A paradox is an apparently contradictory statement that may nevertheless be true. According to the lore of physics, the famous Fermi paradox arose one evening in 1943 during a social gathering at Los Alamos, New Mexico, when the brilliant Italian-American physicist and Nobel laureate Enrico Fermi (1901-54) asked the penetrating question: "Where are they?" "Where are who?" his startled companions replied. "Why, the extraterrestrials," responded the Nobel prize-winning physicist. At the time, Fermi and the other scientists in the group were involved in the top-secret Manhattan Project—the American effort to build the world's first atomic bomb.
Fermi's line of reasoning that led to this famous paradox has helped to form the basis of much modern thinking with respect to SETI. It can be summarized as follows: The Milky Way galaxy is some 13 to 15 billion (109) years old and contains perhaps 100 to 200 billion stars. If just one advanced civilization had arisen in this period of time and attained the technology necessary to travel between the stars, that advanced civilization could have diffused through, or traveled across, the entire galaxy within 50 million to 100 million years—leaping from star to star, starting up other civilizations, and spreading intelligent life everywhere. The Italian-American physicist and Nobel laureate Enrico Fermi
But as scientists on Earth search the (1901-54). (The U.S. Department of Energy/Argonne National heavens, they do not see a galaxy teeming Laboratory)
with intelligent life, nor do they have any technically credible evidence of visitations or contact with alien civilizations, so they logically conclude that probably no such technically superior extraterrestrial civilization has ever arisen in the approximately 15-billion-year history of the galaxy. Here arises the great paradox: Although scientists might expect to see signs of a universe filled with intelligent life (on the basis of statistics and the number of possible "life sites"—given the existence of 100 billion to 200 billion stars in just this galaxy alone), they have collected no credible evidence of such. Are humans, then, really alone? If, on the other hand, we are not alone—where are they?
Many attempts have been made to respond to this very profound question. The "pessimists" reply that the reason human beings have not seen any signs of intelligent extraterrestrial civilizations is because humans really are alone in the galaxy and perhaps in the entire universe. One alternative sometimes suggested is that the human race is actually the galaxy's first technically advanced beings to rise to the level of space travel. If so, then perhaps it is humans' cosmic destiny to be the first species to sweep through the galaxy spreading intelligent life.
The "optimists," on the other hand, hypothesize that intelligent life exists out there somewhere and offer a variety of possible reasons why scientists on Earth have not yet "seen" signs of these alien civilizations. This section of the book discusses just a few of the many suggested reasons. First, perhaps intelligent alien civilizations really do not want anything to do with the human race. As an emerging planetary civilization, humans may be perceived as just too belligerent or too intellectually backward to bother with. Another possibility is that the communications technologies available in humans' planetary civilization simply lie below the minimum communications horizon of advanced alien civilizations. Other optimists suggest that not every intelligent civilization has the desire
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