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Noob Question

  1. Sep 19, 2007 #1
    My understanding is that when you measure an electron's spin on on Axis A, and then on Axis B, the spin on Axis A is "forgotten" and can be something different next time you measure it. Is this correct?

    If it is, then how does this work across entanglement? If you measure electron A's spin at Axis A, then you know electron B's spin is the opposite. But if you then measure electron's A spin again on Axis B, you can potentially "change" electron B's spin on Axis A?
  2. jcsd
  3. Sep 19, 2007 #2
    Who says that would "change" the other electron?

    If A and B were entangled, measuring A once will give you information about both A and B. It will also generally break the entanglement, so further measurements of A tell you nothing further about B.

    For example, if you made a third measurement of A (on axis A for the second time, giving a random result) and then measured B, you would find it correlated with the first (rather than third) measurement of A.
  4. Sep 19, 2007 #3
    Damn, now I gotta think of another way to communicate FTL.
  5. Sep 19, 2007 #4
    Good luck with that.
  6. Sep 20, 2007 #5
    Have a clear answer for the first part: after the mesurement of spin along the B axis the particle will be in an eigenstate of spin pojected on B. Unless A and B are collinear, this is not an eigenstate of spin projected on A, thus if we make a third measuremnet (aling A again), the probability distribution will no longer be a Kronicker-delta
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