SUPERLUMINAL TRAVEL REQUIRES NEGATIVE ENERGIES. Einstein's special theory of relativity asserts that no physical object can travel faster than the speed of light. The theory also holds that mass and energy will have different values depending on your frame of reference. The idea that the mass (energy) density in any one frame would always be at least equal to or greater than zero is called the "weak energy condition." Ken Olum of Tufts (kdo@cosmos5.phy.tufts.edu, 617-628-5000, x2753) follows the reverse tack in arguing that superluminal travel is possible in certain warped versions of space/time but that this would entail the existence of negative energy. In this case the concepts of superluminal motion and of negative energy need to be explored. An object with negative mass would be less massive than empty space. We don't know of any such object, but physicists have detected small regions of space characterized by a very slightly negative energy density (the so called Casimir effect; see Updates 122 and 300). If you combine negative energy with positive energy you get nothing, very different from the explosion you get when you combine matter and antimatter. As for superluminal travel---in Olum's model objects and signals do not actually travel faster than light. Rather, the curvature of a spacetime incorporating a negative-energy density is such that one can arrive quickly at distant places using sub-light speeds. (Physical Review Letters, 26 October 1998.)