"At whatpoint would you abandon the search?"

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

Thequestion seems fair, since SETI's obvious analogs-the historical voyages ofdiscovery made in the centuries following the Renaissance-were completed inconsiderably 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 thatEuropeans didn't even know were places, never mounted an expedition that lastedmore than three years.

But thoseanalogs are false. The South Pacific, for all its watery wastes, iscomprehensible in size. Even Cook's unimpressive Whitby collier, powered by sailcloth, could cross the Pacific in amatter of months, come about, and cross again in a different direction. Hisquarry, the islands peppering the ocean like coins scattered onto a living roomcarpet, signaled their presence by clots of clouds even when the islandsthemselves were below the horizon.

The SETIwilderness is incomparably larger, obviously, and its quarry is cryptic. Evenif there are ten thousand transmitting societies nestled in the arms of theMilky Way, we might need to search millions of star systems before we findone. The actual number of star systems that radio SETI experiments havecarefully examined is fewer than a thousand.

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

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

Andfrankly, it's conceivable that SETI's basic assumptions might be proven wrong. Imagine that the new space-based telescopes (COROT and Kepler) currently beingdeployed to hunt for Earth-size planets around other stars come up empty. Thatwould be a premium-grade bummer. But even if (as widely expected) they dodiscover rocky worlds, it's possible that a decade or so down the line, theirtelescopic successors-atmosphere-sniffing instruments such as the TerrestrialPlanet Finder-might fail to find any extrasolar worlds on which life hastaken hold.

Spacecraftof the future might return to us the news that neither Mars, Europa, nor any ofthe other orbs of the solar system with liquid water have ever produced amicrobe. If these are headlines of the future-if the local cosmic neighborhoodturns out to be as sterile as prime-time television-then that would certainlyput me on the defensive.

But thefact 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 toestimate the chances of SETI success, have-at least until now-become moreencouraging with time, not less. The more we learn about the universe, the moreit seems disposed to house worlds with life. It didn't have to be that way.

Somewhatmore disquieting is the possibility that our approach is wrong. SETI today isoverwhelmingly a search for narrow-band electromagnetic transmissions, or infewer syllables, a hunt for beamed radio or light. We search withstraightforward telescopic techniques, but it's possible that alien broadcastscould be encoded in ways that we're not set up to find. I'm not talking abouthow they construct their messages-or whether they're broadcasting in StandardAmerican 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 aliensmight wield two widely separated transmitters and use quantum mechanicaleffects to encode their messages. The advantage would be that if we opened thistype of alien mail, it would be impossible to tell from which direction itcame, thereby protecting the anonymity of the sender. This sort of approach-stillsomewhat beyond our technical abilities-might make our present receivingschemes seem na?ve.

Inaddition, there's always the chance that the discovery of new physics willreveal some communication mode that's either faster than light and radio, orrequires less energy to use. This doesn't seem likely, but science is allabout surprises.

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