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Geometric wormhole throats

Wormholes and black holes have traditionally been treated a quite separate objects with relatively little overlap. The possibility of a connection arises in that wormholes, if they exist, might have profound influence on black holes, their event horizons, and their internal structure. After discussing some connections, we embark on an overview of what can generally be said about traversable wormhole throats. We discuss the violations of the energy conditions that typically occur at and near the throat of any traversable wormhole and emphasize the generic nature of this result. We discuss the original Morris-Thorne wormhole and its generalization to a spherically symmetric time-dependent wormhole, and also discuss spherically symmetric Brans-Dicke wormholes. We also discuss the relationship with the topological censorship theorem. Finally we turn to a rather general class of wormholes that permit explicit analysis: generic static traversable wormholes (without any symmetry). We define the wormhole throat in terms of a 2--dimensional constant-time hypersurface of minimal area. (Zero trace for the extrinsic curvature plus a ``flare--out'' condition.) This enables us to derive