Our perceptions of reality are framed by notions about cosmic structure. Ancestral mythologies often credited the world's existence to a creator deity, who might un-make what had been made. Or else they envisioned a world whose life cycle, like a person's, might have a foretold end. Nowadays, the Universe as a whole seems safe till the Great Dissipation, yet we peer for patterns that might portend more immediate peril.
One such pattern—formerly confined to science fiction—now inveigles into scholarly argument and even debates over public policy. The “Fermi Paradox” was named in the late 1980s after the celebrated physicist who—thirty years earlier—famously asked: “So, where are all those aliens?” Rough calculations suggested that our Galaxy should be aswarm with signs of earlier civilizations. Their apparent absence—if not exactly a “paradox”—is certainly a puzzle.
As yet, the Search for Extra-Terrestrial Intelligence (SETI) has found no plausible glimmers. After sifting for “needles in the Galactic haystack” across an entire human lifespan with rapidly improving tools and methods, we've glimpsed none of the extravagant radio beacons depicted in the movie CONTACT. No sign (so far) of robotic-interstellar probes. No hints in our Solar System that past visitors altered asteroids or their orbits, or of colonizations in Earth's geological or paleontological record, the sort of spoor that humanity is copiously laying today, amid the Anthropocene Era. This apparent loneliness might end tomorrow, or in a decade or century—the search remains worthwhile—but we do know enough already to say there's no gaudy-welcoming bustle out there. Indeed, we seem like toddlers, venturing forth into a forest that's quiet… maybe too quiet.
Among the many Fermi-hypotheses out there (or “fermis”) are gloomy scenarios issued by the likes of Stephen Hawking (e.g., all ecosystems feature predation, and if you don't know you're a predator, then you are prey)… or author Liu Cixin (in a dark forest, it's prudent to eliminate potential rivals first)… or groups in Oxford and Cambridge who study “existential risk,” suggesting that some Great Filter lies ahead—a minefield of systematically fatal errors that trap nearly all promising, young civilizations.
Everyone, it seems, has their own notion or theory, but only rarely have efforts been made to catalogue and compare, putting some order to this quandary, perhaps the most significant we'll ever face. Back in a 1983 review for Quarterly Review of the Royal Astronomical Society, I listed and ranked hypotheses for what I called “The Great Silence,” before consensus settled on a different name. In 2002, Stephen Webb's catalogue “Where is Everybody? Fifty Solutions to Fermi's Paradox” was another effort at perspective, as were Michael Michaud's substantial Contact With Alien Civilizations and The Eerie Silence by Paul Davies.
Now, Belgrade astronomer Milan M. Ćirković goes even deeper in “The Great Silence: The Science And Philosophy Of Fermi's Paradox,” performing a much-needed critical analysis of the many logical and philosophical fallacies that pervade this field—the most important scientific topic that lacks any known subject matter.
Ćirković pays due homage to the famed “equation” of SETI pioneer Frank Drake which—for all its faults—helped frame our disorderly arguments about ETC (Extra-Terrestrial Civilization). Evidently, one factor or several must be small, in order to explain why the number of known, sapient species in the cosmos still hovers around one. (Depending on your definition of “sapient” and “known.”) Hence, while members of the gloom/risk community expect low values for “L”—the average lifespan of a technological race—others assert we're over-estimating f(L), or the likelihood that life erupts elsewhere in the cosmos. Despite decades spent discovering one easy biochemical step after another, we may yet find that some as-yet undiscovered crucial link was an improbable fluke, leading to just one lonely world that's not sterile, but green.
(As nearby planetary systems are surveyed with advanced spectroscopic tools, f(L) may be the next factor to clarify.)
Or else the low, suppressing Drake factor may be “f(I),” the fraction of fecund, life-bearing worlds that ever host an intelligent, then an ambitiously and detectably technological species.
