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Spin 1/2 particle

  1. Aug 13, 2015 #1
    In my book (Griffiths) it is said :

    A particle is in ↑ state.
    The z-component of the particle's spin angular momentum [Sz] is h bar/2.
    The x- component of the particle's spin angular momentum [Sx] can be either h bar/2 or
    -h bar /2

    He says this is for uncertainty principle...that both Sz and Sx can't have definite value.
    This is where i am having difficulties.

    I mean it is always strictly like this that z component would give certain values and uncertainty would follow
    or just because we have measured z component first...ie, if we had measured x-component first then we would
    have the value h bar /2 for x- component and the uncertainty principle would act on z-component...??
    or because spin is always defined along Z axis...???

  2. jcsd
  3. Aug 13, 2015 #2

    Doc Al

    User Avatar

    Staff: Mentor


    The particle had to have been prepared in the state with respect to the z-axis. It could have been any axis--they just like to pick on the z-axis for some reason.
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