The Wheeler Propagator
  We study the half advanced and half retarded Wheeler Green function and its relation to Feynman propagators. First for massless equation. Then, for Klein-Gordon equations with arbitrary mass parameters; real, imaginary or complex. In all cases the Wheeler propagator lacks an on-shell free propagation. The Wheeler function has support inside the light-cone (whatever the mass). The associated vacuum is symmetric with respect to annihilation and creation operators.   We show with some examples that perturbative unitarity holds, whatever the mass (real or complex). Some possible applications are discussed. 
C. G. Bollini, M. C. Rocca