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176 Brin’s analysis had diverging implications, noted Brian McConnell. On the one hand, the probability of contact may be reduced because some civilizations capable of interstellar communication choose to keep quiet. On the other hand, the probability of contact may rise because some civi- lizations choose to expand. Even if the emergence of technological civiliza- tions is rare, the probability of contact may be high. So why don’t we see them? To McConnell, the Deadly Probe hypothesis was the answer that required the fewest arbitrary assumptions.” O’Neill proposed that we may be alone in this Galaxy as a material- oriented civilization, in contrast to more spiritual orientations. Material orientation may be a brief episode in the evolution of a society, perhaps one that every civilization goes through quickly. Civilizations may evolve to an end state in which intelligent beings are not tied to matter at all; they might have no interest in the physical universe.“ A spiritual orientation might be a path toward eventual extinction. Sta- pledon had suggested the possible implications many years ago: In not a few worlds this way of the spirit was thronged by all the most vital minds. And because the best attention of the race was given wholly to the inner life, mate- rial and social advancement was checked. The sciences of physical nature and life never developed. Mechanical power remained unknown, and medical and biologi- cal power also. Consequently those worlds stagnated, and sooner or later suc- cumbed to accidents which might well have been prevented.’ We might extend Grinspoon’s argument that natural selection can act on a much larger scale than we’re used to thinking about.*° Although the uni- verse as a whole may be hospitable for the evolution of life and intelligence, it may be inimical to their long-term survival at any particular location. Our own time horizon may be much shorter than the billion years between now and an Earth made uninhabitable by a swelling Sun. Other terminators—asteroids, rogue planets, dark stars, gamma-ray bursters, black holes—could end the human adventure. “It is just possible,” con- cluded Cirkovic and Cathcart, “that the total risk function facing civiliza- tions is high enough to explain the total absence of their manifestations.”“° An Optimistic Transition James Annis of the Fermi National Accelerator Laboratory proposed in 1999 that the key terminators of intelligence are gamma-ray bursters. Each one is a mass extinction event on a galactic scale. Bursters were much more common in the past and may have wiped out many evolu- tions to intelligent life. If the timescale for land animals to develop intelligence was long compared to the time between bursts, intelligence could not emerge. Why Don’t We See Them? Other Theories