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The Drake Equation, Take Three The Drake equation was derived from an assumed mode of contact: the detection of electromagnetic signals from very distant stars. It rested on the assumption that technological civilizations do not expand beyond their home systems. The density of such civilizations was determined by the number of separate evolutions to life and intelligence. The equation did not take into account the possibility of interstellar exploration, interstellar colonization, and direct contact. That gap has been challenged forcefully. The Drake equation is wrong, Dyson argued, because it says that the number of extraterrestrial civiliza- tions is equal to the number of independently originating civilizations. In fact, life spreads, diversifies, and speciates. Any community of intelligent creatures adapted to living freely in the vacuum of space will spread and speciate in the Galaxy. One intelligent species let loose in space may become a million intelligent species within an astronomically short time. Consequently, the Drake equation gives only a lower bound to the number of civilized societies.’ Shostak and Barnett, while defending conventional SETI, acknowledged that one star system could seed others if interstellar travel happens. The number of technological societies might be large even if the chance of evolving intelligence at one particular location is small.’ David Viewing of the British Interplanetary Society proposed a revised Drake equation in 1975, introducing factors representing the average number of colonies established by each independent civilization, and colo- nies established by the colonies.’ Brin argued later that the equation needs three new factors when we introduce star travel: V—the velocity at which an interstellar culture grows into space, pausing to settle likely solar systems and rebuild necessary industry before again continuing its expansion L(z)—the lifetime of a zone of colonization into which a species has expanded, after which the settled region becomes fallow again A—an approach/avoidance factor, different for each culture, representing a “cross- section” for discovery by contemporary human civilization. This encompasses 162