Light rays, singularities, and all that

This is the authors' abstract. We don't add key points for this paper.

Starting from the 1960s, numerous results have been obtained on the global structure of solutions of Einstein's general relativity. These include the classic singularity theorems of Penrose and Hawking, which imply that under certain rather general conditions singularities are unavoidable. Light rays play an important role in the proofs of these theorems. This paper reviews the properties of light rays and surveys their implications for the occurrence of singularities in general relativity.