SETI @ Home, revisited

Every day, I help search for extraterrestrial intelligence…or, at least, my computer does.

It’s one of more than 4,287,000 computers worldwide called SETI@Home (SETI stands for Search for Extraterrestrial Intelligence) which constantly examine data collected by the huge Arecibo radio telescope in Puerto Rico for signals that could have come from extraterrestrial civilizations.

Any signals we might pick up from distant civilizations are likely to be so weak by the time they reach us that they could easily be drowned out by both the natural background noise of the universe and the electromagnetic noise our own civilization constantly pumps out. Separating the wheat of an extraterrestrial signal from the chaff of all that other stuff requires enormous computer power. In 1994 David Gedye and Craig Kasnoff, two computer scientists from Seattle, realized that, rather than assign the task to a single large computer, you could instead distribute the work to thousands of smaller computers.

With the help of a founding contribution from The Planetary Society, SETI@Home went online in May of 1999 with a freely downloadable program that kicks in whenever a computer is idle and examines radio telescope data sent to it over the Internet.

Founders anticipated tens of thousands, or several hundreds of thousands, of users at the most; instead, they got millions. The SETI@Home network is effectively the world’s largest supercomputer, sending out over 600,000 work units every day from its servers at the University of California in Berkeley and receiving about the same number of processed data packets back.

Each work unit represents about 107 seconds of observation of a band of frequencies approximately 10 kilohertz wide, taken from a total frequency band 2.5 megahertz wide and centered 1420 megahertz. (That frequency is judged to be a likely one for extraterrestrials to use because it’s the frequency emitted by hydrogen, the most common element in the universe, it coincides with a quiet section of the electromagnetic spectrum, and it lies close to the frequency of water, the fundamental component of life as we know it.)

The SETI@Home software scans the work unit for very narrow-band signals, which are most likely to be artificial (static is a wide-band signal; beeps or continuous tones are narrow-band signals). It also looks for signals that appear, grow stronger and then fade away over12 seconds, which is how long the Arecibo dish is sensitive to any given point in space. It also looks for pulsed patterns, especially three equally spaced pulses, called triplets.

Once the work unit has been examined, the results are sent back to Berkeley. All interesting narrow-band signals, signals that fit the 12-second profile, triplets and pulses are automatically flagged and saved for further analysis and review.

Since 1999, SETI@Home users have devoted more than a million years of computing to the project; my computer alone has put in around 19 1/2 months of computing time.

Out of that work has come a list of 166 promising signals judged worthy of re-examination by the SETI@Home team. Three members of the team, astronomer Dan Werthimer (the team leader), astrophysicist Eric Korpela and physics graduate student Paul Demorest, traveled to Puerto Rico a few days ago to aim the Arecibo telescope back at the spots in the sky where the signals originated. They also pointed the telescope at a few other promising locations, bringing the total number of sites examined to 227.

They didn’t find ET, but they were quick to point out that they could conduct only a rough analysis in real time; the final results of the re-observations will have to wait a couple of months until the data has been analyzed by the SETI@Home network.

Of course, just because we don’t hear extraterrestrials this time around doesn’t mean they’re not there. They could be hiding, using unknown technology, using a different frequency than the ones we’re searching, sending a signal only intermittently, or simply not interested.

Meanwhile, our own radio signals are now reaching thousands of nearby stars. Most TV and radio broadcasts are too weak to be easily detected–aliens likely aren’t watching Milton Berle–but other signals, such as those produced by certain types of military radar, are strong enough to stand out clearly against the universe’s background noise.

If alien computers are running their own version of SETI@Home, some extraterrestrial scientist could be shouting “Eureka!” any day down.

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