In one minute

José Natário revisits the common explanation that Alcubierre's warp drive works by contracting space ahead of the bubble and expanding it behind. He shows this contraction and expansion is only a side effect of Alcubierre's particular choice, and builds a similar warp drive with no contraction or expansion at all. Describing warp spacetimes by a vector field on flat 3-space, he shows they always violate some energy condition and that the volume expansion is the field's divergence. Choosing a divergence-free field gives a volume-preserving warp drive. Simple examples show horizons and infinite blueshifts, and he also discusses the global and optical properties of these spacetimes.

Warp Drive With Zero Expansion