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Can spin be modeled as a spinning vector until "measured" ?

  1. Jan 26, 2016 #1
    I was wondering it the projection of the measurement axes onto a spinning vector modelled by a plane would produce correlations of entangled particles in line with what is found by experiment. Does anyone know of any discussions on this subject? Would this lead to stronger correlations than spin modelled as a vector?
  2. jcsd
  3. Jan 26, 2016 #2
    Spin is sort of like a physical rotation but not in physical space, so no you can't model it as a physical rotation.
  4. Jan 26, 2016 #3
    Usually it's a vector right? I meant that the direction of the vector is rotating.
  5. Jan 26, 2016 #4


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    No. What you propose is local hidden-variable theory.
  6. Jan 26, 2016 #5
  7. Jan 26, 2016 #6
  8. Jan 26, 2016 #7
    You know for a spin 1/2 particle its state is represented by a vector in C^2 and measurement is modeled as a projection. I am not sure what you are really asking but I think reading a book on quantum computation will help clarify your question. Generally any book on quantum computation will talk about spin 1/2, measurement, entanglement etc.
  9. Jan 27, 2016 #8
    I am wondering if it could be modelled as a real vector in three dimensional space spinning very fast in a plane until a magnetic field is introduced, upon which it precesses around the field instead?
  10. Jan 30, 2016 #9
    No, the state of a spin 1/2 particle is a vector in 2d complex vector space rather than a vector in 3d real space.
  11. Jan 30, 2016 #10
    Isn't the state just a representation of what we might find on measurement? Isn't QM silent about what is going on until that point?
  12. Feb 1, 2016 #11
    Thanks for this, I've been look for such a thing. This is pretty much what I thought, that it is essentially circular polarization, but I didn't have the math to back it up.

    The ultrasimple version is derived from an earthy expression of James Carville. Why does the electron spin? Because it can. If a physical system has a degree of freedom, it will use it if it can.
  13. Feb 2, 2016 #12

    A. Neumaier

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    The two descriptions are mathematically equivalent. See https://en.wikipedia.org/wiki/Bloch_sphere
  14. Feb 2, 2016 #13
    I have a problem with the Bloch sphere. The poles represent the z components of spin rather than the spin itself.....
  15. Feb 3, 2016 #14
    Yes but SU(2) is not equivalent with SO(3). In the representation of Bloch vector the global phase is missing,
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