Spin operator for the relativistic particle

A systematic theory of the appropriate spin operators for the relativistic states is developed. This paper discusses it in particle case, i.e., the qu antum mechanics problem. For a massive relativistic particle with arbitrary nonzero spin, the spin operator should be replaced with the relativistic o ne. In the frame of irreducible representation of Poincare group, this spin operator, which is named as moving spin and applied to all the canonical s tates of the particle, is constructed. Further discussion on the concept of moving spin in the quantum field theory will be followed.