Krasnikov tube

A Krasnikov tube is a speculative mechanism for space travel. Spacetime is warped into permanent superluminal tunnels. The rsulting spacetime is analogous to a womrhole or to an immobile Alcubierre drive. Like the traveserable wormhole and the Alcubierre drive, the Krasnikov tube requires exotic matter with a negative energy density.

The idea was proposed in 1995 by  Sergey Krasnikov. 

The structure invovles a tube, a distortion in spacetime, that can be intentionally created. This process would involve hypothetical technology. This tube would be created by travel close to the speed of light.

The traveler is forced to speed along the tube at significant fractions of the speed of light.  

Krasnikov notes that despite the time-machine-likeness of his metric, it won't violate the law causality.  It was proposed by Allen E. Everett and Thomas A. Roman that two Krasnikov tubes running in opposite directions could give rise to a closedtimelike curve. This would violate causality.