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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?

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# Angular momentum of a free Dirac particle

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