Position and Spin Operators, Wigner Rotation and the Origin of Hidden   Momentum Forces
  Using a position operator obtained for spin 1 particles by the present author and Wigner, we obtain a quantum relativistic result for the hidden momentum force experienced by particles with structure. In particular, our result applies to the hidden magnetic forces manifest in some problems of electromagnetism. We also discuss spin and orbital angular momentum operators, as well as Wigner rotation. 
R.F.O'Connell
quant-ph