They gotta be out there. We can find them if we figure out their frequency. It's a matter of looking in the right part of the spectrum, with the right kind of detector and the correct search algorithm. We are not alone; we're just momentarily a bit clueless.

This was the drift of the conversation Friday in an immaculate paneled room on the second floor of the Harvard Faculty Club. On a wall hung a framed portrait of a gray-haired alumnus in a suit and white vest, looking as though he'd just nodded off. Delicate china lined the bookshelves. In a different age one might imagine that explorers and adventurers would gather here, around the fireplace, to discuss the dinosaur bones they'd found in the Rockies, or the latest news about the search for the source of the Nile.

But on this day, everyone talked only of the search for garrulous alien civilizations. These were some of the elites of SETI, the Search for Extraterrestrial Intelligence.

"It's very simple: Are we alone?" said Bruce Murray, chairman of the Planetary Society, the meeting's sponsor. "It's not a matter of theory. It's a matter of observation. In the meantime, it's a matter of intuition."

Faith, even. Many of the people in the room had spent much of their lives listening to the heavens, waiting for that elusive signal. They're a band of optimists, tireless in their quest. What they do is considered goofy by critics, even absurd, but they doggedly search onward, forced by the dictates of the scientific method to admit that they still haven't found what they're looking for. They don't believe in flying saucers and alien abduction. They just believe in the possibilities of life in space.

Among those here were Frank Drake, creator of the famous Drake Equation, the formula for estimating the number of communicative civilizations in our galaxy; Dan Werthimer, who has lured 5 million people to join the cause in a program called SETI@home; Kent Cullers, a physicist whose enthusiasm hasn't waned despite a quarter-century of searches that have yielded only static; and Paul Horowitz, who searches in the optical wavelengths, scanning the sky for an alien laser pulse.

Murray titled the workshop "The Significance of Negative SETI Results." It was a rather provocative title. SETI scientists don't talk much about negative results. You can't prove a negative, they say. You can't discover the absence of the extraterrestrials, because the absence of evidence is not evidence of absence.

But Murray wanted them to consider that, surely, four decades of a null result should tell us something about the hypothesis that there are intelligent, communicative civilizations in space.

Ever since Drake aimed an 85-foot radio telescope in Green Bank, W.Va., toward two star systems in 1960, the SETI faithful have conducted numerous surveys of the night sky. They have known that it was a high-risk, high-reward business. Chances are that they'll find nothing, because our instruments remain primitive (cosmically speaking). But they might possibly make the greatest discovery in the history of mankind.

Though there is no evidence of ETI, the Copernican Principle says that there is nothing particularly special about the Earth, that whatever has happened here might happen elsewhere. There are upwards of 100 billion stars in our galaxy alone, and there are tens of billions of galaxies in the observable universe. Life emerges from ordinary matter, stuff like carbon and oxygen, and there's plenty of it in the cosmos. On the basic question, the people in this room have no doubt: They're out there.

SETI makes another leap, presuming that some of those civilizations will be communicative, and that this period of communication will overlap our own period of listening (it doesn't do us any good if a communicative civilization collapsed 2 billion years ago and its radio transmitters have become buried in the dark green soil of the planet Tralfamadore.)

In the past four decades, there've been tantalizing moments, false alarms, some inexplicable blips, some intriguing anomalies, but nothing that has come close to meeting the standard of scientific proof.

"We just haven't looked at nearly enough stars," Drake told a reporter before the meeting began. "We shouldn't have succeeded yet."

Drake and others concede only that, in the wavelengths they've searched, there doesn't appear to be any nearby intelligent civilization sending an intentional beacon toward Earth ("We've ruled out omnidirectional beacons of more than 10{+1}{+3} watts with a steady duty cycle.")

Project Phoenix, a search sponsored by Drake's organization, the SETI Institute, has been the most sensitive survey so far, examining many possible radio wavelengths, but it targeted only 1,000 nearby stars. That's a tiny portion of the galaxy, never mind all the other galaxies out there.

