Internal space‐time symmetries of massive and massless particles

A unified description is given for internal space‐time symmetries for massive and massless particles. It is noted that the little groups governing the internal symmetries of massive and massless particles are locally isomorphic to the three‐dimensional rotation group or O(3) and the two‐dimensional rotation group or E(2), respectively. It is noted also that the E(2) group can be obtained from a large‐radius/flat‐surface approximation of O(3). This procedure is then shown to be directly applicable to that of obtaining the E(2)‐like little group for massless particles from the O(3)‐like little group for massive particles in the infinite‐momentum/zero‐mass limit.