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Electron spin, why do the pairs have to be opposite?

  1. Jun 30, 2007 #1
    In the experiment where there are two electrons, and one is shot out to a person, the experiment depends on the two having opposite spins, one up one down, though which has which is not determined until observation.

    So person 1 looks at theirs and sees it's an upward spin. This collapses person 2s to downward imediantly. So even if the other person was 2 billion lightyears away at that time, it would still effect instantly. This experiment makes sense, aside from one thing.

    How do they know that one is up spin, and one is down spin to begin with?

    I think I may have learned in chemistry that on the same valance level, electrons must have opposite spins or something, is that it or am I completely off?
  2. jcsd
  3. Jul 1, 2007 #2

    Hans de Vries

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    Science Advisor

    No they do not need, in general, to have opposite spins.

    You are describing an entanglement experiment. Although there seems to
    be an element of randomness in the "collapse of the wavefunction", it is
    not that random. The conservation laws must be all obeyed and in this
    case it's the conservation of spin.

    Another experiment would be where both particles have the same spin, but
    of unknown direction. So if one is up the other must be up as well, and visa
    versa, if one is down the other must be down as well.

    Regards, Hans
  4. Jul 1, 2007 #3


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    Staff: Mentor

    Because the experimental setup is constructed specifically so that the initial state that produces the two electrons (or whatever kind of particle is being used) has a total angular momentum of zero. Therefore, by conservation of total angular momentum, the sum of the particles' angular momenta must also be zero.
  5. Jul 2, 2007 #4


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    Homework Helper

    But still, just to be picky, this does not necessarily imply that the total spin of the two electron system is zero.
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