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Hypothetical entanglement question

  1. Dec 24, 2009 #1
    Hey guys,

    I'm new to to this Physics Forum. In fact, this is my first post ever. So I hope this question hasn't been asked and answered a billion times already.

    Suppose that you have an experiment set up where two particles that are in a vacuum with each other become entangled and then one is sent rocketing off at near light-speeds. If you measured the stationary particle, would the other one, who is flying around near the speed of light, react instantly or would it take longer to react (from the spectator's view point) because it is moving through time slower? Or is an experiment like this even possible?

    Thanks. This has been keeping me up at night!
  2. jcsd
  3. Dec 24, 2009 #2
    1. Distance and speed do not matter
    2. "react" is not the right word. You wont be able to detect any CHANGES in a second particle: you have only 1 change to measure it, and when you get information about the measurement you confirm the entanglements. You can use it to send information FTL
  4. Dec 24, 2009 #3
    Hmm maybe I should rephrase my question then. I'm talking about the difference in time between the two particles. Would you detect the changes immediately even if they were on drastically different 'internal clocks'?
  5. Dec 24, 2009 #4
    No, your vision of entanglement is incorrect.

    Say, I have 2 balls: white and black. I put them randomly into 2 boxes and then send one of them to you. After a while I open my box and I find while ball. Aha, then you have a black one! I say.

    What is a time difference between I open the box and your ball becomes black? Nonsense, right?

    So in fact entanglement has some mysterious components, so my example with the balls is not 100% correct (because it is realistic while Bells shows that it is not), but there is no change.

    You think about second particle in state A then 'changing' into state B. No, this is not how it works.
  6. Dec 24, 2009 #5
  7. Dec 24, 2009 #6
    Yes, they're in superposition. However,

    To detect the change, there has to be some kind of a detector on the rocket. It turns out that, unless you tell the detector the expected result beforehand, the outcome in the rocket appears random to the detector. So we can't detect "the change" at all because nothing actually changes, as Dmitry67 said.

    Edit: I want to clarify what I said a bit more. Two particles together are in a superposition of correlated states, but each particle is in a mixed state.
    Last edited: Dec 25, 2009
  8. Dec 26, 2009 #7


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    Welcome to PhysicsForums!

    There has in fact been an experiment intended to probe this particular phenomenon to a degree. This used particles moving parallel and perpendicular to the Earth's velocity. The result was that there was no difference in the outcomes regardless of the movement of the entangled particles. Because of the speeds involved, the result is not absolutely certain but clearly there were no observable differences.


    This is completely in keeping with standard quantum theory, in which collapse (whatever that is physically, if anything) of the wave function is considered to be instantaneous. This is sometimes called quantum non-locality. Quantum nonlocality should be considered separate and distinct from conventional nonlocality, as quantum nonlocality does not involve faster than light communication or the propagation of forces or causes instantaneously.
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