Explore the vast range of titles available.

There have been many objections to the possibility of time travel. But all the truly interesting ones concern the possibility of reverse causation. What is objectionable about reverse causation? I diagnose that the truly interesting objections are to a further possibility: that of causal loops. I raise doubts about whether there must be causal loops if reverse causation obtains; but devote the majority of the paper to describing, and dispelling concerns about, various kinds of causal loop. In short, I argue that they are neither logically nor physically impossible. The only possibly objectionable feature that all causal loops share is that coincidence is required to explain them. Just how coincidental a loop will be varies: some are really quite ordinary, and some are incredibly unlikely. I end by speculating that the tendency amongst physicists to avoid discussion of causal loops involving intentional action may have been unfortunate, since intentional action is an excellent way to non-mysteriously bring about what otherwise would have been an unlikely coincidence. Hence causal loops may be more likely in a world with beings like us, than in one without.

Miguel Alcubierre's vision of space travel is made possible by the subtleties of Albert Einstein's general theory of relativity. The key to Alcubierre's warp drive is something called exotic matter, which has the curious property of having a negative energy density, unlike normal matter.

Uses space explorations to integrate learning activities in science, language arts, math, social studies, art, foreign language, and physical education. (PR)

First of a new five-volume series, as Donaldson moves away from fantasy and into technological science fiction--where, in the medium-future, faster-than-light journeys (\"crossing the Gap\" by means of other dimensions) are a reality, though some travellers go mad, and space ...