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Quantum entanglement

  1. Apr 10, 2009 #1
    i was watching a video on this and supposedly if an electron was on the other side of the universe it would response instantly to another electron moving.. this didnt make any sense at all because it was spoken as knowlege when theres no way that could be known... so can someone please clarify with me since im a beginner and confused whats knowns about quantum entanglement and how its known? the mains things im not sure about are the distances which it takes place on and how it was figured out. answers = much appreciation
  2. jcsd
  3. Apr 10, 2009 #2
    The thing is that in the 1930's Einstein proposed a thought experiment, through which he claimed that the Heisenberg uncertainty relation can be denied. His argument was on the grounds of assuming that QM is local.
    This is actually called the EPR paradox, you can check it on wikipedia or anywhere else.
  4. Apr 10, 2009 #3
    I think his question was more related to how we know that entanglement effects happen instantaneously. Do we know that? Or is it still theory?
    Sure, we can test with an experiment (two particles, one on the other side of the world) whether the effect is faster than light or not (I thought it was verified that it was indeed faster than light?) but there is no way we can verify that it is indeed instantaneous... Can we?
  5. Apr 10, 2009 #4


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    The effect you are speaking of has been shown to occur at least 10,000 times the speed of light. It might be instantaneous, that is the presumption, and I would expect this lower limit to increase with future experiments.

    As to the distance, so far the maximum distance it has been tested over is on the order of 10s of kilometers. The size of the planet is a limiting factor as it will be difficult to go past that. On the other hand... if I remember correctly, there is a corner mirror on the moon. I wonder if that could be used to do a Bell test that would cross a half million miles?
    Last edited: Apr 10, 2009
  6. Apr 11, 2009 #5
  7. Apr 11, 2009 #6


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    There is such a mirror on the moon, but using it for a Bell test looks to be pretty unlikely. From Wikipedia on retroflectors:

    "Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally-occurring photons challenging..."

    Since a Bell test only produces entangled pairs on the order of hundreds per second (+/- a few orders depending on the setup), while the intensity of a normal laser (the kind referred to above) is well over a million times brighter, that might make it nearly impossible to collect enough photons to do a good test. You might need a million seconds (over a month considering the moon is only visible at certain times) to collect one. Oh well...
    Last edited: Apr 11, 2009
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