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Concept
Krasnikov tube
Summary
A Krasnikov tube 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. The idea was proposed by Sergey Krasnikov in 1995.
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. The Krasnikov tube allows for a return trip that takes travelers back to the time right after they left. Experiencing the effect requires that the traveler race along the tube at speeds close to that 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). This means, for example, that a light-beam message sent along a Krasnikov tube cannot be used for back-in-time signaling.
While one Krasnikov tube can be seen to present no problems with causality, it was proposed by Allen E. Everett and Thomas A. Roman of Tufts University that two Krasnikov tubes going in opposite directions can create a closed timelike curve, which would violate causality.
For example, suppose that a tube is built connecting Earth to a star 3000 light years away. The astronauts are travelling at relativistic velocities, so that the journey through this Tube I only takes 1.5 years from their perspective. Then the astronauts lay down tube II rather than travelling back in tube I, the first tube they produced. In another 1.5 years of ship time they will arrive back on Earth, but at a time 6000 years in the future of their departure.
Official source
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