THE FERMI PARADOX is an avenue of cosmological speculation popularly attributed to Italian physicist Enrico Fermi, who developed the world’s first nuclear reactor as part of the Manhattan Project and who is thus considered (alongside men like Oppenheimer, Einstein, and Szilard) one of the fathers of the atomic bomb. Around 1950, during one of his visits to Los Alamos, Fermi pondered the possibility of alien life in the galaxy with his lunchtime companions and asked the unexpected question, “Where are they?”

Fermi’s question comes down to this: given what we know or expect to find out about the age of the universe; the number of stars within it; the number of those stars likely to have planets; the number of those planets likely to support life; the amount of time it took life to evolve; the amount of time it took human life to evolve; the relatively short leap from the invention of agriculture to fully technologized, world-transforming civilization; and the relatively short distances between stars, the Earth should have already been visited or colonized by alien life (and perhaps many times over across its four-billion-year history). And yet there is no serious indication that this is the case. Moreover, there is no significant indication from any of our observations — either in 1950, or in any of the years since that we have spent searching for extraterrestrial intelligence — that there are any aliens out there either. Occasional unexpected and intriguing blips like the Wow! signal or odd astronomical formations aside, we have found no evidence of any alien civilizations whatsoever, much less the sorts of vast galactic empires imagined by such familiar science fictional franchises as Star Wars or Star Trek. If they were out there, we would expect to see some traces of them, or of the tremendous energy footprint thrown off by their activities; instead, outer space seems, unsettlingly, to be completely empty.

Why “unsettlingly”? Because to resolve the Fermi Paradox, we have to find something wrong in the chain of assumptions that suggests alien civilizations should be sufficiently common as to be visible from Earth. If we assume our planet and solar system are typical, then in our galaxy alone — with 100 thousand million possible chances — there should be millions of planets on which life has evolved. Our civilization went from rudimentary agriculture to the nuclear bomb in just 12,000 years; in a universe almost 14 billion years old, that’s a lot of chances for someone else to make the big leap. It seems impossibly unlikely that we’d be first.

The “Great Silence” thus implies what Robin Hanson has called a “Great Filter” located somewhere in our near future rather than in our past. Perhaps there is something — some inexorable natural tendency towards species collapse — that intervenes between our current level of technological development and the level of technological development required to spread across the galaxy that stops civilizations from achieving it. Whatever that Great Filter is, the news doesn’t seem good; given that exploration of the stars appears tantalizingly within our own species’ grasp, the most obvious sorts of potential “Filters” involve imminent civilizational collapse or ecological catastrophe, and perhaps even near-term human extinction. Perhaps extinction-level collisions with astronomical bodies are much more common than they seem to be; perhaps fossil-fuel sources of energy always run out before anyone manages to get and stay off-world; perhaps civilizations always blow themselves up, or always crash their planetary ecosystems, or always retreat into the orgiastic comforts of virtual reality instead, before they hit the threshold where they become truly “galactic.” Maybe what the Fermi Paradox tells us is that there’s no future, at least not a future that looks very much like an extension or expansion of our present.


Cixin Liu’s The Dark Forest takes up the Fermi Paradox as one of its central narrative and thematic problems. The Dark Forest is the second novel in the trilogy that began with The Three-Body Problem, which was published in China in serial form in 2006 and as a novel in 2008 before being translated into English by Ken Liu in 2014, and which won both the Chinese Science Fiction Galaxy Award and the 2015 Hugo. (If you haven’t read The Three-Body Problem yet, I highly recommend that you close this page and come back when you’ve done it. Go now!)

The Three-Body Problem concerned a SETI-like Chinese military project devoted to the search for extraterrestrial intelligence, comprised of scientists and academics who were discredited and exiled during the Cultural Revolution. When one of these researchers, Dr. Ye Wenjie, actually makes contact with an alien race, the result is extraordinary: The alien researcher on the other side of the communication warns her that its society is utterly twisted and that she must never make contact again, lest they invade Earth:

Do not answer!
Do not answer!!
Do not answer!!!


I am a pacifist in this world. It is the luck of your civilization that I am the first to receive your message. I am warning you: Do not answer! Do not answer!! Do not answer!!!

There are tens of millions of stars in your direction. As long as you do not answer, this world will not be able to ascertain the source of your transmission.

But if you do answer, the source will be located right away. Your planet will be invaded. Your world will be conquered.

Do not answer! Do not answer!! Do not answer!!!

Ye Wenjie has been so embittered by her own experiences that she decides to reinitiate contact with the aliens anyway, on the grounds that the human race deserves to face obliteration. Either humanity will evolve to face the threat, or else it will die — and either solution seems preferable to the bleak continuation of what already exists. “Come here!” she writes back. “I will help you conquer this world. Our civilization is no longer capable of solving its own problems. We need your force to intervene.”

