Of course, we are still in the early days of SETI, and the lack of success to date cannot be used to infer that ET civilizations do not exist. The searches have so far covered only a small fraction of the total "parameter space"--that is, the combination of target stars, radio frequencies, power levels and temporal coverage that observers must scan before drawing a definitive conclusion. Nevertheless, initial results are already beginning to place some interesting limits on the prevalence of radio-transmitting civilizations in the galaxy [see box on next page].

The Fermi Paradox becomes evident when one examines some of the assumptions underlying SETI, especially the total number of galactic civilizations, both extant and extinct, that it implicitly assumes. One of the current leaders of the field, Paul Horowitz of Harvard University, has stated that he expects at least one radio-transmitting civilization to reside within 1,000 light-years of the sun, a volume of space that contains roughly a million solar-type stars. If so, something like 1,000 civilizations should inhabit the galaxy as a whole.

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