Krasnikov tube

A Krasnikov tube[1] is a speculative mechanism for space travel involving the warping of spacetime into permanent superluminal tunnels.

The resulting structure is analogous to a wormhole or an immobile Alcubierre drive (and like them requires exotic matter with negative energy density) with the endpoints displaced in time as well as space.

[2] The tube is a distortion of spacetime that can be intentionally created (using hypothetical technology) in the wake of travel near the speed of light.

Krasnikov argues that despite the time-machine-like aspects of his metric, it cannot violate the law of causality (that a cause must always precede its effects in all coordinate systems and along all space-time paths) because all points along the round-trip path of the spaceship always have an ordered timelike separation interval (in algebraic terms, c2dt2 is always larger than dx2 + dy2 + dz2).

That is, vacuum fluctuation would grow exponentially, eventually destroying the second Krasnikov tube as it approaches the timelike loop limit, in which causality is violated.[who?]

This spacetime diagram shows the causal structure of a Krasnikov tube; the U-shaped line is the boundary of the tube, while the diagonal lines represent the forward light cones of the dots.