The attempt to detect artificial transmissions from other civilizations in space. The first such attempt, called Project Ozma, was made in 1960 by the American radio astronomer Frank Donald Drake (1930– ), who observed two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, with the 26-m dish at the National Radio Astronomy Observatory, West Virginia. Since then many more searches have been made with increasing sensitivity and greater coverage.
Most searches have concentrated on that part of the radio spectrum where the background noise is a minimum, between the radio spectral lines of hydrogen (H) at 21 cm wavelength and hydroxyl (OH) at 18 cm. Since H and OH together make H2O, this region of the radio spectrum is termed the water hole. Radio signals deliberately sent to attract attention over interstellar distances are expected to be of narrow bandwidth, 1 hertz or less, and specialized receivers have been developed to detect them. Another possible mode of communication is by brief (nanosecond) pulses of laser light which could outshine their parent star by 10 000 times.
SETI can succeed only if there are civilizations elsewhere in the Galaxy transmitting towards us. The Drake equation attempts to estimate how many such communicative civilizations there might be. Early estimates were optimistic, suggesting that perhaps tens of thousands of them existed in the Galaxy. Most present-day estimates are more conservative, no more than a few hundred, and some astronomers suggest that we might even be the only advanced civilization in the Galaxy at present.
http://www.seti.org/ Official SETI Institute website.