As we begin this discussion, it occurs to me that no review of faster than light travel could be complete without a review of the Alcubierre Warp Drive.
According to all sources, the Alcubierre Drive is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable energy-density field lower than that of vacuum - that is, negative mass - could be created. Proposed by theoretical physicist Miguel Alcubierre in 1994, the Alcubierre drive is based on a solution of Einstein's field equations. Since those solutions are metric tensors, the Alcubierre drive is also referred to as Alcubierre metric.
Most scientists agree that objects cannot accelerate to the speed of light within normal spacetime; however, the Alcubierre drive shifts space around an object so that the object would arrive at its destination more quickly than light would in normal space without breaking any physical laws.
Although the metric proposed by Alcubierre is consistent with the Einstein field equations, construction of such a drive is not necessarily possible. The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter or manipulation of dark energy. If exotic matter with the correct properties does not exist, then the drive cannot be constructed. At the close of his original article, however, Alcubierre argued, following an argument developed by physicists analyzing traversable wormholes. that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive.
Another possible issue is that, although the Alcubierre metric is consistent with Einstein's equations, general relativity does not incorporate quantum mechanics. Some physicists have presented arguments to suggest that a theory of quantum gravity, which would incorporate both theories, would eliminate those solutions in general relativity that allow for backward time travel, see the chronology protection conjecture, and thus make the Alcubierre drive invalid.
