Arthur C. Clarke famously encapsulated a quandary about the question of life on other planets: “Two possibilities exist: either we are alone in the universe or we are not. Both are equally terrifying.” After reading Seth Shostak’s wide-ranging and comprehensive survey of the search for extraterrestrial intelligence — SETI, to those in the know — I’d be willing to bet that Shostak would respectfully disagree. This optimistically persevering scientist firmly believes that we are not alone — “In short, you’ll be reading about the discovery of an alien signal in the next two dozen years” — and that this finding will be a joyous boon to mankind. As senior astronomer at the SETI Institute in Mountain View, California, he’s dedicated his life to the quest for an indisputably artificial signal from space — be it radio waves, light pulses, or less plausible and more exotic media such as gravity waves or neutrino streams. Shostak concisely charts the timeline and milestones of this relatively young field, which extends back only to the early 1960s. He charmingly portrays the characters of the researchers involved, including himself. What the SETI folks believe might be out there, and how they go about looking for life, as well as the germane hardware, physics and cosmology are all conveyed in a breezy yet scrupulously scientific manner. (Likewise, occult, saucerian, and new age beliefs are demolished.) Provoking and fascinating, this book skates deftly over its central, retroactively apparent lacuna — which is identical, ironically enough, to that behind UFO studies. For all of the field’s logical speculations, stringent methodologies, thrilling near-misses and computer-parsed data accumulation, it has no actual successes to point to. At the core of SETI is something as numinous as the Grail, which only faith has yet made conceivable.