Rood and Trefil (Astronomy & Physics, Univ. of Virginia) provide engaging counterarguments to the prevailing inclination to believe in extraterrestrial intelligences (ETI galore. Maybe we are alone, they are saying. (All alone, says Trefil; perhaps another world or two, says Rood.) They base their thinking on estimates derived from the ""Green Bank"" equation, the formula Frank Drake and others developed in the early 1960s when astronomers (at Green Bank, W.Va.) first began to search for extraterrestrial life (SETI). All you have to do is to change the variables according to optimistic, conservative, or pessimistic views, and N--the number of intelligence worlds--can vary from 1 on up. The book is more than mathematical formulas, however. To give meaning to the calculations the authors must discuss rates of star and planetary formation, the evolution of life forms, the duration of intelligent civilizations. They must also speculate on whether intelligent life is at all interested in looking for fellow galactic travelers and is willing to put its money where its mouth is. Maybe nobody out there cares. Maybe they do--but for aggressive reasons. In any case the question remains, Where are they? Why haven't we heard? Trefil says it's because we really are something special. Rood--agnostic--says maybe so, but he's not so sure. The two also disagree on certain related topics, speaking with separate voices in one chapter that speculates about future computers, space travel, and the like. On the other hand, both agree on the legitimacy of pursuing ETI questions. The variables in the Green Bank equation apply in practical and theoretical ways to a technological society, leading to discussions of how to tap new energy sources, colonize space, and avoid planetary holocaust or the decay of civilization. Meantime the authors' open minds would support modest SETI programs for their potentially useful fallout. In sum, you'll find these doubting Thomases a refreshing antidote to unalloyed Saganism.