New formulation of a $\mathrm{}(1+1)\mathrm{}$-dimensional field theory constrained to a box

We consider a $\mathrm{}(1+1)\mathrm{}$-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.