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?