BERKELEY-- Since Monday, May 17, more than 200,000 people from more than 100 countries have downloaded the software necessary to let them join the largest search for extraterrestrial intelligence in the world.

"In the last three days we've done a century of computing," said Dan Werthimer, project scientist for SETI@home and a research physicist at the Space Sciences Laboratory at UC Berkeley. "SETI@home is now the world's largest supercomputer."

Monday was the day the SETI@home group made the software available to anyone in the world with a desktop computer and the desire to particpate in one of the most portentious projects today, the search for intelligent life in space. Before that, some 12,000 people had been involved in testing the software, racking up about 200 years' worth of computing time since last November.

"I'm amazed at the extreme eagerness of people to use this," said computer scientist David Anderson, SETI@home project director. Before the software became available Monday, he was getting email every day -- in all languages -- saying, 'Give me the software NOW!'

Many others have volunteered their time to do programming, help build a website and even translate instructions into other languages, Werthimer added.

Software is available at their website for Windows machines, Macintosh computers and 30 varieties of the Unix operating system.

On Windows and Macintosh machines, the software acts like a screensaver -- whenever the computer is idle, the SETI@home software takes over and begins analyzing data in search of strong

spikes or repetitive patterns in radio signals from space. The data come from the large radio telescope at Arecibo in Puerto Rico and are broken into small chunks that are sent to individual computers for analysis.

As the computer works away at the data, the screen displays a three-dimensional graph charting the signal analysis. Participants also can view maps showing where the SETI@home project is searching and who is taking part in the project.

After the computer is finished with the analysis, it sends the results back to UC Berkeley through the internet and grabs another chunk of data.

"This project lets us do SETI a lot, lot faster, with 10 times more sensitivity and exploring more thoroughly the spectrum of radio frequencies we scan," Werthimer said. "Plus, it's a fun and educational project, a global science project."

"Never before has there been an opportunity for anyone, anywhere in the world to join the scientific search for intelligent beings elsewhere in our universe," said Louis Friedman, executive director of the Planetary Society, which provided early seed money and is a principal sponsor for the project. "This is a grand experiment - in science, in technology and in society - a global cooperative effort at the frontiers of knowledge."

The project also is an experiment in "distributed computing" -- a way of breaking down a problem requiring lots of computation into small chunks that can be done by many small computers distributed anywhere in the world. The SETI@home project is the first distributed computing project to offer the general public the opportunity to participate in important research.

"This will be the largest distributed computing project ever," Anderson predicted.

The project was launched three years ago with the Planetary Society (http://planetary.org), in cooperation with Paramount Pictures, providing $100,000 for development of the publicly available software. Sun Microsystems also donated computing equipment and the University of California provided matching funds of $180,000 from its Digital Media Innovation Program (http://www.dimi.ucsb.edu/).

The idea of linking computers in a global network to analyze radio data from space originated with David Gedye, a UC Berkeley computer science graduate and a former student of Anderson.

"SETI@home is a way of harnessing all the idle computers to increase our computing capacity and our chance of finding extraterrestrials," Werthimer said.

Werthimer and other UC Berkeley physicists operate several ongoing SETI projects, including a 20-year-old project, the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP), whose newest instrument, SERENDIP IV, piggybacks on the Arecibo telescope. But the computer capacity available to SERENDIP is sufficient to look for only the most obvious signals from extraterrestrial civilizations, Werthimer said.

SETI@home will let SERENDIP physicists analyze more thoroughly the data they receive daily from their ongoing survey of the sky using the large radio dish at Arecibo.

The radio data is broken down into small chunks -- a 10 kilohertz range of wavelengths in a strip of sky visible from Puerto Rico -- through which the screen saver program can search for patterns that may indicate a deliberate broadcast from a distant civilization. The data downloaded to each desktop computer takes up only about 250 kilobytes of computer memory, though the computer must have 32 megabytes of RAM (random access memory) to run the screensaver software.

"You can download enough data through the internet in five minutes to keep the computer analyzing for several days," Anderson said. "The computer then sends back a summary of the interesting stuff it found and gets another chunk of data."

Whatever interesting signals may turn up from SETI@home must be checked by project staff to make sure they are not due to radio interference from Earth or orbiting satellites.

"We're not asking people to call the press when they see a spike on the screen," Werthimer said. "We get strong signals all the time and have to sift through them."

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