Allen Everett

As is typical of popularizations with this degree of specificity, the book's first half is taken up with basic background material, although it must be said that the explanations of relativity are very good and are not devoid of some of the associated mathematics. Then come the possible ways in which the restriction against superluminal travel might be bypassable and in which closed timelike curves might in principle be constructible (e.g. spacetime wormholes, hypermassive cylinders, Gott's cosmic strings, Mallett's helical light pipes). By emphasizing Hawking's theorem to the effect that either infinite size or negative energy ("exotic matter") must be involved, and reporting on recent research into how enormously problematic the amassing of negative energy would be, the authors give the impression that physics is getting closer to ruling out "time machines" once and for all. Great stuff, and a good update on earlier books by Thorne and Davies.… (mais)