The universe seems determined to maintain radio silence, so SETI astronomers involved in the search for extraterrestrial intelligence are broadening their hunt and looking for light signals directed at Earth. This method has been tried before but previous experiments have been plagued by false alarms.

?This is perhaps the most sensitive optical Seti search yet undertaken,? says Frank Drake, chairman of the board of the privately funded, non-profit Seti Institute. Drake, who in 1960 conducted the first modern hunt for evidence of extraterrestrial intelligence, is usually associated with radio Seti, an approach in which large antennas are connected to specialized, multi-million channel receivers. ?This is different,? he says. ?We are looking for very brief but powerful pulses of laser light from other planetary systems, rather than the steady whine of a radio transmitter.? They have already examined about 300 individual star systems, as well as a few star clusters.

Light detection has the disadvantage of only being able to detect a signal deliberately targeted at Earth, but it is less susceptible than radio to terrestrial interference. The new system uses a trio of light detectors and was designed and built by researchers at the University of California. Other systems have used fewer detectors and have consequently suffered from frequent false alarms.

Another optical Seti project, a collaboration between Princeton and Harvard Universities, is working on installing a light detector, while the optical Seti project at Columbus, Ohio, already operates such a system.

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