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I What's so strange about entangled particles?

  1. Sep 3, 2016 #1
    In a thought experiment, there is a spin-0 source emitting particles. (I suppose you already know what is that experiment.) Two observers in opposite sides along the same axis measure opposite spin components. If one observer measure, say, spin up, then the second observer will certainly measure spin down. Then it's said that the measurement by one observer affects what the another observer will find. But why don't conclude that the system (composed by the two particles) was already set up for that particular pair of results (spin up to one particle and down to the other particle) before the first observer interact with the system?
    Last edited by a moderator: Sep 4, 2016
  2. jcsd
  3. Sep 3, 2016 #2
    You're thinking of spin-1/2 particles because the spin can take two values. Anyway, the explanation you gave falls flat on its face when you consider more general measurements on each particle. This is called Bell's theorem. But you are right that if you measure the spin of both particles in the same direction, then it's no different than classical correlation.
    Last edited by a moderator: Sep 4, 2016
  4. Sep 4, 2016 #3
    I got this. Thank you.
  5. Sep 4, 2016 #4
    The correlation depends on the basis in which both the measurements are made. The basis can freely be adjusted at any time. So since the basis can be freely adjusted and the results depend on the basis, then if the results are predetermined, the choice of basis must be predetermined, and experimenters have no free will.
  6. Sep 4, 2016 #5
    So why is quantum entanglement considered mysterious?
  7. Sep 4, 2016 #6
    I guess some find that the correlation between the measurement outcomes of a pair of separated particles looks like transfering of information faster than light (non-locality), which is impossible. However, since there is no causal relationship, relativitiy is not violated. The absence of causality between correlated outcomes is mysterious too. :smile:
  8. Sep 4, 2016 #7


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    Staff: Mentor

    David Mermin gave a simple example of a hypothetical Bell's Theorem experiment in this article:

    Is the moon there when nobody looks? Reality and the quantum theory (PDF file)

    Read the description carefully, study the results, and see if you can come up with a "non-mysterious" explanation.
  9. Sep 5, 2016 #8


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    Good question. There's nothing mysterious with it on the level of the pure physics part of quantum theory, but it is the very phenomenon that makes quantum theory very different from classical physics, and we are used to classical physics in our everyday experience (except for the fact that matter is stable, which is not understandable at all within classical physics).
  10. Sep 5, 2016 #9
  11. Sep 12, 2016 #10
    Because, according to QM, the outcome could have been the opposite (spin down to one particle and up to the other particle).
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