The Dark Forest, translated by Joel Martinsen and published in English earlier this year, begins where The Three-Body Problem left off, spiraling the story in wonderfully unexpected directions. Ye Wenjie makes a brief appearance at the beginning of the story as a very old woman, but the focus of the novel is an entirely new set of characters wrestling with the dire existential consequences of her decision. It is now generally known that the Trisolaran fleet is on its way to Earth, and that it will arrive in a few hundred years; the Trisolarans are considered a completely superior military force, and, moreover, they have sent advanced devices called sophons ahead of themselves that stymie Earth’s research into advanced science and thus prevent significant technological advancement in the intervening centuries. It falls to the human species to plan some type of defense in the name of surviving the incursion, even as any such plan seems totally hopeless in the face of such an overawing threat, and even as two groups within the human species pose internal threats to the collective human project: those who would seek to align themselves with the Trisolarans in exchange for power or influence (or just mere survival), and those “Escapists” who would seek instead to build spaceships for some tiny remnant to flee the solar system in advance of the Trisolarans. Both groups are considered loathed traitors, even as all hope of actually successfully resisting the invaders seems pathetically miniscule.

While the UN develops a global space fleet in an attempt to repel the Trisolarans (a prospect that even its proponents believe is doomed), a secondary strategic plan emerges. The Trisolaran sophons can read all computer data and all printed texts, and they transmit the information back to the incoming Trisolaran fleet, including any and all plans made by the defense force; they can also overhear communications between individuals. The one thing the sophons can’t read — and what the Trisolarans furthermore can’t really understand, having evolved as a species to deploy direct telepathic communication with each other’s minds — are the secret thoughts hidden inside an individual human brain. And so the UN chooses, more or less, the four best strategic thinkers in the world, and gives each of these “Wallfacers” a portion of the global defense and an essentially unlimited budget to attempt to execute some complex anti-Trisolaran subterfuge that they develop entirely within their own minds and reveal to no one, in the hopes that one of these four plans will somehow work. The four Wallfacers are encouraged to behave strangely and erratically on every level, in the hopes of disguising their true agenda from the Trisolarans until the trap is sprung; they are opposed in this endeavor by the “Wallbreakers,” pro-Trisolaran traitors who scrutinize every aspect of the Wallfacers’ lives in order to anticipate and unravel their secret scheme, rendering it moot.

Positing a hibernation technology that will allow the Wallfacers to survive the long centuries until the moment the invasion fleet arises, The Dark Forest is structured around the twists and turns of each of the Wallfacers’ labyrinthine plots, as each one in turn is exposed by their Wallbreaker until only the last remains. This aspect of the book is wonderfully exciting on its own terms; each move and countermove is perfectly paced against the backdrop of overawing species crisis and existential threat. As good as The Three-Body Problem was — and it seemed quite difficult to improve on! — The Dark Forest is better, turning many of the familiar assumptions of Anglospheric science fiction on their ears as the games intensify and the Trisolarans draw ever nearer.

The book became truly impossible for me to put down in its last hundred or so pages, as the quickening of the plot and the gambits of the last Wallfacer unexpectedly merged with the more philosophical concerns that had dominated the first book in the trilogy. As Ye Wenjie suggests in her brief appearance in the novel’s prologue, the characters in the novel are all participating in a kind of theoretical and speculative science called cosmic sociology, which attempts to axiomatically derive from first principles the basic sociobiological laws of civilization that must govern any possible societies that emerge from all intelligent life, anywhere in the universe. On a meta level, this is of course the larger task of the novel as well, and indeed of all science fiction: imaging the similarities and differences that would link human sociality, as we know it on Earth through reflection on our species’ own tragic history, to the other forms of sociality that might exist on alien worlds or in some future time. Given the huge scope of its cosmic-sociological vision, The Dark Forest may well be the dark contemporary answer to Olaf Stapledon’s transcendent 1937 novel Star Maker, sharing with that book the ambition to understand and schematize all possible societies while diverging sharply from its careful communitarian optimism.

In the prologue, Ye Wenjie suggests the first two axioms of cosmic sociology: “First: Survival is the primary need of civilization. Second: Civilization continually grows and expands, but the total matter in the universe remains constant.” The two, taken together, flatten the idea-space of science fiction dramatically into a strictly Darwinian and imperialist paradigm: civilizations need to expand to gather more resources if they hope to continue to survive. And thus one of the first conclusions of cosmic sociology is that all other civilizations must be, more or less, exactly what ours has been: ravenous, violent, expansionist, shortsighted, and devoted to its own continuation at any cost. Even a society that attempts to change its own noxious DNA — as ours has, in fits and starts, with greater and lesser successes since the end of the Second World War — must come to realize that, in the cosmic scheme of things, the true material core of all sociality, everywhere in the universe, is robbery and murder. Or the society doesn’t learn — and thus finds out the hard way.

This becomes, in the end, the core of Liu’s unimaginably tragic solution to the Fermi Paradox, his sad final pronouncement both on the exhilarating SF fantasy of the alien encounter and on the utopian hope that, in some other space or time, history might have gone some other way. (Final, at least, until Death’s End, the third book in the trilogy, comes out this year.) “The universe,” Liu writes,

is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds other life — another hunter, an angel or a demon, a delicate infant or a tottering old man, a fairy or a demigod — there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people.

So, where are they? Why is the universe so uncannily, so eerily, so terribly quiet? Because in the dark forest, anything that makes a sound gets eaten.


Gerry Canavan teaches 20th- and 21st-century literature and culture at Marquette University.