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188 time, if we have tried hard enough to find them, then shouldn’t we see evi- dence of extraterrestrial life?* The Probability of a Paradox Any assumption must have a probability of being correct. Stuart Clark described how astronomer Jean Heidmann developed this idea. If a conclusion is based on two assumptions that are each 90% certain, that conclusion is only 81% correct (0.9 x 0.9 = 0.81). Heidmann identified 112 assumptions in Barrow and Tipler’s interstellar expansion argument that lead to their conclusion that extraterrestrials do not exist. If each of these assumptions is 90% correct, then their conclusion is 0.9 times itself 112 times. This means that Barrow and Tipler’s conclusion is only 0.0007% certain. Jill Tarter challenged the whole logical construct, because it requires that we take as a fact that extraterrestrials are not here. We can’t say for sure that there isn’t some long, slow spacecraft orbiting the asteroids and chopping up raw materials.‘ We have not explored our neighborhood thoroughly enough to rule out the presence of extraterrestrials or their works. As the chances of finding a small alien artifact by accident are almost zero unless it draws attention to itself, we cannot exclude the possibility of a presence in our solar system. To jump to the conclusion that they are not there simply because we don’t see them easily is to make the same mistake people made about microscopic life, argued McDonough. Until Leeuwenhoek invented the microscope, it was thought that nothing smaller than an insect or a mite lived. The world was crawling with zillions of microscopic beasties, but because nobody had seen them, they did not exist.° “Some would argue,” Webb acknowledged, “that until we can rule out that possibility, there is no Fermi paradox.” The implication is clear: SETI should include searches of our solar system to test the probe and coloniza- tion hypotheses.° The only way we can be sure that there are no signals is by conducting an all-sky, all-frequency, all the time search—and that would answer the question only for that slice of time when the search was active. The only way we can be sure that there are no alien artifacts in our solar system is by a thorough search of the Sun’s empire, with resolution fine enough to pick up the remains of small interstellar probes. Russian astronomers L.M. Gindilis and G.M. Rudnitskii concluded that there is no paradox at all. Although the idea of this “astrosociological” paradox was useful in stimulating discussions and active searches for an answer, we should acknowledge the degree of our ignorance and moderate Reformulating the Problem