SETI searches for extraterrestrial intelligent life using radio telescopes, with other methods proposed. A discovery would have profound implications, while a negative result suggests we are alone. Detecting signals is challenging, but man-made and natural sources can be ruled out. Alternative communication methods and physical probes may be necessary.
SETI, or Search for Extra-Terrestrial Intelligence, is a project that aims to detect extraterrestrial intelligent life. SETI primarily uses radio telescopes to search for extraterrestrial signals, although optical telescopes and other methods have been proposed. Any discovery of intelligent alien life by SETI would have profound implications for the future of humanity; a negative result would imply that we are alone in the universe and that the rise of intelligent life was a rare and random event.
The sky, as a whole, is not a strong radio emitter; terrestrial sources of radio waves are already much stronger than anything observed with radio telescopes. Any civilization that discovered radio waves would have to emit a constant stream of information into space, just as we did with radio and television broadcasters. Detecting these signals, if they exist, is a huge challenge given all the potential sources of noise.
Many SETI researchers expect man-made signals to use a narrow range of frequencies, and any signals from outside the solar system should remain in one spot in the sky; these criteria help rule out man-made radio sources and natural sources such as pulsars. SETI radio astronomers tried using huge supercomputers to scan these signals into telescope data, and the task was eventually entrusted to a distributed computing network, SETI@HOME. However, SETI has yet to announce the discovery of signals believed to come from alien life.
Some have theorized that an alien civilization would not use radio communications at all, after discovering a superior alternative. Optical astronomers have searched the sky for any artificial laser beams in the visible range, but nothing has come up so far. Other more exotic possibilities include communication via gravitational waves, neutrino explosions or some unknown quantum entanglement phenomenon. If communication is impossible, a SETI project may have to send physical interstellar probes to discover intelligent life, which would be even more difficult.
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