Hyper-fast travel without negative energy

Abstract

Olum (PRL 81 3567-3570, 1998) has defined "superluminality" as the ability of a signal path to carry information faster than any neighbouring signal path, and has suggested that this requires a negative energy-density. However, this condition can be created without exotic matter if we are only sending information along the delivery path in one particular direction, and restrict ourselves to experiments that do not involve the speed of any counterpropagating or return-trip signals. Although negative energy-densities may be required for enhanced transit speeds in both directions along a single path at the same time, a traveller will normally not need (or want!) to travel in two opposing directions at once, so the condition of bidirectionality that gives rise to the negative energy condition may be unnecessarily restrictive.