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Another question about bell inequality

  1. Jan 20, 2013 #1
    I imagine that some topics and questions keep reappearing since it is hard to track through all past posts even with the query tool. So apologies if this has been covered before (as it probably has). I just want to check my intuitive understanding of the Bell experiment, having heard an excellent lecture by Alain Aspect here in Montreal recently. If I understand correctly, the key to the experimental test is that one chooses an axis of polarization for one of a pair of entangled particles (or photons), and that choice effectively randomizes the other axes for that particle. However the other particle is far away and cannot obviously know which axis in 3-dimensions was chosen for the first particle. Thus the second particle should be randomized in all three dimensions, but in fact it is not (that is, they are not when experimentally measured over many particles), it/they remain correlated with respect to the chosen axis (with appropriate corrections for geometry of the apparatus etc). Is this a reasonable very non-mathematical statement of the experiment and results? If so, it seems pretty much the same as an interference fringe experiment with double/multiple slits, measuring polarization angle rather than simply amplitude. If so, why is the Bell experiment usually presented as more definitive than a double slit? Thanks in advance!
  2. jcsd
  3. Jan 20, 2013 #2


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    Welcome to PhysicsForums, mes314159!

    1. Yes, well said. Must say I am jealous that you saw Aspect, he is one of my heroes and I have not had the opportunity to see him.

    2. I don't exactly see the direct connection here, although I see an indirect one.

    The Bell idea followed EPR's ingenious attack on Quantum Mechanics. That requires a way to predict spin with certainty at any angle, something that is not possible with a single particle. But is possible with a pair of suitably entangled particles.
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