The distance between stars is almost unimaginably vast. Travel between them, while not theoretically impossible, appears to be so stupendously difficult, hazardous, time-consuming and energy-expensive as to suggest that even ultimately advanced civilizations may choose to interact almost entirely through information exchange rather than matter transfer. In any event there is no convincing evidence that "they" are, or have ever been, here. At least for our time, serious search for extraterrestrial intelligence (SETI) can only be done through appropriate looking and listening.
Not surprisingly, there are skeptics who oppose such work. Two principal streams of criticism approach SETI from opposite directions. First, if intelligent life is as ubiquitous as extreme SETI proponents would hope, then it is very hard to understand why at least one such life form, over some period in the past billions of years, would not have spread itself or its robot surrogates throughout our galaxy. Alternatively, ...
