Classical perspectives on the Newton-Wigner position observable

This paper deals with the Newton-Wigner position observable for Poincaré-invariant classical systems. We prove an existence and uniqueness theorem for elementary systems that parallels the well-known Newton-Wigner theorem in the quantum context. We also discuss and justify the geometric interpretation of the Newton-Wigner position as “center of spin”, already proposed by Fleming in 1965 again in the quantum context.