Recent solutions to the Einstein Field Equations involving negative energy
densities, i.e., matter violating the weak-energy-condition, have been
obtained, namely traversable wormholes, the Alcubierre warp drive and the
Krasnikov tube. These solutions are related to superluminal travel, although
locally the speed of light is not surpassed. It is difficult to define
faster-than-light travel in generic space-times, and one can construct metrics
which apparently allow superluminal travel, but are in fact flat Minkowski
space-times. Therefore, to avoid these difficulties it is important to provide
an appropriate definition of superluminal travel.Comment: 15 pages, 3 figures, LaTeX2e, Springer style files -included.
Contribution to the Proceedings of the Spanish Relativity Meeting-2001
(Madrid, September 2001