them. There is a pair of electrons quantum entangled at spot X and Y. Before leaving A told B to kill himself if the electron that he measured was spin down. After a certain pre-decided interval A and B both make the measurement. A measures that his electron is spin up. Therefore, B is dead. Does that violate special relativity?
  2. jcsd
  3. Mar 21, 2009 #2


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    Ignoring the irrelevant information in that rather convoluted question, all that's being asked is whether entanglement violates special relativity. Which there is already a https://www.physicsforums.com/showthread.php?t=299687" on right now.

    The general consensus is no, because entanglement does not transmit any usable information in itself.
    Last edited by a moderator: Apr 24, 2017
  4. Mar 21, 2009 #3
    This doesn't refute my friend's thought experiment. Please tell me exactly which part of it breaks down in its argument for violation of special relativity.

    Also, I did not post it in that thread in fear of it getting pushed to the back of the discussion. I understand that no information is transferred, but in this case isn't there information being transferred in that you know the other guy will be dead after you measure it?
  5. Mar 21, 2009 #4

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    Forget quantum mechanics. You can do the same thing with two socks, one black and one red. A sees he has a red sock, and then instantaneously knows B has a black sock. That doesn't violate SR either.
  6. Mar 21, 2009 #5
    In fact there is a reference frame where A knows that B is dead BEFORE B commits suicide.

    However, it does not allow to makeany 'i killed my own father' paradoxes, so relativity is still valid.
  7. Mar 21, 2009 #6
    thanks guys, i REALLY understand it now, i know what's going on here

    i'm not just making stuff up to get jollies going, i'm planning on being a theoretical physicist myself, i'm majoring in math and physics, so i'm trying to be serious here, and a good physicist should be able to pick this thought experiment apart by saying where it fails
  8. Mar 23, 2009 #7
    There's nothing to prevent knowledge about something far away appearing instantly. (sock example)

    There is no information transfer: Information transfer require B be able to send some sort of message to A, which he can't. A will instantly know something, but B has no control over what that is. No information being sent, no violation of SR.
  9. Mar 23, 2009 #8
    I believe it is more complicated: there is some information sent using superlimunal (entanglement) channel (S), but it cant be decoded without another part, sent traditionally (T).

    S can not be decrypted without T, but T alone is not enough, so it proves that S contains some informations.

    It is like sending an enrypted ZIP file instantly but sending a password by DHL. Without paper mail you can open ZIP, but still password alone does not contain all information.
  10. Mar 23, 2009 #9
    I think in this situation there is nothing at all being transferred (at least nothing usable). But you're right in that something (which is not information) can be "sent" using entanglement. Combined with a classical channel, this is quantum teleportation.
  11. Mar 26, 2009 #10
    You have received serious answers. Think about the socks. For your example, they are sufficient as an explanation.

    The mystery of quantum entanglement is more complex than your simple example. See ilja-schmelzer.de/realism/game.php for some example which cannot be explained with socks.
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