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175 for the apparent absence of extraterrestrials (this is taken from the more popularized version of his original paper). Solitude. We are unique in evolving technological intelligence. Habitable planets may be rare; some “spark” may be needed to initiate life, or some “software miracle” is required for intelligence. The average lifespan of a technological civilization may be short due to some “inevitability” of self-destruction, or because robotic probes spreading throughout the galaxy are programmed to wipe out other intelligent life. Magical Technology. Technological species may discover techniques that make radio and even colonization irrelevant. Quarantine. Other intelligences may purposely avoid contact with us. Benevolent species might let “nursery worlds” lie fallow for long periods to nurture new sentience; we may be in a preserve or “zoo;” alien observ- ers may be awaiting humankind’s social maturity; or we may be quaran- tined as dangerous. As our neighbors won’t remain near us due to the galaxy’s differential rotation, the quarantine hypothesis would appear to call for some degree of cultural uniformity in the galaxy. Macrolife. Advanced civilizations may abandon planet-dwelling. There might be selective pressures favoring those suited to life in starships. Spaceborne sophonts might greedily destroy terrestrial planets by mining them for their natural resources, or they might cherish nursery worlds they do not need as real estate. Seniors Only. Spacefaring intelligences might graduate to other interests. Achieving immortality might promote conservatism and an aversion to the dangers of spaceflight. Low Rent. Earth might be inaccessible or undesirable. Alien life may be biologically incompatible with terrestrial life. We may be in a region of the galaxy where interstellar travel is more difficult. Migration Holocaust. Occupation of a nursery world by an expansionist species might cause extinction of its higher life forms, delaying the local emergence of intelligence. Earth might be the first nursery world to recover sufficiently after the last wave of colonization passed this way. Brin offered a grim scenario in which colonizing cultures leave behind them wastelands emptied of intelligence; the Great Silence may be the sound of sand sifting up against monuments. He added other possibilities: Aliens might be cautious about contact to avoid becoming targets for an aggressive species or its probes, or they might be here, contacting govern- ments or secretly meddling in our affairs. Brin concluded on a hopeful note. A noble race may have taught a tradi- tion of respect for the hidden potential of life in all subsequent spacefaring species. “It might turn out that the Great Silence we’re experiencing is like that of a child’s nursery, wherein adults speak softly, lest they disturb the infant’s extravagant and colorful time of dreaming.” The Great Silence