  Boundary condition changing operators in conformal field theory describe
various types of "sudden switching" problems in condensed matter physics such
as the X-ray edge singularity. We review this subject and give two extensions
of previous work. A general derivation of a connection between the X-ray edge
singularity, the Anderson orthogonality catastrophe and finite-size scaling of
energies is given. The formalism is also extended to include boundstates.
