The end of the world

Liu Cixin’s space-opera trilogy Remembrance of Earth’s Past revolutionized Chinese science-fiction, and creates a vision of the future as relevant and communicative as any in the history of the genre.

Liu’s avowed poetics for the work, at least as represented for English-speaking audiences, are given in an essay published in 2014, around when the first part of the the trilogy, The Three-Body Problem, was first translated. The essay concludes with his claim, “I wrote about the worst of all possible universes in Three Body out of hope that we can strive for the best of all possible Earths.”

The trilogy certainly takes a dark view of some of the ways humans relate to one another and to their world, but human nature is what Liu takes for granted: humans would act this way even in a Liebnitzian “best of all possible universes.” The “worst” universe Liu creates isn’t the human characters, it is the plot he embeds them in, which is so full of misfortunes that it borders on sadistic. Humans’ difficulties with one another and with their world are the subject that he wants to examine, and hopefully improve, if our actual universe turns out to be less awful than the fictional one he has drawn.

The center of the awfulness is the game-theoretic principle (invented by Liu) that gives the second book in the trilogy its title: Dark Forest Theory. Spoilers follow.

Let’s talk about game theory for a second. The Prisoner’s Dilemma is probably the most famous game theory exercise (other than… chess), perhaps in part because its ‘inspiration’ dramatizes the concerns of a strategic game so interpretably. Consider two prisoners in separate rooms, both facing the same two choices: maintain their collective innocence (or silence, depending on how you imagine the criminal justice system), or confess their collective guilt. If neither confesses, there isn’t enough evidence for a major conviction, and they both serve 1 year in prison. If one confesses and the other doesn’t, the traitor gets to go free (plea bargain, say), and the other prisoner serves a five-year sentence. If they both confess, they both serve two years: sentences reduced for cooperation.

The usual representation of strategic games is a decision matrix like the one below, in which action “A” would be staying silent, and action “B” would be “confessing,” also referred to as “defecting.”

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The Prisoner’s Dilemma is interesting precisely because “rational” action leads to a suboptimal outcome: a rational actor should always defect, because so doing both gives a chance at receiving no penalty, and bounds the worst-case outcome to -2. More quantitatively, the average outcome for defecting is -1, and the average outcome for not defecting is -3. However, this means that two rational actors will always defect, and therefore always pay a cost of -2. Two non-rational, “hopeful” actors, meanwhile, will never defect, and therefore do “better” than the rational players.

A modified, iterated version of the Prisoner’s Dilemma is sometimes used to model geopolitics, and therefore gets called the “Peace-War Game.” If a group of players plays the Prisoner’s Dilemma repeatedly, the history of their decisions becomes a factor in how the other players in the game treat them. Specifically, if a player never defects, other players can with confidence choose not to defect when playing them, so that their long-run outcome averages close to -1 per game, rather than the -2 per game of the always-defecting player. On the other hand, if a player has a history of defecting, even peaceful players should defect when playing them, because they can predict that the player will continue their pattern of “bad behavior.” In the context of geopolitics, this is roughly equivalent to saying that the best policy is to remain at peace with one’s neighbors, but respond with force to any neighbors who attack others.

Dark Forest Theory is essentially the Peace-War Game with one major modification. The penalty you pay when your opponent defects is not a small multiple of the penalty if they do not—it is infinite: total civilizational destruction.


When Liu says that he wrote about “the worst of all possible universes,” he means essentially that the universe of Remembrance of Earth’s Past is one in which Dark Forest Theory holds: the laws of physics and the distribution of civilizations in the galaxy are such that it is overwhelmingly likely that the first civilizations we encounter will have the ability to destroy humanity in its entirety, and that within a century or less, we will attain technology sufficient to draw the attention of such civilizations.

Although the some of the specific future-physics Liu uses to sketch this reality are either far-fetched or outright impossible (for instance, using the Sun as a resonator for a massive radio antenna in The Three-Body Problem), the narrative as a whole has an air of chilling plausibility, in large part because of how it discovers, in Dark Forest Theory, an explanation of the so-called Fermi Paradox.

This thought experiment, posed by the physicist Enrico Fermi, asks why, if there are hundreds of millions of solar systems in the galaxy, and the galaxy is only a hundred-thousand light-years wide, have we received no communication of any kind from another civilization? Even if we assume that intelligent life is quite rare, it seems inevitable that the probability of receiving no alien communication whatsoever over the course of human history must approach 0—statistical impossibility.

Dark Forest Theory proposes a succinct explanation for this paradox: communicating with other civilizations means entering the Dark Forest “game,” in which civilizations that can act rationally must attempt to destroy their counter-parties as swiftly as possible. We have not heard from alien civilizations, in other words, because any civilization that attempts to contact its galactic neighbors will be swiftly destroyed, with a probability approaching 1.

“Worst of all possible universes” starts to sound like an understatement.


The second of the books in Liu’s trilogy, The Dark Forest, dramatizes humanity’s discovery of this grim reality. Rather than being the “darkest before the dawn” second act that is now common in modern science fiction semi-serialization (Empire Strikes Back, The Dark Knight, etc), this is instead something of a triumph. Dark Forest ends with a discovery of the principles behind humanity’s defeat in act one, The Three-Body Problem, which are also the principles that will lead inevitably to humanity’s destruction in act three, Death’s End.

