The search for extraterrestrial intelligence, called SETI, poses something of a paradox for astronomers. If they could prove that alien civilizations exist, it would clearly rank as one of the greatest discoveries of science. But the odds against finding extraterrestrials are huge. In the four decades since the search began, only a handful of scientists have taken part—and they haven't turned up anything at all.
Still, the odds of picking up an alien signal have lately gotten better, thanks in part to an improbable crew of several eminent astrophysicists, a schoolteacher, a rock musician, a salesman, and about a dozen other enthusiasts. Since last November, they have been operating a search out of a control room on the edge of the Princeton University campus, in New Jersey, in an observatory so spartan there's no bathroom in the building. Yet even with these drawbacks, it could well be that here, less than five miles from where Orson Welles's fictional Martians landed in the celebrated 1938 War of the Worlds radio broadcast, the first evidence of an alien civilization could be confirmed. The biggest drawback to the search, acknowledged but downplayed by its champions ever since Frank Drake did the first systematic looking in 1960, is the assumption that aliens communicate by radio. That may not be a bad guess: Radio waves are easy to generate, and at certain frequencies they would sail across the galaxy without much interference. Besides, most stars generate very weak radio waves, so if you're broadcasting from one solar system to another, you wouldn't have to compete with your home star for a clear channel. Nonetheless, scientists are just guessing about radio waves; they don't really have a clue about how aliens might choose to communicate. And for the 40 years they've been searching, they've ignored the vast remainder of the electromagnetic spectrum, including infrared, optical light, ultraviolet light, X rays, and gamma rays. Not anymore. Astronomers at a half-dozen observatories around the world are now looking to see if extraterrestrials might be communicating with visible light, such as that emitted by lasers, instead of radio frequencies. "It has taken us a while," admits Harvard physicist and longtime radio-wave SETI searcher Paul Horowitz, "but we're now taking optical very seriously." OSETI, as the optical search is called, was first proposed by physicist Charles Townes, the inventor of the laser, back in 1961, just a year after Drake's first radio search. Lasers were so new and unexpected that physicists weren't even sure what they were good for, but Townes, who garnered a Nobel for his invention in 1964, had plenty of ideas, including interstellar communication. He coauthored a paper in the journal Nature, arguing that observers should look for extraterrestrial laser light as well as radio waves. Nobody paid much attention, largely because the amount of power required to make a laser outshine a star seemed absurdly large at the time. "We in the radio mafia," admits Horowitz, "had sold our own technique to ourselves so well that we didn't take anything else seriously."
