Real Time Propagator in the First Quantised Formalism

We argue that a basic modification must be made to the first quantised formalism of string theory if the physics of `particle creation' is to be correctly described. The analogous quantisation of the relativistic particle is performed, and it is shown that the proper time along the world line must go both forwards and backwards (in the usual quantisation it only goes forwards). The matrix propagator of the real time formalism is obtained from the two directions of proper time. (Talk given at the Thermal Fields Workshop held at Banff, Canada (August 1993).)