Realizing that a computer might as well be doing something in its spare time, hundreds of thousands of personal computers (and their owners) are involved in the search for extraterrestrial intelligence (SETI).

Telescope time is expensive. With hundreds of billions of stars in our galaxy, the cost of such a search could quickly become astronomical.

Two organisms living together are in symbiosis. If one organism benefits at the expense of the other, the symbiotic relationship is parasitic; if both come away winners, it is mutualistic; and if one benefits while the other is neither helped nor harmed, commensal.

Analyzing radio signals that astronomers are collecting anyway for other projects, SETI@home is a commensal SETI project.

Not unlike a commercial advertiser, if a civilization wished to maximize the chances of someone hearing its signal, it would broadcast at a frequency with the greatest number of listeners.

More than 90% of the atoms in the universe are those of hydrogen, the simplest atom possible. Hot hydrogen emits a nice reddish glow, but most of the hydrogen in the universe is stone cold, and dark. Fortunately, even cold hydrogen emits radio waves, at a frequency of 1419 megahertz (MHz).

KVMR's signal vibrates at 89.5 MHz; to pick up neutral hydrogen you need to tune above the FM band, in the microwave region of the spectrum. A radio wave from KVMR's transmitter measures about 11 feet long; the wave from hydrogen is shorter, about 8 1/3 inches, or 21 cm. If there is any wavelength that any radio astronomer anywhere in the universe tunes to frequently, it's 21 cm.

Radio waves are much longer than light waves (a wave of orange-red light from a neon sign measures 25 millionths of an inch), so radio telescopes must also be large. At Arecibo, Puerto Rico, astronomers have employed a bowl-shaped valley to support a dish a thousand feet across, over an acre in area. Arecibo spends a lot of its time tuned to 21 cm.

The raw computing power needed to extract a needle of intelligent signal from a haystack of noise is as daunting as the resources needed to detect the signal; the cost of time on a large computer can also become astronomical. An alternative is distributed processing, coordinating the work of many smaller computers. In the largest-ever experiment in distributed computing, SETI@home participants around the world download data and the software to process it. Once they've crunched the numbers, they send the results back, and ask for more. Each computer involved in SETI@home analyzes a chunk of data recorded as Arecibo scanned the sky for roughly a minute and a half. The graph on the screen summarizes the results.

Drawn in perspective as if it goes into the screen, the "z-axis" breaks the minute-and-a-half chunk into smaller increments, from the beginning (closest to you) to the end (deepest into the screen).

While SETI@home centers its attention on the 21 cm line, it has to look at wavelengths on either side of that line.

Standing over the freeway in the parking lot of the Nevada City Post Office, one hears a subtle shift in pitch - a Doppler shift. Sound waves from a passing vehicle are "squeezed together" as the vehicle approaches, "stretched apart" after it's passed.

Earth moves around its axis, around the sun, and around the galaxy. All other planets are also in motion, and an interstellar signal would thus be Doppler shifted. The computer therefore analyzes a chunk of the radio spectrum roughly 10 kHz (10,000 cycles per second) wide. The actual frequencies analyzed can be read off the "x-axis" of the graph, running from left to right on the screen; add the number on the part of the graph that interests you to the "base frequency," displayed in the "Data Information" box.

The goal of the project is to find a spike, a sharp increase in signal strength, represented by the height of the graph along the "y-axis." Before one gets too excited when such a peak shows up on the screen, realize that the vast majority (all?) will later be screened out by computers in Berkeley when they're found to have come from such sources as terrestrial aircraft, spacecraft, refrigerators, etc.

While color is often used in scientific displays to help the eye discriminate among the data, in this case the colors in the graph are added to make the whole thing look pretty; it is, after all, a screen saver.

In mapping space, we've adopted a slight conceit: we've placed Earth at the very center of the universe. Extending Earth's equator out into space delineates the universe's "celestial equator;" it cuts through space very close to the belt of Orion, the well-known winter constellation. SETI@home data come from a swath of the galaxy extending from just above the celestial equator to roughly a third of the way to Polaris.

Ultimately, the odds of our having company in the galaxy depend on a number of factors: the prevalence of potentially life-supporting stars and planets; the likelihood of life, intelligence and technology evolving on such planets; and finally, the odds that, with its new-found technological powers, such a civilization doesn't destroy itself.