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312 others. Assumptions about distance also may be wrong if some advanced technological civilizations choose to expand, planting colonies or radiating their presence outward in interstellar arks. The extraterrestrials we detect may be much farther away than the average—or much nearer. Two schools of thought imagine that interstellar expansion, once started, will continue until the Galaxy is occupied. The colonization school, often driven by population growth models, foresees humans or their alien coun- terparts planting settlements around all suitable stars; their colonies will generate more colonies that will continue the expansion. The self- reproducing probe school foresees machines creating artificially intelligent progeny that journey on to other systems, perhaps seeding them with biological life. Both models are questionable because they assume uniformity and con- tinuity in both purpose and action over millions of years. History tells us that purposes change over much shorter spans of time; policy decisions and the commitment of resources may be even more short term. Waves of colo- nization may be temporary, for us and for others; they could stall for a variety of reasons. So might expansion by machines. Tipler and Barrow claimed that there would be no resistance to the expansion of the volume of stars colonized by Von Neumann probes. What happens if the machines of an expanding civilization encounter another with the technological means to resist? The colonists, or the probes, might be defeated. Let’s return to the most disturbing model, Dyson’s technological cancer sweeping through the Galaxy. Would those intelligences, perhaps in the form of sentient machines, remain ruthless conquerors through the mil- lennia? Or might they vary their behavior as they evolve? As Sagan and Newman put it, “where are they?” depends powerfully on the politics and ethics of advanced societies.’ “Colonizers” Versus “Imperialists” Tipler and Barrow, who visualized an aggressive interstellar expansion and colonization program, attempted to draw a distinction between that idea and the more pejorative concept of interstellar imperialism. First, they declared that there was no reason to expect imperialism. Then they acknowledged that the existence of “imperialists” would motivate “colonizers” to speed up their occupation of previously unoccupied Assumptions: After Contact Expansion Will Be Relentless