  The energy conditions of general relativity permit one to deduce very
powerful and general theorems about the behaviour of strong gravitational
fields and cosmological geometries. However, the energy conditions these
theorems are based on are beginning to look a lot less secure than they once
seemed: (1) there are subtle quantum effects that violate all of the energy
conditions, and more tellingly (2), there are also relatively benign looking
classical systems that violate all the energy conditions. This opens up a
Pandora's box of rather disquieting possibilities --- everything from negative
asymptotic mass, to traversable wormholes, to warp drives, up to and including
time machines.
