Classical perspectives on the Newton--Wigner position observable

Abstract

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 `centre of spin', already proposed by Fleming in 1965 again in the quantum context.

28+5+4 pages (main text, references, appendices), to appear in International Journal of Geometric Methods in Modern Physics. v2: Added references, corrected typos, restructured references v3: Corrected typos, changed MSC class (better-fitting class in MSC2020)