In exobiology, the widely used acronym SETI means the search for extraterrestrial intelligence (SETI). The major goal of most publicly and privately funded SETI programs has been to listen for evidence of radio frequency (microwave) signals that are generated by intelligent extraterrestrial civilizations. A less frequently encountered companion acronym in the field of exobiology is CETI, which means communication with extraterrestrial intelligence. While the two terms appear comparable, they involve distinctly different approaches to interstellar contact and, therefore, could result in significantly different social consequences on a planetary scale. (See also chapter 11.)
SETI represents a more conservative, passive scientific approach to interstellar contact in which scientists on Earth patiently look for signs or listen for signals that may be indicative of the presence of intelligent alien beings elsewhere in the Milky Way galaxy. If a nonnatural, artifact signal is ever successfully received and decoded as part of some contemporary or future SETI effort, the people of Earth would be under no obligation to respond, and the sender of this signal would (in all likelihood) have no way of knowing that the message was intercepted by intelligent beings on a life-bearing planet called Earth that orbits around a main-sequence yellow star called Sol.
CETI involves a less conservative, more active technical approach to achieving interstellar contact. In the practice of CETI, terrestrial scientists intentionally send out specially prepared radio frequency signals or information-laden artifacts into the galaxy—announcing the presence of the human race to whomever or whatever out there is capable of receiving and interpreting these active attempts at interstellar communication. (See chapter 9.) In its most comprehensive form, CETI also implies that the people of Earth would respond to an alien signal, should one ever be received and successfully decoded.
The large radio telescope is an important tool of modern astronomy that under certain operational conditions also supports both CETI and SETI activities. When used as a powerful radio frequency transmitter to beam a specially prepared signal to some interesting portion of the galaxy, the radio telescope becomes an important tool in the practice of CETI. When used as a very sensitive receiver to search for and collect faint "artifact" radio signals as may emerge from among the stars, the radio telescope becomes an important tool in the practice of SETI. The task for scientists, then, becomes one of skillfully separating the natural cosmic radio signals of interest to astronomers from any possible artificial (alien-produced artifact) signals of interest to exobiologists.
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