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170 perspective, it only takes an extended thought experiment to fill the Galaxy. Is galactic expansion the working of some universal law about the course of intelligent life, or is it just a figment of the imagination of a technologi- cally presumptuous but still adolescent species? Has a general principle governing the evolution and expansion of life been discovered, or is this just a reflection of the hubris to which we humans are so given? Our own history does not support the Tiplerian premise, Finney concluded; expan- sion is not automatic.”° The proposed models of interstellar colonization ignore the differences between the exploration of the Earth and the exploration of the Galaxy, argued Sagan and Newman. Earth is uncolonized by extraterrestrials not because interstellar spacefaring societies are rare, but because there are too many worlds to be colonized in the plausible lifetime of the coloniza- tion phase. For a reasonable population growth rate, the Galaxy is too big; they’re not here yet (Sagan calculated that the establishment of a “galactic hegemony” would take a billion years). A culture that gave a wide berth to planetary systems in which life is evolving would pose no contradiction to the apparent absence of extraterrestrials in our solar system; we would be none the wiser if such a civilization occupied all of the remaining plan- etary systems in our spiral arm of the Galaxy. Sagan and Newman introduced a political dimension into the debate, claiming that cultures aggressive enough to plan galactic colonization will destroy themselves before they get far. Although there are no very old civi- lizations with a consistent policy of conquest of inhabited worlds, there may be abundant groups of worlds linked by a common colonial heritage.”’ Goldsmith and Owen too emphasized the vastness of our Galaxy. Even if a million civilizations exist in the Milky Way and even if 1% of these civilizations are devoted to interstellar exploration on a grand scale, there still would be 40 million stars to explore for each civilization. A galaxy- wide program of colonization requires alternating expansionist and “mature” periods; it may turn out that no civilization will embrace enough cycles to spread through a galaxy. The absence of colonists from other civilizations here on Earth does not prove that there are no long-lived civi- lizations in the Milky Way.”* Galactic Vandals Challenging Tipler’s assumption that a technological civilization would send out self-replicating probes, the Clarks argued that such galactic vandalism is an extremely unlikely motivation for interstellar travel. Eric Jones and Barham Smith questioned the idea that any self- respecting product of Darwinian evolution would gift the Galaxy to a swarm of sentient machines. Such machines would mutate, Benford warned; they could malfunction and change their motivations, becom- ing competitors to their creators. Why Don’t We See Them?