"Earthlings are just getting into the game," said Werthimer, a fast-talking University of California-Berkeley professor who is chief scientist for SETI@home. He urged against making too many assumptions about the way extraterrestrials would want to communicate. About half of all great discoveries are purely serendipitous, he said. The breakthrough might not even come from a SETI search. "It'll be somebody doing a dark matter experiment, or a gravity wave experiment," he said.

Werthimer showed a slide with a newspaper headline: "30 Trillion Fruitless Tries." That's how many different wavelengths from different stars have been examined by SETI searches. But Werthimer said that's still only a start, that new generations of computers and spectrum analyzers will allow searchers to examine far more carefully the radiation coming to Earth.

"I think we need another factor of a million or a billion. So that's 20 or 30 years away," he said.

Cullers, who is blind, told the group, "You wouldn't be very impressed if I looked all over the sky and didn't see anything. Because I can't see." Current SETI searches are still virtually blind, he said. We need bigger telescopes, maybe one on the far side of the moon. Keep trying for 100 years, maybe 200, and we'll find them, he said.

"Anybody who spends his or her life devoted to this has to be very hopeful," Murray told a reporter. "My personal analog to this is monks working on an illuminated manuscript in the Middle Ages. They work on one page almost their entire life. They don't expect to see the completed Bible."

The discussion had large dollops of radio-astronomy jargon -- this many megahertz, this wavelength. Gravitational lenses. Any such discussion is an odd mixture of hard science, advanced engineering and speculative exo-psychology. We shouldn't presume too much, the visionaries will say, but then a moment later they will reveal their latest hypothesis about ET motivations. There's a lot of interstellar mind-reading going on.

Ideally we would simply eavesdrop on the cosmos and pick up leakage from alien TV stations. But leakage is faint and extremely hard to detect. Drake and Murray argue that we should look for beacons, the intentional messages from the ETs. But this raises some squishy notions of alien behavior. Why would any intelligent civilization want to shout into the night sky?

"Intelligent creatures do weird things," Drake answered. "They would construct a beacon because it is emotionally or philosophically important."

They might belong to a religious cult, he said.

Geoff Marcy, the famed planet-hunting astronomer, steered the conversation toward the number of potentially habitable worlds in our own galaxy. In the past nine years he and his colleagues have found more than 100 planets outside our own solar system (he announced another six Saturday). Many are huge, hot Jupiter-like planets with eccentric orbits close to their parent stars, making them unlikely candidates for the emergence of intelligent life. But Marcy's data show that smaller planets are more plentiful than larger ones, and the trend line strongly suggests that the most common ones could be small, rocky planets like Earth, Venus and Mars.

In our galaxy alone, he said, "There must be 20 billion Earth-sized rocky planets with liquid water on the surface right now."

But Marcy has a hunch that intelligence is rather rare in our galaxy. He asked the group why we have found no artifacts, such as robotic probes, from an intelligent civilization. This echoes the famous Fermi Paradox, after the physicist Enrico Fermi, who more than half a century ago asked the question, "Where is everybody?"

Cullers offered one answer: We haven't looked very hard.

"Our searches for gadgets are terrible," he said.

But Marcy said there might be other factors at work. Perhaps there is something about life itself, or intelligence, that makes it rare. This was a line of reasoning that got little attention in the Harvard Faculty Club. The origin of life is a mystery still. It took 3 billion years for life to evolve into something as complex as a worm. No one understands how consciousness emerges from the brain.

The Drake Equation is still largely a string of unknowns. We don't if it's common for a planet to remain habitable for 4 billion years, or if Earth just got lucky. We don't know how long a technological civilization typically survives. We have to wonder how long we'll be around.

Werthimer had the best line of the day: "Your best guess is your worst enemy."

At the end of the meeting, Drake thanked everyone for the collegial nature of the event. And it was true: Everyone had been polite, thoughtful, friendly. Searching for the aliens is a difficult, challenging, mind-boggling enterprise, and people need lots of support and encouragement.

When you're in a strange business like this, it's nice to know you're not alone.

Astronomer Frank Drake says we haven't found alien life because "we just haven't looked at nearly enough stars."Geoff Marcy says, "There must be 20 billion Earth-sized rocky planets with liquid water on the surface."