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."
"With laser technology already at hand, Earth can produce a brief flash of light thousands of times brighter than the sun," says Harvard astrophysicist Paul Horowitz, left, with grad student Andrew Howard in the Oak Ridge Observatory outside Boston. "This astonishing fact has motivated the search to detect visible flashes from other civilizations that may inhabit the Milky Way."
Then at a workshop sponsored by the California-based SETI Institute in late 1997, Townes once again made his case, in what Horowitz remembers as "a wonderful, offhanded, understated way. And some of us finally listened and realized, 'Hey, this is a really neat idea!'" What helped make the difference was that lasers had become much more powerful, and, more important, physicists had concluded that the total energy required for intergalactic communication via brief laser pulses wasn't insurmountable. By the late 1990s, scientists were able to pack up to a million billion watts of power into laser bursts of a trillionth of a second. At that level, a laser is a whopping 5,000 times brighter than the sun. In the meantime, lasers will likely keep getting more powerful, and presumably the advanced civilizations we're looking for have already surpassed us in laser power. Moreover, says Horowitz, designing detectors capable of seeing ultrashort bursts of laser light "turned out to be incredibly easy." About six months after the 1997 workshop, Horowitz began an optical SETI search using Harvard's 61-inch telescope at the Oak Ridge Observatory outside Boston. Rather than shoehorn their project into the telescope's schedule, Horowitz opted for what's called piggybacking: Whenever his Harvard colleagues David Latham and Robert Stefanik are using the scope for their own research—looking for planets around nearby stars similar to our sun—Horowitz and crew siphon off about a third of each star's light and divert it into two identical detectors. That's to prevent false alarms: "Just by the nature of electronics," says Horowitz, "the detector itself will generate spikes of current that mimic a signal." These spurious spikes still happen with two detectors but almost never at the same time; a spike has to show up in both detectors at once to be counted.
earth, do you read me? OSETI detectors are designed to intercept brief pulses of laser light that might indicate extraterrestrials are trying to communicate with us. To test their detector, the Harvard team took time exposure oscilloscope readings of successive flashes from a light-emitting diode (LED). The small bumps, lower left, are single-photon flashes, and the larger bumps, lower right, are multiple-photon flashes. "Light from astrophysical sources like stars arrives with the photons spread out in time and does not produce double pulses," says Andrew Howard. "Signals from a large laser on another planet would stand out because of artificial characteristics: large amplitude in a short time and successive pulsing."
Even so, the system can be fooled. The optics that siphon starlight from the telescope and split it can create internal reflections, causing flashes that look identical in both detectors. Beyond that, the sky will sometimes flash on its own: Charged particles speeding through space sometimes smash into the upper atmosphere. These cosmic rays generate quick bursts of light known as Cerenkov radiation, as well as high-energy particles called muons that could be mistaken for alien signals. But cosmic-ray flashes are purely local; if you had a second telescope pointed at the same star, and both scopes saw the flash at the same time, it couldn't be Cerenkov radiation or muons—and it couldn't be a glitch within the telescope or detector. That's where the Princeton group comes in, led by astrophysicist David Wilkinson, a 2001 winner of the prestigious James Craig Watson Medal for lifetime achievements from the National Academy of Sciences. In recent years, the university offered to free him from all teaching duties so he could pursue whatever project tickled his fancy. He had also been diagnosed with lymphatic cancer, requiring periodic chemotherapy that left him exhausted and prone to bronchial infections. Nevertheless, Wilkinson refused to relax. He kept up his active involvement as a leader in a project to map in unprecedented detail the leftover radiation from the Big Bang and in 1998 volunteered to teach a freshman seminar on "Searching for Life in the Galaxy" as well. As it happened, two different guest speakers during the semester mentioned Horowitz's project. That triggered a flash of Cerenkov light in Wilkinson's brain. He knew that the dilapidated Fitz-Randolph Observatory building, abandoned in the 1970s, had an equally dilapidated 36-inch telescope sitting inside that could serve as Horowitz's backup. So Wilkinson called his friend Horowitz to get the technical specifications. He also needed a team to refurbish the building and the telescope, build the electronic detectors, and then operate the whole thing. Major funding for a fringe project like looking for aliens was not immediately forthcoming, but that didn't deter Wilkinson, a pickup-truck-driving, motorcycle-riding scientist who promptly rolled up his sleeves and went looking for volunteers. His eclectic crew initially included a couple of Princeton chemists who also happened to be avid amateur astronomers, fellow physicists, a computer specialist, and some students—all of them willing to donate sweat and expertise to the enterprise. The university and an alumnus kicked in $20,000 in starter funds. And that was just fine with Wilkinson. "I love the fact that we're doing real, potentially very important science on a shoestring," he said last summer. "I'm having a great time—probably spending more time on it than I should."
Mark Lopez, an amateur astronomer, gets ready to spend a night at Princeton's Fitz-Randolph Observatory looking for optical signals from aliens. The telescope is a Cassegrain reflector, with a 36-inch primary mirror. A dehumidifier, right, is used to keep condensation from forming on the mirror when the telescope is not in use.
The searchers are having a great time as well. Every clear night, one or two volunteers show up to take the controls, check in with the Harvard group for instructions on where to look, and watch as the data roll in. "I still can't believe high-powered scientists like Dave Wilkinson have trusted me to run this telescope myself," says Aaron Schomburg, who does a solo midnight-to-sunrise shift every Tuesday and Friday before showing up at his regular job as an elementary-school science teacher. "But I think that whether we're alone in the universe may be the biggest scientific question there is. We may not learn the answer with this search, but when we do, I'll know that I was involved at the beginning." And it's still the beginning. So far, the Harvard-Princeton project has logged seven "coincidence" detections, in which flashes hit both telescopes within three minutes of each other. The closest pair of flashes were still 16 seconds apart, and that's nowhere near simultaneous enough to count as a signal. According to Wilkinson, "we'd need to see them within two milliseconds or less to take them seriously." Of course, no one can predict when or if a real signal will ever be detected. It could happen tomorrow, or it might not come in for thousands of years. But tragically, David Wilkinson won't see it happen. On September 5, 2002, lymphoma finally defeated him at age 67. "He was the quintessential gentleman of science," says Chip Coldwell, a member of the Harvard team who helped the Princeton crew get up and running. "His leadership and technical expertise will be sorely missed."
"I've had a good run in this business," said astrophysicist David Wilkinson, at Princeton's Fitz-Randolph Observatory just two weeks before his death. "It's time for the younger people to take over."
