The irreducible representations of the inhomogeneous Lorentz group which describe, respectively, particles with and without mass are qualitatively different. Nevertheless, the zero-mass representations can be obtained from the representation with mass by letting the mass go to zero. Particles of sufficiently small mass or sufficiently high energy behave like particles of zero mass.
- Received 22 October 1962
- Published in the issue dated March 1963
© 1963 The American Physical Society