Finding many planets in orbit around other stars has provoked new interest in the long-running debate about the existence of extraterrestrial intelligence. Now we are asked to think about what rational, practical steps we can take to prepare for discovering evidence of sapient aliens, an event whose timing and exact nature are unpredictable. Our preparations must be based on probabilities, analogies, and disciplined speculations rather than on confirmed evidence.

Many of those interested in this debate expect that what they see as the final blow to anthropocentrism – discovering extraterrestrial intelligence – is just a matter of time and effort. If that discovery occurs, it might lead to a change in the way we see our status and our position in the universe. Such a discovery might not be the kind of paradigm break that Thomas Kuhn discussed in his famous book, The Structure of Scientific Revolutions (Kuhn 1970). Kuhn was writing about breaking scientific paradigms: replacing one set of physical laws with another. The new paradigms were not just different; they were better. The anticipated discovery of extraterrestrial intelligence is part of a different kind of paradigm break. Contact with an alien civilization would involve much more than science, raising philosophical and societal questions where laws are less certain or non-existent.

Evidence, scenarios, choices

Standards of evidence

A century ago, in the wake of Percival Lowell and the controversy over the canals of Mars, many humans believed that the red planet was inhabited by an alien civilization (Crowe 1986). Subsequent observations discredited that idea; the theory failed the empirical test. Beginning in 1947, thousands of people reported seeing exotic craft in our atmosphere, even on the Earth's surface (Jacobs 1975; Peebles 1994). While large numbers of our fellow humans believed that those objects were visitors from other worlds, most analysts concluded that nearly all UFOs actually were IFOs – identifiable flying objects. As astronomer Seth Shostak pointed out, there still are no artifacts to examine (Shostak 1998, 135). Others have suggested that the sightings most difficult to explain might be unknown phenomena that are unrelated to alien visitors. Again, the empirical standard has not been met.