If aliens are likely, why haven't we met them?
Enrico Fermi may never have said it the way it is quoted, and the physicists who were at that lunch remembered it differently. The question outlived the argument about who asked it.
▶ Start the storyBecause a simple argument says they should be everywhere, and nothing we observe matches it. The Fermi paradox is the seeming inconsistency between the lack of evidence of extraterrestrial civilizations and the apparently high likelihood of their existence. If interstellar travel is possible, even the "slow" kind near the reach of Earth technology, it would take only 5 million to 50 million years to colonize the galaxy, which is brief on a geological scale, let alone a cosmological one, and many stars are older than the Sun.
5–50 million years
The name comes from a lunch at Los Alamos in 1950, where Enrico Fermi, talking with Emil Konopinski, Edward Teller and Herbert York, blurted out a question that Konopinski remembered as "But where is everybody?" According to Teller, everyone laughed because they understood at once that he meant extraterrestrial life. Fermi never published it, though, and Robert Gray argues that the real discussion was about whether interstellar travel is feasible, not about aliens at all.
The answers fall into three classes: few intelligent civilizations ever arise, civilizations exist but we see no evidence, or their lifetimes are short. A book published in 2015 elaborated on 75 different explanations.
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Recap
The paradox pairs a short colonization clock with a silent sky, and its solutions are sorted by whether civilizations are rare, hidden or short-lived.
💡 A trick to remember it · Rare, Hidden, Short: three words to sort 75 ideas into three drawers.
Surprising fact · Colonizing the galaxy would take only 5 to 50 million years, even at slow speeds.
Sources (2)
No source, no claim. Every fact in this lesson (23 claims) cites at least one of these.