Extraterrestrial intelligence refers to hypothetical intelligentextraterrestrial life. 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. In June 2020, astronomers from the University of Nottingham reported the possible existence of over 30 "active communicating intelligent civilizations", or Communicating Extra-Terrestrial Intelligent civilizations in our own Milky Way galaxy, based on the latest astrophysical information.
There has been a search for signals from extraterrestrial intelligence for several decades, with no solid results. Active SETI 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 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." The Wow! signal remains the best candidate for an extraterrestrial radio signal ever detected.
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, an historical precedent being the Columbian Exchange. Such meetings have generally led to the destruction of the civilization receiving 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.
