A Simple Five-Dimensional Wave Equation for a Dirac Particle
  A first-order relativistic wave equation is constructed in five dimensions. Its solutions are eight-component spinors, which are interpreted as single-particle fermion wave functions in four-dimensional spacetime. Use of a ``cylinder condition'' (the removal of explicit dependence on the fifth coordinate) reduces each eight-component solution to a pair of degenerate four-component spinors obeying the Dirac equation. This five-dimensional method is used to obtain solutions for a free particle and for a particle moving in the Coulomb potential. It is shown that, under the cylinder condition, the results are the same as those from the Dirac equation. Without the cylinder condition, on the other hand, the equation predicts some interesting new phenomena. It implies the existence of a scalar potential, and for zero-mass particles it leads to a four-dimensional fermionic equation analogous to Maxwell's equation with sources. 
N. Redington and M.A.K. Lodhi