||The first modern, scientifically based search for extraterrestrial intelligent radio signals (SETI) was conducted at Green Bank in 1960. The Tatel 85 foot telescope was used, along with a custom-made receiver system based on an experimental Microwave Associates parametric amplifier. The receiver had a system noise temperature of about 350 K, a bandwidth of 100 Hz, and operated at the 21 cm line frequency. A dual feed system was used on the telescope with the receiver switching via a Dicke switch between the two feeds so as to allow the identification of RFI and to provide an easy test for extraterrestrial origin of any candidate extraterrestrial signals. Two stars were searched for signals with negative results. However, the experiment stimulated much interest in hypothetical extraterrestrial life in both the scientific world and the general public. A large number of similar experiments, using ever more powerful instrumentation, have followed this first effort, so far with negative results. The experiment played an important role, along with other pioneering experiments, in motivating research in the subject of extraterrestrial life, a subject which has become the major active scientific field of astrobiology.
Discoveries of recent years, particularly of many other planetary systems, and possible surprising potential habitats in such places as Mars and Europa, have made much more plausible the existence of widespread life, including intelligent life, in the universe. At the same time, these and other discoveries have made SETI more challenging. No longer are single G stars the prime targets of search. F, K, and M stars and binary stars must also be considered potential abodes of life. The ultimate instrument for SETI is an all-sky, all-frequency, all-the-time sensitive radio telescope. This will deal with the transient signal challenge. Although the basic technology for such an instrument exists, the present cost is so high, particularly because of the enormous required computer capability that is probably decades away. It has become plausible that optical signals, particularly very brief laser pulses, might be a detectable extraterrestrial technology. It has been recognized that SETI signals could well be transient and detectable only a small fraction of the time. To deal with these possibilities, sensitive optical searches are now underway for nanosecond-time-scale optical signals. The Allen Telescope Array, an array of—eventually—350 six-meter antennas, is far along in its construction and will provide the ability to conduct continuous multi-beam SETI searches.