From a poisoned and desperate 1998, a group of particle-physicists at the Cavendish Laboratory in Cambridge are employing recent (i.e., future) discoveries about the temporal properties of tachyons to try to beam a warning back to the previous generation. Meanwhile, in sunny 1963, a young physicist named Gordon Bernstein is trying to get rid of the odd interference ruining a graduate student's experiment and causing frowns in the department at La Jolla. So, while Gordon resolves the noise into unfathomable messages about marine plankton and some sort of pesticide no one has ever heard of, the Cambridge SOS-broadcasters helplessly watch the 1998 spread of an appalling biochemical contamination from the oceans to the atmosphere. But as the light is going out for the 1998 world, the very reception of the tachyon-signal is paradoxically deflecting Gordon's 1963 world into an alternate future in which the major horrors of the years after 1963 will indeed be averted. At his best (Gordon's wrestlings with the inexplicable signal; bouts of departmental flak) Benford gives the phrase ""science fiction"" a new meaning as the art that brings science to fictional life. And for the most part his people (notably a clever and self-serving English bureaucrat) are solid enough to be cared about. True, there's too much self-conscious detail here--on British class rivalries, 1963 politics, and Gordon's Jewishness. But that flaw only slightly muffles the power of this admirable, important work from a major voice in science fiction.