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167 species could colonize the entire Galaxy within a time far shorter than the Galaxy’s age. Eric Jones of the Los Alamos Scientific Laboratory came up with one of the lowest estimates of the time required: about 5 million years. As we see no evidence of such colonization, he concluded that no other technological, space-faring civilization has arisen in our Galaxy."* Jones argued later that descendants of such a civilization should be found near virtually every useful star in a time much less than the current age of the galaxy. Only extreme assumptions about local population growth rates, emigration rates, or ship ranges can slow or halt an expansion. If interstellar travel is practical at a few percent of light speed, it is virtually certain that our solar system would have been settled by non-natives long ago. Unless we discover that interstellar travel is impractical, we are prob- ably alone in the Galaxy. Jones conceded one key point. If intelligent beings can live in interstellar space and need not be clustered around stars, estimates of settlement times may be meaningless; the absence of obvious signs of settlements in our solar system would not be significant.!* Astronomers Thomas Kuiper and Mark Morris picked up the argument in 1977. If interstellar travel and colonization are possible, there are two possible outcomes: Technological civilizations that last long enough to begin the colonization process are rare, in which case the Galaxy is essen- tially unpopulated, or there are several such civilizations, in which case the galaxy is fully explored or colonized. They offered three possible explana- tions for the lack of contact: The Earth has been a preserve for a long time; alien technology has advanced beyond the stage where a planetary base is needed; Earth’s biology is incompatible with or even hostile to that of the species that dominate our part of the Galaxy.’ David Schwartzman proposed that although colonization is not a prob- able extraterrestrial strategy, surveillance and eventual contact are. The present surveillance of Earth by extraterrestrials may be the best reconcili- ation of those who are optimistic and those who are pessimistic about the number of alien civilizations.”” Hart and astronomer Ben Zuckerman organized a conference at the University of Maryland in 1979 to look at the Where Are They question, subtitling the meeting “A symposium on the implications of our failure to observe extraterrestrials.” Although the papers presented a variety of views, there was widespread agreement that interstellar travel is feasible. Physicist Stephen Webb later commented that it was difficult to read the proceedings of that conference without concluding that extraterrestrial civilizations have the means, motive, and opportunity to colonize the Galaxy. Yet, Zuckerman found that, to astronomers who work with optical and radio telescopes, the universe appears to be a gigantic wilderness area untouched by the hand of intelligence (with the possible exception of God’s).'* If They Could Expand, They Must Not Exist