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EPR Paradox and implications for QM

  1. Jun 1, 2009 #1


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    Can somehow confirm that my understanding of the EPR Paradox is correct. Here goes...

    Essentially you can entangle two photons, and send them in opposite directions. If we detect one of the photons, we can find out its properties, and since the properties of the two photons are linked, also that of the other photon.

    Quantum mechanics says the photon we didn't measure does not even have a certain state, and so we shouldn't be able to tell anything about it. This would suggest quantum mechanics is wrong.

    The argument from the quantum mechanics side is that measuring the first photon causes both photons to take a definite state. This requires information to be transferred from the first photon to the other photon. Now, since, the photons being light, are travelling in opposite directions at the speed of light, we'd need the information to travel faster than the speed of light. This would go against special relativity.

    First of all, is the above correct?

    Secondly how do we explain the last bit so that it agrees with relativity?
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  3. Jun 1, 2009 #2


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    You are correct, assuming that the photons are in some way entangled (for example, they were created in some process by which we know for sure that they must have orthogonal polarizations).

    This seems to be in contradiction with special relativity, on of whose postulates is commonly stated as "nothing can travel faster than the speed of light."
    However, the exact postulate is " Light in vacuum propagates with the speed c (a fixed constant) in terms of any system of inertial coordinates, regardless of the state of motion of the light source." Basically, that means that information cannot be transmitted faster than a certain speed c.
    The resolution of the paradox lies in the fact that you cannot use the EPR experiment to get information from one place to another faster than you can send a light signal.
  4. Jun 1, 2009 #3


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    Even FTL information transfer is not incompatible with relativity. It is only the combination of FTL, relativity, and causality that is incompatible. You can have any two of the three, and right now it looks like causality and relativity.
  5. Jun 8, 2009 #4
    As I understand it, Bell's theorem which was verified by experiment, admits only 2 possibilities. Either causality is violated or photons do not exist in a definite state when not being measured. QM agrees with Bell's findings even though his findings do not depend on the correctness of QM. EPR which requires causality AND a definite state for photons has to be wrong.
  6. Jun 9, 2009 #5


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    No, you can only assume that if you measure the same property of the other particle, the result of the first particle's measurement allows you to predict with certainty the result of the second particle's measurement. However, you cannot assume that if you measure a different property of the second particle, then the second particle's "hidden" value for the property you measured on the first property was still the same even though you didn't measure it. This would be a "local hidden variables" explanation for the correlation that's seen when you measure the same property, but it's inconsistent with the statistics seen when you measure different properties. Please see my post #2 on this thread for an analogy involving lotto cards that I like to use to explain this.
    No, the idea that the particle did have a certain state even if you didn't measure it corresponds to a "local hidden variables" picture, and the results of QM show that such a picture doesn't actually work. Entanglement certainly doesn't suggest quantum mechanics is wrong, it's exactly what QM predicts!
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