  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).)