The most wrenching parts of the book are the scenes in which humanity discovers Dark Forest Theory on a small scale: in two small groups of interstellar ships, each leaving the Solar System faster than any human-produced object has ever traveled, and the ships each group separated from one other by only a few thousand kilometers. As they reckon with their new reality—only able to put off destruction by the Trisolarans by continuing to accelerate into deep space, and uncertain of having the fuel or food reserves to make it to another habitable planet—they realize suddenly that they have entered a Dark Forest game. Each ship in a group has the armament to kill every person on board the others, an incentive to do so in the food and resources to be gained thereby, and no means to deflect an attack once it has been launched.

Within minutes of this realization, the inevitabilities of Dark Forest Theory set in, and one ship in each group destroys the others, harvesting resources from the hulls that remain. Several light-days later, the hue and cry of each battle finally reaches Earth, where humanity as a whole feels the horror of a Dark Forest universe sink in.

A solar-system defense fleet assembled over centuries was just destroyed in a matter of seconds, an occupation force is bearing down on Earth from inside the orbit of Jupiter, and humanity has just realized that even being members of the same species is not enough to ensure cooperation in the Dark Forest.

At this cliff of greatest despair, Liu delivers the moment of greatest triumph in his trilogy: Luo Ji has found a way, despite the Trisolarans’ technological dominance, to enter a Dark Forest game with them, by creating a trigger that will broadcast the locations of Earth and of their home planet across the galaxy. They can still invade the Earth, but he can reveal them to the larger universe, and in the Dark Forest, to be known is to be doomed. The Trisolaran occupation force starts accelerating out of the Solar System, and humanity suddenly has an indefinitely extended future, freed suddenly from the certain execution delayed only by relativistic physics which threatened through all of The Dark Forest.

The plot of Liu’s trilogy is a combat between the brutality of the Dark Forest and the technological achievement, and therefore agency, of humanity. Luo Ji’s sudden countermove at the end of The Dark Forest is the one moment where the latter briefly seizes the high ground. Unlike our heroic scifi trilogies—Star Wars, Pullman’s His Dark Materials, and the Nolans’ Batman—in which the second installment ends with the grimmest reversal in an overall arc of triumph, Liu’s trilogy of the “worst of all possible universes” ends its keystone novel with the one triumph in an overall arc of despair.

Luo Ji’s triumph is an sudden and wholly unlooked-for stay of execution, exactly because that is the only kind of respite that is possible in a Dark Forest universe. Each society creeps forward through history attempting to escape the notice of all others, and doomed to certain destruction once discovered; the only certain stay on a clash between two civilizations who have discovered each other is for an autonomous and benevolent third party to suddenly appear with a gun pointed at the head of both societies, forcing them for a while to work together in order to stay an individually assured destruction.


Such a respite is exposed for the fragile thing it is in Death’s End, where the human-Trisolaran détente collapses the very second Luo Ji hands off the trigger to Cheng Xin. Concomitant with Dark Forest standoffs in Liu’s worst of all universes is a despair in human institutions and societies under such stress: when the survival of two entire worlds is balanced on the edge of a game-theoretic knife, a human society of compolitized individualities will never be able to play a perfect game for long. Brought to such a precipice, the Brownian motion of politics and emotions will inevitably push us off the equilibrium sooner or later.

In his essay, while explaining his view of the history of human societies, and sketching what would be meant by “the worst of all possible universes,” Liu alludes meaningfully to the experience of American aboriginal peoples, on contact with a technologically superior civilization invading from Europe. “Their experience with contact with an alien civilization,” he says, “seems far closer to the portrayal in Three Body.” In our own history, we have already glimpsed this worst of all universes, in the moments when one human civilization was willing to contemplate (and effectuate) the total destruction of another.

This is the heart of Liu Cixin’s thesis: we have already seen in our history that when two human societies are so alienated from one another to consider destroying one another, the destruction of one of them follows as a certainty, almost as soon as it becomes technologically possible. The insistent march of Manifest Destiny across North America is a repeated confirmation of this observation: early modern technology didn’t allow the sudden domination of an entire continent, but each territory that found itself at the border of the two civilizations remained free only until the politics of the technologically ascendant Europeans decided on another step westward, and that decision never once failed to come, over three hundred years and three thousand miles of history. In the worst of all possible universes, humanity as a whole finds itself thrust inexorably into confrontation after existential confrontation until the loaded dice roll finally comes up snake-eyes, and then, suddenly, there is no humanity left.


To “strive for the best of all possible Earths,” then, humanity must develop a way to see other societies as like instead of as other, and must recognize the risk implied by introducing ever-more-destructive technologies into social and societal confrontations that don’t base themselves on that identification of common purpose. This spring the United States declared that its economic self-interest was more important than the risk of an ecological catastrophe that could destroy the entire modern world. God willing, this is our darkest moment on an arc that bends towards light.

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