Strings, strong fields and boundaries

String theory in spacetimes with boundaries is shown to be equivalent, in some instances, to string theory on orbifolds. This equivalence is used to calculate the Casimir energy of the string between two infinite parallel plates in spacetime. The calculation mimics some features of string theory in Rindler and Schwarzschild spacetimes and supports the conjecture that string theories avoid the singularity problem by giving particle production rates that diverge in the presence of large, but finite, fields.