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Searching for Intelligence I have no doubt that there are many other inhabited worlds, and that on some of them beings exist who are immeasurably beyond our mental level. We would be rash to deny that they can use radiation so penetrating as to convey messages to the Earth. Probably such messages now come. When they are first made intelligible a new era in the history of humanity will begin. The means for contact with extraterrestrial intelligence are potentially within our hands. 5 What a noble endeavor SETI is! A bountiful cosmos, advanced technology, and dedicated researchers: everything is in place. —Andrew J.H. Clark and David H. Clark, 1999° Searching our solar system through optical telescopes failed to provide persuasive evidence of extraterrestrial intelligence. To pursue the question further, searchers broadened the quest with new technologies and extended it outward to the stars. This expanded enormously the volume to be searched. What evidence can we look for at interstellar distances? Although intel- ligent life is likely to be much less common than nonintelligent life— perhaps orders of magnitude less common—it can be much easier to detect. We can observe the consequences of its actions, meaning its use of bank ate technologies. Asimov proposed a definition of intelligence within the context of such a search: A species is intelligent if it can develop a complex technology. This definition makes it unnecessary to delve into psychology and philoso- phy. Instead of trying to divine the inner being and its inner thoughts, one 33 —Bishop Barnes of Birmingham, 1931! —Carl Sagan, 1973 Radio Days