"At what point would you abandon the search?"

That's a question I get relatively frequently from folks who think that SETI may be a quixotic quest, as futile as searching for the Seven Cities of Gold. After all, modern efforts to find signals from extraterrestrial transmitters are now in their fifth decade. Could it be that those of us who still hope to tune in other worlds may be missing some writing on the wall? Some dead-obvious, chiseled text with a simple, if disappointing message: "There are no aliens"?

The question seems fair, since SETI's obvious analogs-the historical voyages of discovery made in the centuries following the Renaissance-were completed in considerably less time than SETI has been beating the cosmic bushes. Columbus spent five weeks finding North America (and he wasn't even looking). Captain Cook, a true paragon of explorers, and a man who mapped places that Europeans didn't even know were places, never mounted an expedition that lasted more than three years.

But those analogs are false. The South Pacific, for all its watery wastes, is comprehensible in size. Even Cook's unimpressive Whitby collier, powered by sailcloth, could cross the Pacific in a matter of months, come about, and cross again in a different direction. His quarry, the islands peppering the ocean like coins scattered onto a living room carpet, signaled their presence by clots of clouds even when the islands themselves were below the horizon.

The SETI wilderness is incomparably larger, obviously, and its quarry is cryptic. Even if there are ten thousand transmitting societies nestled in the arms of the Milky Way, we might need to search millions of star systems before we find one. The actual number of star systems that radio SETI experiments have carefully examined is fewer than a thousand.

It's a simple truth, although one not universally acknowledged, that SETI is still in its early stages. Consequently, many of its practitioners will tell you that this is a multigenerational experiment, akin to building cathedrals in medieval Europe. In other words, a lot of SETI scientists will answer the question that began this article by saying "not in my lifetime, nor in that of my children or grandchildren."

Fighting words, but could they be hyperbolic? To begin with, SETI experiments will have examined millions of star systems within a generation. And within two, we could carefully check every star in the Galaxy. The SETI ship has a lot of ocean to cover, but thanks to new technologies, it's picking up speed. So clearly, if we haven't found something by mid-century or so, it will be hard to argue that it's still "early stages."

And frankly, it's conceivable that SETI's basic assumptions might be proven wrong. Imagine that the new space-based telescopes (COROT and Kepler) currently being deployed to hunt for Earth-size planets around other stars come up empty. That would be a premium-grade bummer. But even if (as widely expected) they do discover rocky worlds, it's possible that a decade or so down the line, their telescopic successors-atmosphere-sniffing instruments such as the Terrestrial Planet Finder-might fail to find any extrasolar worlds on which life has taken hold.

Spacecraft of the future might return to us the news that neither Mars, Europa, nor any of the other orbs of the solar system with liquid water have ever produced a microbe. If these are headlines of the future-if the local cosmic neighborhood turns out to be as sterile as prime-time television-then that would certainly put me on the defensive.

But the fact is that none of this incites me to break out the worry beads. Not yet. The various factors in the well-known Drake Equation, which is often used to estimate the chances of SETI success, have-at least until now-become more encouraging with time, not less. The more we learn about the universe, the more it seems disposed to house worlds with life. It didn't have to be that way.

Somewhat more disquieting is the possibility that our approach is wrong. SETI today is overwhelmingly a search for narrow-band electromagnetic transmissions, or in fewer syllables, a hunt for beamed radio or light. We search with straightforward telescopic techniques, but it's possible that alien broadcasts could be encoded in ways that we're not set up to find. I'm not talking about how they construct their messages-or whether they're broadcasting in Standard American English or a lilting Klingon dialect-but the technical scheme they use. For instance, Walt Simmons at the University of Hawaii has suggested that garrulous aliens might wield two widely separated transmitters and use quantum mechanical effects to encode their messages. The advantage would be that if we opened this type of alien mail, it would be impossible to tell from which direction it came, thereby protecting the anonymity of the sender. This sort of approach-still somewhat beyond our technical abilities-might make our present receiving schemes seem na?ve.

In addition, there's always the chance that the discovery of new physics will reveal some communication mode that's either faster than light and radio, or requires less energy to use. This doesn't seem likely, but science is all about surprises.

Indeed, my personal feeling is that if SETI hasn't turned up something by the second half of this century, we should reconsider our search strategy, rather than assume that we've failed because there is nothing-or no one-to find. Would I ever conclude that we've searched enough? Would I ever truly give up on SETI's bedrock premise, and tell myself that the extraterrestrials simply aren't out there? Not likely. That would be to assume that we've learned all there is to know about our universe, a stance that is contrary to the spirit of explorers and scientists alike. We might yearn, or even need to believe that we are special, but to conclude that Homo sapiens is the best the cosmos has to offer is egregious self-adulation.