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267 With prodigious energy resources, Sagan speculated, technologically advanced civilizations should be able to rework the cosmos. Unexplained or “unnatural” events such as bizarre energy sources or mysterious cavities in the interstellar medium might have explanations consistent with our present science—or they might not. As long as we cannot understand these phenomena, we cannot exclude the possibility that they are manifestations of extraterrestrial intelligence.*! We should keep a sharp eye out for anomalous order of any kind, Grin- spoon proposed. This could include nonequilibrium mixtures of gases (or, conversely, too much equilibrium in places where other known processes are creating disequilibrium), strange mechanical shapes and assemblages, or rhythmic environmental changes without any obvious cause. White sug- gested looking for “entropy pools”—areas of entropy reduction surrounded by regions of increased entropy.” If we find ordered structures without a known natural cause, can we be ahoaat toa atau an . sure that they indicate extraterrestrial intelligence? Even very smart scien- tists have misinterpreted evidence; Kepler thought that lunar craters were cities in circular form. It is difficult to judge how best to search for intelligent life when we cannot even be sure what the dominant form of intelligence on Earth will be. “What prospect could we have,” Rees asked, “of envisaging what might be spawned from another biosphere with a billion-year head start on us?” We probably do see evidence of alien activity, Gindilis and Rudnidski proposed, but we are unaware of it. Thomas Kuiper predicted that detec- tion will be the result of an accumulation of phenomena that are hard to explain.™ Mind-Stretcher. What we will observe will be their star wars, argued space expert James Oberg. Man’s greatest efforts have been military; the same may be true elsewhere in the Galaxy.** We might detect the energies released by colossal battles among the Galaxy’s titans, the Gods of War. We may have seen their signs already, interpreting them to be astronomi- cal phenomena. The Great Silence may say more about our own limitations in concep- tualizing intelligence and its works than the ability of the universe to produce them, suggested Grinspoon. Clarke told us many years ago that any sufficiently advanced technology would be indistinguishable from magic.*° The only type of intelligence we could detect would be one that employed a technology that we can recognize; that might be a minor and atypical fraction of all extraterrestrial intelligence.*’ This leads us toward a limiting concept: Intelligent beings may only recognize the signs of other intelli- gences near their own technological level. We Will Recognize Their Signs