Angular momentum of a free Dirac particle

[DOTDO]

Hi

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?

 

[Orodruin]

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

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.