# 5: Center of Mass Position, Internal Angular Momentum, Helicity

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In this chapter we define operators for the center of mass position and internal angular momentum of a Lonentz invariant physical system. These operators are defined in terms of the Poincare generators for the system. The motivation for defining center of mass position and internal angular momentum comes from experience in classical mechanics and in nonrelativistic quantum mechanics when it has been found useful to separate the motion of a system into motion of the center of mass and motion of the system relative to the center of mass. We carry out the same separation for relativistic quantum mechanics. Conditions on the center of mass position and internal angular momentum and given in Section 6.1 and definitions are given in Section 6.2. The helicity of a system is defined in Section 6.3 and some Poincare transformations are given in Section ó.4. Some derivations are given in Section 6.5.