Topology change and causal continuity

The result that, for a scalar quantum field propagating on a "trousers" topology in 1 + 1 dimensions, the crotch singularity is a source for an infinite burst of energy has been used to argue against the occurrence of topology change in quantum gravity. We draw attention to a conjecture due to Sorkin that it may be the particular type of topology change involved in the trousers transition that is problematic and that other topology changes may not cause the same difficulties. The conjecture links the singular behavior to the existence of "causal discontinuities" in the spacetime and relies on a classification of topology changes using Morse theory. We investigate various topology changing transitions, including the pair production of block holes and of topological geons, in the light of these ideas. [S0556-2821(98)07020-9].