Other once-mysterious factors have surrendered to science. Twenty years ago, we knew of no extra-solar planets; now more than 4000 are confirmed, and that figure should soon leap by an order of magnitude. Now add another discovery, that our solar system contains not one ice-roofed ocean world (Europa), but as many as ten! This implies that life may exist near any and every star, not just those with a balmy “Goldilocks Zone.”
Casting a wide, encompassing net, Ćirković looks critically at the family of “Zoo Hypotheses” —proposing that ETCs do exist out there but have deliberately masked themselves from our sight for some reason. Variations are myriad and the author cites several—e.g., that advanced ETCs might enforce a Star Trek-like “prime directive” banning interference in primitive civilizations such as ours. Among potential motivations for such interdiction may be ethics, or pity, or preservation of a scarce resource. (Perhaps young, struggling societies like ours provide a rich supply of drama, helping to stave off cosmic ennui.) Or the drive may be economic; so long as we blare free samples of our rich culture, they feel uncompelled to reciprocate. Alternatives abound.
Ćirković takes the reader on a journey, appraising claims that there's something special about our place in the Galaxy (just far enough from the Galactic core to escape intermittent incineration) or this particular time (our third-generation solar system has plenty of metals). While some protest that “special circumstance” contravenes the Copernican Principle of Mediocrity, Ćirković has a stronger objection—that time-or-place selection seems to violate Non-Exclusivity. Any effect or process that merely reduces the number of ETCs linearly won't suffice—not by itself—because it is exceptions that will spread far and wide, inheriting the galaxy. In order to have explanatory power, a solution to Fermi's Paradox must be fierce and nonlinear—allowing few exceptions—or else work in tandem with others to cull the number of ETCs way down into single digits. Or just one.
What about such effects working in tandem? Some combination of traits may have been anomalously favorable on Earth. Some propose that our comparatively large moon may have been significant. I've mentioned the way our planet skates along the very inner edge of the sun's Continuously Habitable Zone, perhaps making it atypically dry and oxygen rich, for an open-ocean world. Ćirković speculates that a combination of many factors may be responsible both for earlier silence and then for a following astrobiological “phase transition,” for which it's statistically possible for humanity to be a vanguard.
A related matter—the controversy over METI, or Messaging to ETI—is covered in less depth, since it is primarily about grownup behavior (or its lack) down here on Earth… whether small groups of zealots should bypass all institutions, peer critique, risk appraisal or public opinion, to shout “yoohoo” into a potentially hazardous cosmos. Ćirković's book offers plenty of grist for discussion and consensus-seeking, before rushing to force a fait accompli on our children.
From the Gaia Hypothesis to Rare Earth models, to various “great filters,” to a Galaxy dominated by machines… all the way to a putative attractor state of “transcendence” that might pull advanced ETCs away from lifestyles we'd observe… the truly remarkable thing here is how many concepts are now fodder for legitimate discussion that were formerly restricted to science fiction. (Full-disclosure, I've used nearly all of them as plot drivers in varied stories and novels.) Ćirković shows that the topic is so broad, with so many implications, that it ultimately distills down to an appraisal of us. Of contemporary humanity in all our complexity and contradiction, our obduracy and brilliance in confronting a tsunami of accumulating knowledge… and awareness that all these speculations will (must) seem childishly naïve from the perspective of our heirs.
That's all right. It is our verve and curiosity and eager gregariousness that make us interesting, propelling speculative extravaganzas like The Great Silence. Moreover, even if it all serves as reality-show entertainment for bored, alien-voyeur couch potatoes, well, fine. We still want our royalties.
And we're coming.
David Brin is an astrophysicist whose novels include The Postman, Earth, and Existence. He serves on advisory boards (e.g., NASA's Innovative and Advanced Concepts program or NIAC) and speaks or consults on topics ranging from AI and SETI to privacy and national security. His nonfiction book about the information age—The Transparent Society—won the Freedom of Speech Award of the American Library Association.
