In one minute

Everett and Roman extend Krasnikov’s two-dimensional geometry into a four-dimensional spacetime tube that a spacecraft would establish during a subluminal outward journey. Their model has a flat interior, curved walls, and light cones opened to permit rapid travel in one direction relative to outside observers. The first outward trip retains the usual light-speed limit, while the round-trip time measured on Earth can become arbitrarily short. One tube does not create a closed timelike curve, but two separated tubes running in opposite directions can create one. The calculated stress-energy tensor requires negative energy density. Applying quantum inequalities over short sampling times, they find extremely thin walls and enormous negative-energy requirements for useful superluminal travel in their chosen model.

Superluminal subway: The Krasnikov tube