  It is well known that boundary conditions on quantum fields produce
divergences in the renormalized energy-momentum tensor near the boundaries.
Although irrelevant for the computation of Casimir forces between different
bodies, the self-energy couples to gravity, and the divergences may, in
principle, generate large gravitational effects. We present an analysis of the
problem in the context of quantum field theory in curved spaces. Our model
consists of a quantum scalar field coupled to a classical field that, in a
certain limit, imposes Dirichlet boundary conditions on the quantum field. We
show that the model is renormalizable and that the divergences in the
renormalized energy-momentum tensor disappear for sufficiently smooth
interfaces.
