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174 The interstellar imperative—the bottom line of starflight—is that ours should become a civilization that can outlive its star. —Eugene Mallove and Gregory Matloff, 1989*° Some intelligent beings would realize, sooner or later, that changes in their stars would make their planetary systems uninhabitable. The fore- knowledge of impending doom, even if it were millions of years in their future, might drive some technological civilizations to migrate toward more hospitable locales. Those outward leaps might begin at very different times in different systems, depending on the lifetimes of host suns. Some hot stars may support zones of habitability for only a few hundred million years; cooler ones may remain stable for tens of billions of years. Over time, the Outleap might acquire legendary status like the great Trek of the Boers in South Africa, or the Long March of the Chinese Commu- nists in the 1930s. However, an outward migration on this scale may not be a single event that includes all of a given planetary population. Different groups may depart at different times, possibly in different directions. A very ancient civilization that was forced to make its first jump billions of years ago might face the prospect of a second migration if its new home became unlivable. However, no second jump would be necessary for a species prepared to do without stars. A society capable of building inter- stellar arks that do not require the energy of a sun could migrate to any location in interstellar space. They could be present throughout the vast- ness of our Galaxy, yet be invisible to us. This model of interstellar migrations separated by eons of time implies a much slower rate of expansion than those foreseen by Tipler, Jones, and others. Although the infrequency of such expansions would limit the number of locations where technological societies exist, migrations in dif- ferent directions could raise that number. This minimum model might offer a solution to the Paradox—if we had any way to confirm it. However, we should be cautious about applying any particular model to all technological civilizations that choose to establish colonies beyond their home systems. Even if this model applies to most, there may be statistical outliers. The model may not apply at all to unin- habited probes, which might be sent even when there are no plans for colonization. David Brin published a major analysis of the paradox in 1983, entitled “The Great Silence.” Convinced that the case for some version of slow interstel- lar travel was growing stronger, he categorized some of the possible reasons Why Don’t We See Them? A Minimum Model of Interstellar Expansion The Great Silence