Extraterrestrial intelligence

Extraterrestrial intelligence is intelligent life originating beyond Earth, whether or not it has been detected or communicated with humanity.

Drake equation: A framework for estimating the number of communicating civilizations in the Milky Way by multiplying factors governing their emergence and detectability. It makes the unknown factors behind the possible number of civilizations explicit.
Search for extraterrestrial intelligence: Scientific efforts to detect evidence of technological civilizations beyond Earth, especially through electromagnetic signals or artifacts. It is the organized search most directly aimed at detecting intelligent life.
Intelligence: The capacity to learn, reason, solve problems, and adapt behavior to achieve goals. Any claim of extraterrestrial intelligence depends on how intelligence is defined.
Rare Earth hypothesis: The hypothesis that complex life is uncommon because its emergence requires an unusual combination of planetary and astronomical conditions. It contrasts with expectations that habitable worlds often produce intelligent civilizations.
Contact (1997 film): A 1997 science-fiction film, based on Carl Sagan's novel, about scientists detecting and interpreting an extraterrestrial message. It dramatizes the scientific and personal consequences of a confirmed signal.
Fermi paradox: The tension between the apparent abundance of opportunities for extraterrestrial civilizations and the absence of confirmed evidence for them. It frames the central puzzle posed by a silent sky.
Technosignature: An observable effect that could reveal extraterrestrial technology, such as a signal, atmospheric pollutant, or unusual waste heat. It broadens detection beyond deliberate radio messages.
Origin of life: The transition from nonliving chemistry to the first forms of life. Intelligence requires life to originate before it can evolve.
Mediocrity principle: The assumption that Earth's situation is not unusual unless evidence indicates otherwise. It motivates estimates that Earth's life and intelligence may not be exceptional.
Pale Blue Dot: A photograph of Earth taken by Voyager 1 in 1990 from roughly six billion kilometers away. Its view of Earth as a tiny world informs reflections on humanity's cosmic significance.

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