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Angular momentum of a free Dirac particle

  1. May 19, 2015 #1

    I read that

    for Dirac equation, [ L , H ] =/ 0 ,

    so Dirac found a operator S such that

    1. [ S , H ] = - [L, H] ---> [ J, H ] = 0 where J = L + S , the total angular momentum.

    2. S gives an eigenvalue of spin 1/2 ---> solutions of Dirac equation describe fermions.

    The total angular momentum is constant in motion...

    but still, L and S varies as time elapses although there is no external force... why?

    And if there exists an external force, do L and S both change?
  2. jcsd
  3. May 19, 2015 #2


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    This is an internal exchange of angular momentum and deals with internal interactions. If you had an interaction with an external system you might change the total angular momentum.
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