Fermi paradox

The apparent mismatch between estimates that extraterrestrial civilizations should be common and the lack of evidence that any have been detected or contacted humanity.

Drake equation: A formula that estimates the number of communicating extraterrestrial civilizations in the Milky Way from several uncertain factors. It makes explicit the assumptions behind expectations that detectable civilizations should exist.
SETI: The scientific search for signals or other evidence of extraterrestrial intelligence. It tests whether technological civilizations have produced detectable signals.
Zoo hypothesis: The speculation that extraterrestrial civilizations deliberately avoid contact with Earth, perhaps to observe humanity without interference. It explains silence through intentional noninterference rather than absence.
Enrico Fermi: An Italian-American physicist who made foundational contributions to nuclear physics and the first controlled nuclear chain reaction. His lunchtime question about extraterrestrials gave the paradox its name.
Abiogenesis: The process by which living organisms arise from nonliving matter. Its unknown probability leaves the frequency of life’s beginnings uncertain.
Rare Earth hypothesis: The hypothesis that complex life is uncommon because it requires an unusually favorable combination of planetary and astronomical conditions. It resolves the paradox by arguing that technological civilizations are exceptionally rare.
Breakthrough Listen: A large SETI program that searches for artificial radio and optical signals from nearby stars and galaxies. Its observations illustrate how modern searches investigate the paradox.
Dark forest hypothesis: The hypothesis that civilizations remain silent because revealing their locations could invite destruction by hostile civilizations. It treats silence as a rational response to danger.
Los Alamos National Laboratory: A United States research laboratory established during the Manhattan Project in New Mexico. Fermi’s famous question is associated with a conversation among colleagues at Los Alamos.
Evolution of human intelligence: The biological and cultural processes that produced human cognitive abilities and complex societies. Earth offers only one known path from life to technological intelligence.

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