be brought together without causing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. (See Time travel using wormholes and the Chronology protection conjecture.) It has been suggested that a similar mechanism would destroy time-machine Krasnikov tubes. 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. See also Wormhole Time travel Wheeler–Feynman absorber theory References External links The Krasnikov Tube: A Subway to the Stars by John G. Cramer Exposition and criticism of the Krasnikov Tube by Allen E. Everett and Thomas A. Roman Superluminal motion in (semi)classical relativity by Serguei Krasnikov (In Russian) Warp drive theory