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Theory of electron spin

  1. Aug 28, 2008 #1
    QM texts explain electron spin as an intrinsic spin with intrinsic angular momentum of electron.

    And this theory predicts (or postdicts maybe?) splitting of silver or hydrogen atom or electron beams in Stern Gerlach experiments.

    a few questions:

    1) intrinsic angular momentum with its magnetic moment is invoked to explain the beam splitting in a nonuniform magnetic field. But many electrons in a sivler atom also have non zero angular momentum and associated magnetic moment. Why does this not also cause beam splitting in SG experiment into more than two beams ??

    2) Does QED predict electron spin?

    3) Does the Dirac equation predict electron spin or handle it, and how or any link to an explanation would be good.

  2. jcsd
  3. Aug 28, 2008 #2
    what is the spin of a 'lone pair'?
  4. Aug 29, 2008 #3


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    QED and Dirac eq. are based on the existance of spin, spin is found experimentally before one developed theory.
  5. Aug 29, 2008 #4
    In other words, QED and the Dirac equation do not derive or predict the existence of spin, but assume it as an empirical fact.
  6. Aug 30, 2008 #5
    Dirac wanted an equation for describing the quantum-relativistic motion of electron. He found that for obtaining an hamiltonian linear in mass and momentum he must working in (at least) 4-dimensional space. Those additional dimensions provide the existence of spin and antimatter.

    Althought spin was alreay known (and so surely it had to appear in the equation) there is littley logically-derived in the formalism.

  7. Aug 30, 2008 #6


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    Spin was already formulated by W. Pauli before Dirac did this equation, the thing Dirac did was to extend Paulis eq. into special relativity, and hence he got the negative energy solution, which he postulated were the existence of antimatter.
  8. Aug 30, 2008 #7
    After Dirac italian Maiorana has derive a Dirac like equation for the photon with spin 1.

    If you know the experiment results you can slant almost every theory to these results, remember Ptolemeus Universe :)
  9. Aug 30, 2008 #8
    i think you are right, thanks for precisation.

  10. Aug 30, 2008 #9


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    also recall the relation between Dirac's gamma matrices and Paulis spin matrices.
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