Extraterrestrial intelligence

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Extraterrestrial intelligence (often abbreviated ETI) refers to hypothetical intelligent extraterrestrial life.

The question of extraterrestrial intelligence and the existential question "Are we alone in the universe?" is one of the oldest and most popular in science and is a popular theme in science fiction.[1] The basic assumption behind the existence of extraterrestrial intelligence is inferred from the existence of human intelligence and the size of the known universe.


The Copernican principle is generalized to the relativistic concept that humans are not privileged observers of the universe.[2] Many prominent scientists, including Stephen Hawking[3] have proposed that the sheer scale of the universe makes it improbable for intelligent life not to have emerged elsewhere. However Fermi's Paradox highlights the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilization and humanity's lack of contact with, or evidence for, such civilizations.[4]

The Kardashev scale is a method of measuring a civilization's level of technological advancement, based on the amount of energy a civilization is able to utilize.[5]

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.[6]


There has been a search for signals from extraterrestrial intelligence for several decades (SETI), with no solid results.[7]

The U.S. government's position is that "chances of contact with an extraterrestrial intelligence are extremely small, given the distances involved."[8][9]


The extraterrestrial hypothesis is the idea that some UFOs are vehicles containing or sent by extraterrestrial beings (usually called aliens in this context).[7] As an explanation for UFOs, ETI is sometimes contrasted with EDI (extradimensional intelligence), for example by Allen Hynek.[10]


  1. ^ Are we alone? Peter Spinks. May 21, 2013.
  2. ^ Peacock, John A. (1998). Cosmological Physics. Cambridge University Press. p. 66. ISBN 0-521-42270-1. 
  3. ^ Hickman, Leo (25 April 2010). "Stephen Hawking takes a hard line on aliens". The Guardian. Retrieved 24 February 2012. 
  4. ^ Krauthammer, Charles (December 29, 2011). "Are we alone in the universe?". The Washington Post. Retrieved January 6, 2015. 
  5. ^ Kardashev, Nikolai. "On the Inevitability and the Possible Structures of Supercivilizations", The search for extraterrestrial life: Recent developments; Proceedings of the Symposium, Boston, MA, June 18–21, 1984 (A86-38126 17-88). Dordrecht, D. Reidel Publishing Co., 1985, p. 497–504.
  6. ^ Zaun, Harald (1 November 2011). "Es war wie eine 180-Grad-Wende von diesem peinlichen Geheimnis!" [It was like a 180 degree turn from this embarrassing secret]. Telepolis (in German) (Heise). 
  7. ^ a b "The search for ET is a detective story without a body" by Nigel Henbest, New Scientist, March 9, 2013, p. 53.
  8. ^ Larson, Phil (5 November 2011). "Searching for ET, But No Evidence Yet". White House. Retrieved 2011-11-06. 
  9. ^ Atkinson, Nancy (5 November 2011). "No Alien Visits or UFO Coverups, White House Says". UniverseToday. Retrieved 2011-11-06. 
  10. ^ Fuller, Curtis (1980). Proceedings of the First International UFO Congress. New York: Warner Books. , pp. 157-63.