FROM SCIENCE FICTION WRITER AND SPACE PHYSICIST DAVID BRIN: Way back, two-thirds of a century ago, shortly after the Manhattan Project to develop the atom bomb, physicist Enrico Fermi and his colleagues took a break, over lunch, to discuss the notion of other life in the cosmos.
Many of Fermi’s colleagues enjoyed science fiction stories. They held that a richly endowed cosmos, replete with trillions of stars, should contain a vast plurality of living worlds, and—on some of them—minds like ours, capable of insight, curiosity, intellectual advancement. Within a galaxy as old as this one, there must have been many, many races that preceded us, some of them now millions of years older, and commensurately more advanced. Indeed, some of these alien intelligences would surely have set forth, expanding and exploring across space. How interesting a future we would have, with such fascinating others to talk to, when our descendants finally made contact!
Fermi listened patiently to these expressions of confidence in a fecund universe, teeming with conversation, as vivid and enthusiastic as the discussion they were having at that moment, in a Chicago cafeteria. Only then, the great physicist shook his head and asked:
“So? Shouldn’t they already have been here by now? Should we not have heard their messages? Or seen their great works? Or stumbled upon residue of past visits to our planet? These wondrous ancients of yours, where are they?”
Across all the years since Fermi posed that challenge, it has been called many names. The Great Silence, the Seti Dilemma, and so on. But most often the “Fermi Paradox.” And every passing year, while enthusiasts still scan for signals, the sky’s eerie hush steadily gets more and more bothersome.
How many life-bearing worlds are out there? So curious are we—members of an expansive, eager, enlightenment civilization—that we are willing, even in rough economic times, to fund ambitious efforts like NASA’s new Kepler Orbiting Observatory, with the mission of putting solid numbers behind some of our best estimates. So far, it seems that planets are common in the cosmos. Now we hope to get a handle on what fraction may be a bit like Earth. But even if we find that figure to be high, it will take later, more advanced instruments, to detect glimmering spectroscopic indications that life appeared on those distant worlds.
And from there the conundrums will continue! What portion of these Life Worlds will develop intelligent, technological beings? (It only happened once on Earth, across 4 billion years.) And what further sub-fraction might eventually overcome all obstacles and hazards to start spreading across the stars?
These estimates have been crunched and probed and argued-over for more than two generations. Earnest calculations claim there ought to be neighbors out there. We shouldn’t be alone.
And yet, somehow, we appear to be! At least, as far as we can tell, so far.
Then it began to sink in. This wasn’t just a theoretical matter, anymore. The appearance of scarcity has implications, disturbing ones.
Something must be suppressing the outcome. A “filter” of some sort may winnow down the number of sapient races. To a number that is low enough to explain our apparent isolation. Our loneliness. Perhaps even reducing the total down to just one.
This is no place to get into the full-pitch debate. But over 10 dozen pat “explanations for the Fermi Paradox” have been offered. More than a hundred! Each of them pushed with great fervor by this or that person or group, each of them fervently convinced that the (skimpy) evidence supports just one possible conclusion.
Some claim that our fertile and lush planet must be unique. (And, so far, nothing like Earth has been seen, though life certainly exists out there.) Or, it is suggested that most life worlds suffer lethal accidents—like the one that ended the reign of the dinosaurs—far more often than our planet has. In other words, Earth has been luckier than most.
Other theories suppose that intelligence must be a rare fluke, and that fact alone would bring the numbers down to a level compatible with observation.
Note that these are actually the optimistic explanations! Because they suggest that the “great Fermi filter”—the thing that’s been keeping the numbers down—lies behind us. Not ahead.
But what if life-bearing planets turn out to be common? And suppose intelligence arises frequently? Then the filter—whatever process it is, that winnows down the numbers—may lie ahead of us. Perhaps some terrible mistake that all sapient races make. An error so alluring that all of them stumble into it. Some catastrophe that now awaits us, around the next bend.
Or perhaps several. A veritable minefield of possible ways to fail.
Want to hear something strange? The so-called "optimists" in the SETI (Search for Extraterrestrial Intelligence) community are more likely to believe something like this. Carl Sagan thought that most alien civilizations destroyed themselves through nuclear winter, unless they managed first to cure themselves of all aggressiveness ... which might then explain why such species did not travel far or make colonies or fill the cosmos with their progeny. A pretty dour version of "optimism"!
That is the dour context of our story. And the implications go far, far beyond mere sci-fi musings about contact with aliens. In the end, all of this carries huge and important implications about us! Because now we have to wonder, each time we face some worrisome step along our road toward maturity ... from avoiding war to becoming planetary eco-managers, to genetic engineering and so on. ... All the time, we have to ask ourselves "Could this be it? The mistake? The big one that all (or almost all) alien sapient races make? The error, the trap, that keeps the numbers down and that keeps us asking Fermi's question?"
It is the specter lurking at our banquet. The shadow that slinks around the edges of both reflection and foresight, as we turn to examine all the conceivable threats to our existence.
At least, all of those we can now see.

—Excerpted from David Brin's next novel, Existence, which he hopes readers will see completed in 2010. His previous novels include Earth, The Postman (the basis for the Kevin Costner film of the same name), Foundation's Triumph, and The Life Eaters (a graphic novel that became an international sensation).

David Brin appears with Jill Tarter, Doug Vakoch, Frank Drake, Ray Kurzweil, Francisco Ayala, and Steven Dick in "Where Are They, All Those Aliens?" the 29th episode in the Closer to Truth: Cosmos, Consciousness, God TV series, hosted and created by Robert Lawrence Kuhn. The series airs Thursdays on the PBS HD network and many other PBS stations. Every Friday, participants will share their views on the previous day's episode.