Optical SETI is a particular branch of the search that involves setting up ground-based telescopes to look for very short pulses of light that are hitting our planet.

"This is perhaps the most sensitive optical SETI search yet undertaken," said Frank Drake, Chairman of the Board of the SETI Institute and a co-investigator on the new experiment.

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," noted Drake. "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."

While optical SETI has been undertaken before, it is only recently that major experiments, scrutinizing hundreds or even thousands of star systems, have been initiated.

This is largely the consequence of a study conducted by the SETI Institute during the years 1997 - 1999 which showed that new technology has made optical SETI an appealing approach for finding technologically sophisticated civilizations. However, unlike its radio counterpart, optical SETI requires that any extraterrestrial civilization be deliberately signaling precisely in our direction.

Why would aliens, if they exist, send laser signals? For one, lasers can carry immense amounts of information, meaning that aliens interested in communicating with other advanced civilizations might build lasers for the task. Powerful lasers can also be transmitted with relatively inexpensive equipment and can travel from one planet to another -- across the galaxy -- without much interference.

"Lasers have the advantage that they can send a lot of information," Berkeley researcher Dan Werthimer, who heads up Berkeley's alien-search effort told SPACE.com last year. "You could send your whole Library of Congress, all your music, poetry, literature, science, medicine."

The new experiment is unique in exploiting three light detectors (photomultipliers) to search for bright pulses that arrive in a short period of time (less than a billionth of a second). Of course, light from the central star will trigger the detectors as well, but seldom will all three photomultipliers be hit by photons within a billionth of a second time frame. The expected number of false alarms for the stars being looked at is about one per year.

Other optical SETI experiments use only one or two detectors and have been plagued by false alarms occurring on a daily basis. Starlight, cosmic rays, muon showers, and radioactive decays in the glass of photomultiplier tubes can all contribute confusing "events" to optical SETI searches.

Dan Werthimer and Richard Treffers of UC Berkeley designed the hardware and software for the new, three-tube system. It was built by Shelley Wright, an undergraduate physics student at UC Santa Cruz, under the direction of principal investigator Remington Stone, a research astronomer at Lick Observatory. The astronomers expect that the new approach will produce a clean experiment that can be run automatically, and for which the results will be far less ambiguous.

So far, the experiment at Lick Observatory has examined about 300 individual star systems, as well as a few star clusters. The intention is to continue the search at least on a weekly basis for the coming year. The project is being sponsored by the SETI Institute.

"One great advantage of optical SETI is that there's no terrestrial interference," comments Drake. "It's an exciting new field."