  Using concepts developed in string theory, Cohen, Moore, Nelson and
Polchinski calculated the propagator for a relativistic point particle.
Following these authors we extend the technique to include the case of closed
world lines. The partition function found corresponds to the Feynmann and
Schwinger proper time formalism. We also explicitly verify that the partition
function is equivalent to the usual path length action partition function. As
an example of a sum over closed world lines, we compute the Euler-Heisenberg
effective Lagrangian in a novel way.
