Rotations and e, $\nu$ Propagators, Part I
  Rotation symmetry is less constraining than space-time symmetry. The free electron propagator is a projection operator that we show can be constructed from rotation symmetric projection operators. Rotation-based identifications of time, space, energy, momentum, polarization matrices, and the positron hypothesis are determined by the constraints that turn rotation symmetric projection operators into the electron propagator.   PACS: 11.30.-j, 11.30.Cp, and 03.65.Fd 
Richard Shurtleff