Generalized Dirac operators on Lorentzian manifolds and propagation of   singularities
  We survey the correct definition of a generalized Dirac operator on a Space--Time and the classical result about propagation of singularities. This says that light travels along light--like geodesics. Finally we show this is also true for generalized Dirac operators. 
Paolo Antonini
math.DG math-ph math.AP math.MP