Schwinger's non-commutative coordinates and duality between helicity and Dirac quantisation conditions

Abstract

The helicity operator of massless particles has only two polarisations like it takes place for photons and gravitons. For them not all of the 2s+1 spin magnetic quantum states exist, with two exceptions, and the spin operator ceases to be defined properly and consistently. The problem was solved by Schwinger, who introduced non-commutative space coordinates that completely eliminate the spin operator and ensure that only helicity operator appears explicitly. We further investigate the violation of the associativity relation of the momentum translation operator that emerges due to the failure of the corresponding Jacobi identity. The associativity relation is broken by a phase factor which satisfies a 3-cocycle relation. The associativity is restored when a 3-cocycle is an integer number, and leads to the quantisation of massless particle's helicity. We discuss the correspondence (duality) between the helicity and the Dirac quantisation conditions. The relation for the minimal space cell volume, similar to the minimal phase-space cell of Heisenberg is suggested.