Extraterrestrial intelligence (often abbreviated ETI) refers to hypothetical intelligent extraterrestrial life. No such life has ever been proven to exist in the Solar System except for humans on Earth, and its existence on other star systems is still speculative. The question of whether other inhabited worlds might exist has been debated since ancient times. The modern form of the concept emerged when the Copernican Revolution demonstrated that the Earth was a planet revolving around the Sun, and other planets were, conversely, other worlds. The question of whether other inhabited planets or moons exist was a natural consequence of this new understanding. It has become one of the most speculative questions in science and is a central theme of science fiction and popular culture.

Intelligence
Intelligence is, along with the more precise concept of sapience, used to describe extraterrestrial life with similar cognitive abilities as humans. Another interchangeable term is sophoncy, first coined by Karen Anderson and published in the 1966 works by her husband Poul Anderson.

Sentience, like consciousness, is a concept sometimes mistakenly used to refer to the concept of extraterrestrial sapience and intelligence, since it does not exclude forms of life that are non-sapient.

The term extraterrestrial civilization frames a more particular case of extraterrestrial intelligence. It is the possible long-term result of intelligent and specifically sapient extraterrestrial life.

Probability
The Copernican principle is generalized to the relativistic concept that humans are not privileged observers of the universe.  Many prominent scientists, including Stephen Hawking 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.

So far, there is no proof of extraterrestrial life, including intelligent extraterrestrial life.

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

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

Search for extraterrestrial intelligence

There has been a search for signals from extraterrestrial intelligence for several decades, with no solid results. Active SETI (Active Search for Extra-Terrestrial Intelligence) is the attempt to send messages to intelligent extraterrestrial life. Active SETI messages are usually sent in the form of radio signals. Physical messages like that of the Pioneer plaque may also be considered an active SETI message.

Communication with extraterrestrial intelligence (CETI) is a branch of the search for extraterrestrial intelligence that focuses on composing and deciphering messages that could theoretically be understood by another technological civilization. The best-known CETI experiment was the 1974 Arecibo message composed by Frank Drake and Carl Sagan. There are multiple independent organizations and individuals engaged in CETI research.

The U.S. government's position is that "chances of contact with an extraterrestrial intelligence are extremely small, given the distances involved." This line of thinking has led some to conclude that first contact will most likely be made with extraterrestrial artificial intelligence, rather than with biological beings.

The Wow! signal remains the best candidate for an extraterrestrial radio signal ever detected.

On 14 June 2022, astronomers, working with China's FAST telescope, reported the possibility of having detected artificial (presumably alien) signals, but cautions that further studies are required to determine if some kind of natural radio interference may be the source. More recently, on 18 June 2022, Dan Werthimer, chief scientist for several SETI-related projects, reportedly noted, “These signals are from radio interference; they are due to radio pollution from earthlings, not from E.T.”.

Potential cultural impact of extraterrestrial contact 

The potential changes from extraterrestrial contact could vary greatly in magnitude and type, based on the extraterrestrial civilization's level of technological advancement, degree of benevolence or malevolence, and level of mutual comprehension between itself and humanity. Some theories suggest that an extraterrestrial civilization could be advanced enough to dispense with biology, living instead inside of advanced computers. The medium through which humanity is contacted, be it electromagnetic radiation, direct physical interaction, extraterrestrial artefact, or otherwise, may also influence the results of contact. Incorporating these factors, various systems have been created to assess the implications of extraterrestrial contact.

The implications of extraterrestrial contact, particularly with a technologically superior civilization, have often been likened to the meeting of two vastly different human cultures on Earth, a historical precedent being the Columbian Exchange. Such meetings have generally led to the destruction of the civilization receiving contact (as opposed to the "contactor", which initiates contact), and therefore destruction of human civilization is a possible outcome. However, the absence of any such contact to date means such conjecture is largely speculative.

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

See also
 Extraterrestrial: The First Sign of Intelligent Life Beyond Earth, 2021 popular science book by Avi Loeb
 Extraterrestrial life
 Rare Earth hypothesis
 Cosmic pluralism
 Fermi Paradox
 Extraterrestrials in fiction
 First contact (science fiction)
 Contact (1997 film)
 Messaging to Extra-Terrestrial Intelligence (METI or Active SETI)
 Sapience

References

External links
 An astrophysicist's view of UFOs (Adam Frank; New York Times; 30 May 2021) 

Extraterrestrial life
Intelligence
Biological hypotheses