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50 years of seeking E.T.

Click for video: The Drake Equation estimates the likelihood
of alien intelligence, based on assumptions about life in the
universe. Click on the image to watch a Space.com video
featuring the SETI Institute's Seth Shostak and Frank Drake.

It's been almost 50 years since scientists first came up with the idea of looking for radio signals from extraterrestrial civilizations - and although there have been a couple of curious blips, we haven't yet definitively heard E.T.'s cosmic call. Now the experts in the search for extraterrestrial intelligence, or SETI, are wondering whether we've been looking in the wrong places for the wrong kinds of signals.

Or maybe we just haven't been looking long enough.

All of those possibilities are considered in "Confessions of an Alien Hunter," a new book from Seth Shostak, the SETI Institute's senior astronomer.

Shostak's "confessions" are actually Shostak's arguments for why the SETI search makes sense - leavened with dramatic accounts of the effort's best-known false alarms (including an episode that Shostak wrote up for msnbc.com a decade ago) and folksy metaphors that would put Dan Rather to shame (including this one: "Life is as durable as Christmas fruitcake").

The California institute where Shostak works is the primary standard-bearer in the search for alien signals. That search dates back to 1960, when astronomer Frank Drake (now the SETI Institute's president) aimed an 85-foot radio telescope in West Virginia skyward in hopes of tuning in the extraterrestrials.

After his initial foray, Drake and his fellow seekers moved on to bigger and better telescopes, including the old 140-foot Green Bank Telescope and the 1,000-foot Arecibo Observatory. But the strategy was pretty much the same: Check one star for an unnaturally steady radio signal, then move on to the next star.

Now that's changing. The Allen Telescope Array, a joint venture involving the SETI Institute and the University of California at Berkeley, will allow bunches of stars to be studied at once. Like microchips, the efficiency of SETI has been improving at a geometric pace in agreement with Moore's Law. If that pace continues for the next two dozen years, more than a million stars will be checked for signs of on-air life, Shostak notes.

Shostak and Drake say that sampling should be big enough to result in contact - assuming first that the aliens exist, and then that they think like we do. The first assumption is big, but the second one is even bigger. If the search for signs of microbial life on Mars and more distant worlds requires a correct understanding of astrobiology, the search for intelligence beyond our own solar system requires something more: astropsychology, perhaps?

Over the decades, the strategy for SETI has by necessity been dictated by a cosmic Golden Rule: We look for communication in the channels that we use to communicate. A generation ago, that might have been the analog television signals that carried "I Love Lucy" out to the cosmos. Today, Drake speculates that the aliens might be transmitting digitally, with lasers instead of monster radio antennas.

During a weekend talk in Seattle, Drake pointed out that the just-completed National Ignition Facility can focus the light of 192 lasers to create a pulse that lasts just a few nanoseconds but far outshines the sun. "Those lasers can make pulses of light which are visible to very small telescopes all across the galaxy," he noted.

Shostak theorizes that E.T. might have two types of transmitters going: one that flashes such pulses of light toward a long list of target planets that might be habitable - including us - and another "low-power, omnidirectional broadcast that tells you how to join their book club, or whatever." For that reason, SETI searchers have started conducting surveys for those tiny flashes of light as well as for sustained radio traffic.

So where should we look? Historically, the SETI Institute's target list has favored Earthlike planets where life as we know it might have taken root. But in "Confessions of an Alien Hunter," Shostak suggests that on the basis of what we're learning about artificial intelligence, the most likely aliens to send signals would actually be artificially intelligent machines.

If E.T. is a big shiny robot, the strategy of targeting Earthlike worlds orbiting sunlike stars may turn out to be "a very antiquated idea," Shostak acknowledged during a weekend interview. "A world on which the whole thing can rust might not be the best place for it," he said. A better place, from the machine's point of view, would be in orbit around a star hot enough to provide the prodigious power required for the big broadcast.

But Drake said the other end of the stellar scale shouldn't be overlooked, either. It turns out that about three-quarters of the stars in our galaxy are red dwarfs, which are dimmer than the sun but still could provide a home for E.T. Those stars have been overlooked in past SETI searches.

The bottom line, Drake said, is that "our simple picture was really way too simple" when it came to visualizing the kinds of places in the universe where life might lurk.

That's one of the reasons why Drake isn't discouraged that the SETI quest has come up empty, even after 50 years. He pointed out that only a thousand stars or so have been studied, over bandwidth that accounts for just a few percentage points of the potential spectrum. "We've looked at something like 10-5 of the possible combinations," he said.

Shostak said he felt confident that solid evidence of life beyond Earth will be found within two dozen years - either by continuing with SETI, or by analyzing exoplanet atmospheres, or by digging into the dirt on Mars or the ice on Europa or Enceladus. Drake, meanwhile, had a longer timetable in mind. "I don't think 2025 is going to happen unless we're very lucky," he told me. "Maybe it'll take twice as long - maybe 2050."

Other experts have suggested time frames of 100 to 200 years.

Of course, such timetables assume that SETI efforts around the world will continue to attract followers and funding. SETI efforts in the United States are funded privately rather than publicly, and Drake said it's getting tougher to raise money. "As long as the recession keeps going on, we have to move that [timetable] back," he said.

Could there ever come a point when the experts decide there's no E.T. out there to phone home? What would Shostak do if he hasn't heard from the aliens after a century of searching (other than celebrating the fact that he's still alive in the year 2060, that is)?

"I don't think I would be ready to say that they're just not there," he said, "but I might be inclined to say that we're barking up the wrong arboreal fixture ... that there's something fundamentally wrong with what we're doing."

How common do you think extraterrestrial intelligence could be? Plug some figures into the Drake Equation and come up with your own estimate. Search for extraterrestrial intelligence on msnbc.com. Then weigh in with your comments below.