  We study the real time formalism of non-equilibrium many-body theory, in a
first quantised language. We argue that on quantising the relativistic scalar
particle in spacetime with Minkowski signature, we should study both
propagations $e^{i(p^2-m^2)\tilde \lambda}$ and $e^{-i(p^2-m^2)\tilde \lambda}$
on the particle world line. The path integral needs regulation at the mass
shell $p^2=m^2$. If we regulate the two propagations independently we get the
Feynman propagator in the vacuum, and its complex conjugate. But if the
regulation mixes the two propagations then we get the matrix propagator
appropriate to perturbation theory in a particle flux. This formalism unifies
the special cases of thermal fluxes in flat space and the fluxes `created' by
Cosmological expansion, and also gives covariance under change of particle
definition in curved space. We comment briefly on the proposed application to
closed strings, where we argue that coherent fields and `exponential of
quadratic' particle fluxes must {\it both} be used to define the background for
perturbation theory.

High Energy Physics - Theory