With billions of planets capable of supporting intelligent life, why no visits to earth? This is "The Fermi Paradox." Any civilization, with modest rocket tech & immodest funds, could rapidly colonize the Milky Way...
Extraterrestrial life should be common. Discussing this with colleagues over lunch in 1950, physicist Enrico Fermi asked: "Where are they?" or "Where is everybody?" The paradox is between evidence for its likelihood, and lack of evidence for its existence
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