The Casimir effect in string theory

Abstract

We discuss the Casimir effect in heterotic string theory. This is done by considering a [Formula: see text] twist acting on one external compact direction and three internal coordinates. The hyperplanes fixed by the orbifold generator [Formula: see text] realize the two infinite parallel plates. For the latter to behave as “conducting material,” we implement in a modular invariant way the projection [Formula: see text] on the spectrum running in the vacuum-to-vacuum amplitude at one-loop. Hence, the relevant projector to account for the Casimir effect is orthogonal to that commonly used in string orbifold models which is [Formula: see text]. We find that this setup yields the same net force acting on the plates in the context of quantum field theory and string theory. However, when supersymmetry is not present from the onset, finiteness of the resultant force in field theory is reached by adding formally infinite forces acting on either side of each plate, while in string theory, both contributions are finite. On the contrary, when supersymmetry is spontaneously broken à la Scherk–Schwarz, finiteness of each contribution is fulfilled in field and string theory.