Light-Cone Kaluza-Klein Geometry: Confined Propagation as a Particle   Model
  A modified Kaluza-Klein theory is proposed in which propagation takes place only at the speed of light. The propagation can be confined to a small volume, forming a particle with rest mass. The usual four space-time coordinates locate the confinement volume, and Kaluza's fifth coordinate is replaced by an internal degree of freedom. Electromagnetism corresponds to a gauge field on the phase of the internal motion. Self-gravity might create the confinement, as in a geon, but the particle would have a Planck mass. This large mass could be made closer to the masses of observed particles if gravity were allowed to increase in strength within the confinement volume. 
Donald E. Jennings (NASA Goddard Space Flight Center)