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Why would entangled particles light years away need to be able to cancel?

  1. Apr 28, 2009 #1
    Ok, here's my problem... In the EPR experiment it is described that if entangled particles are required to be able to cancel each others spin's then no mater how far apart these particles are, if one is measured you can instantly infer the state of the other particle. Why would the other particle's state matter if it is thousands of light years away? The particles need to be ABLE to cancel but they cant cancel if they don't come in contact with each other so the state of the particles doesn't really mater until they touch so their states should be able to be whatever they want until they touch. Am i missing something?
     
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  3. Apr 28, 2009 #2

    Fredrik

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    Some people think of the wavefunction as a mathematical representation of the properties of the system, and some think of it as a mathematical representation of our knowledge about the system. I don't think either of those "interpretations" make much sense.

    What everyone agrees on is that we can think about the wavefunction as a measure of the probability that a measurement will have a certain result. The never ending debate is about whether it should, or could, be interpreted as something more than that. As I said, I don't think it can.

    If it's just a measure of the probabilities of possible results of experiments, then there's nothing truly disturbing about the EPR scenario, except perhaps that this would imply that QM doesn't actually describe the world.
     
  4. Apr 28, 2009 #3

    Matterwave

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    Usually, the particles are required to "cancel" each other as you say because of some conservation law. If you created 2 photons from an atom that had no angular momentum and they had angular momenta which didn't add up to 0 then you've just violated conservation of angular momentum.
     
  5. Apr 28, 2009 #4
    As long as when the two particles touch they can cancel, i dont see how what they do when they don't touch really maters as long as the resulting differences in their states doesn't allow energy to be created or destroyed somewhere else. Its like saying that because a roller coaster speeds up as it goes down a track that energy is being created... no because once it reaches the height it fell from again (as long as there is no friction involved) it will be going the same speed that it was when it left that height initially. The coaster's state changed but as long as when it goes back to the height it fell from it is going the same speed as it was before it's initial drop, newton is happy. This is like the particles... as long as they CAN cancel they should be able to do whatever they want and because they cant cancel unless they are touching... their states don't mater as long as they are able to cancel by the time they touch. This means that even if you measure one of the particles, the other can still be in a state of superposition... in fact... the only time information about one of the particles must travel instantly is when the two are touching but there would be no distance between them anyways!
     
  6. Apr 28, 2009 #5
    I'm not developing some crazy idea... I'm actually fairly confident that I'm missing something but i just don't see why the particles would need to be able to cancel even if they couldn't cancel because they aren't touching.
     
  7. Apr 28, 2009 #6

    DrChinese

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    Entangled particles act "as if" they remain in contact with each other. No one knows exactly what happens or how it is done. However, the general scenario you describe is ruled out experimentally.

    Please keep in mind that in the particular example you provide, there does not appear to be any obvious problem. If you measure the 2 entangled photons at the same angle setting, say 0 degrees, they will in fact cancel (in terms of conservation). But if you measure at certain other settings - say one at 0 degrees and the other at 22.5 degrees - you get results that make no sense in conventional (classical) terms.

    The best place to start is to learn about EPR and Bell's Theorem. You can learn about this from one of my web pages on the subject: Bell's Theorem with Easy Math which also provides some relevant background. You can also see the original references.
     
  8. Apr 28, 2009 #7

    Fredrik

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    "Touching" isn't a concept that's used in quantum physics. The term "interacting" is used, but what we're talking about here isn't an interaction.
     
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