"... but nature seems uncooperative ... For a discussion of the apparent impossibility of time machines in general relativity, see "Singularities and Causality Violation," by Frank J. Tipler in Annals of Physics, Vol. 108, pp. 1-36, 1977. Tipler is open-minded; in 1974 he had argued
the other side of the case."
--Eric Drexler, Engines of Creation