  We consider a 1+1 dimensional field theory constrained to a finite box of
length L. Traditionally, calculations in a box are done by replacing the
integrals over the spatial momenta by discrete sums and then evaluating sums
and doing analytic continuations. We show that it is also possible to do such
calculations using an analogy to finite temperature field theory. We develop a
formalism that is similar to the closed time path formulation of finite
temperature field theory. Our technique can be used to calculate spatially
retarded green functions, without evaluating sums or doing analytic
continuations. We calculate the self energy in a simple scalar theory as an
example.
