There is a paradox at the heart of The Dark Forest, the second novel in Cixin Liu’s bestselling, multi-award-winning science fiction trilogy, Remembrance of Earth’s Past: Given the age and vastness of the universe (13.8 billion years and counting, 10 trillion galaxies), mathematical probability suggests that intelligent life should have emerged on many planets throughout the universe. Even if the frequency of such emergence were infinitesimally small, given the number of stars and planets involved there could be millions, or even billions of intelligent civilizations distributed throughout the universe. And yet, from our vantage on Earth, we have so far observed no evidence of extraterrestrial intelligent life; we seem to be alone in the universe. This is the Fermi Paradox, named after physicist Enrico Fermi, who popularized it in the 1950s, famously asking, “Where is everybody?”
Liu poses Fermi’s question obliquely in the opening pages of The Dark Forest, in a prologue that overlaps the conclusion of the trilogy’s sublime first book, The Three-Body Problem. Luo Ji, a young sociology professor and former astronomer, visits the gravesite of a friend, a brilliant physicist who has recently taken her own life. There he encounters Ye Wenjie, the physicist’s mother, and one of the key characters in The Three-Body Problem. To Luo Ji’s surprise, and seemingly apropos of nothing, Ye Wenjie suggests that he become the founder a new field of study, cosmic sociology, based on the statistical assumptions of the Fermi Paradox:
But what’s the point of cosmic sociology, Luo Ji wonders, if Earth is the only civilization in the universe? Ye Wenjie, who has already glimpsed a possible solution to the paradox through her experiences in the first novel, is undeterred. She provides Luo Ji with two axioms upon which to build the discipline:
In addition to these axioms, Ye Wenjie introduces two other important concepts Luo Ji will need to consider if he is to develop a theory of cosmic sociology: “chains of suspicion and the technological explosion.” There isn’t time to explain any of this before Ye Wenjie exits stage left to rejoin the plot of The Three-Body Problem, leaving Luo Ji with the promise that he is clever enough to figure things out on his own—a promise he will struggle to fulfill throughout The Dark Forest.
After the prologue, the novel picks up where The Three-Body Problem left off: humanity has made first contact with a hyper-advanced alien race called the Trisolarans, and the results aren’t what Carl Sagan hoped for. There will be no interstellar cultural exchange, no transcendent communication by harmonic modes. This is a story of existential crisis: Desperate to escape their inhospitable planet, which is doomed to be consumed by one or more of the three suns in its solar system, the Trisolarans have dispatched a thousand warships to wipe out humankind and claim Earth as their new home. The fleet will take 400 years to reach Earth. In the meantime, the Trisolarans have placed an unbreakable quantum lock on human technological progress by putting proton-based supercomputers, called “sophons,” into orbit around Earth (one of the most astonishing and original conceits of the first book). The sophons disrupt fundamental scientific research, thus preventing humanity from progressing beyond its current (primitive) understanding of the universe. Through quantum entanglement, the sophons also allow the Trisolarans to observe and communicate with Earth in real time. The Trisolarans’ message to humanity: “You’re bugs!”
As he did with the first book, Liu constructs The Dark Forest as a sprawling, complex story that juxtaposes cosmic scale with human frailty and resilience. The nations of Earth respond to the looming threat of annihilation first with defiance, then with despair. Against an enemy so technologically advanced, there is no hope of a military victory, not without paradigm-shifting advances in fundamental science—advances that are now impossible, thanks to the sophons. If defeat is a certainty, then only one logical course of action remains: humanity must escape from the solar system before the Trisolarans arrive. But Escapism raises an impossible ethical dilemma: how to decide who will be granted the privilege of survival, and who must stay behind to perish on Doomsday?
In less enlightened times, answering this question would have been simple. The nobles would have invoked the divine right of kings, packed up their kin and the crown jewels, and blasted off into the stars without so much as a conciliatory wave, while the rest of us commoners waited around to be vaporized by alien death-rays.
Now, thanks to the Renaissance, the American and French Revolutions, and the triumph of humanism, the matter of who will go and who will stay is not so simple. Faced with the ultimate existential crisis, the very ideals of fairness, equality, and human dignity that enabled us to progress socially as a species become a trap. These ideals demand that either everyone goes, or everyone stays. And since not everyone can go, no one may go. Escapism is declared a crime against humanity. The human race will hold to its highest ideals and defend them even unto extinction.
This conflict between individual self-interest and social cooperation based on empathy and mutual benefit is the novel’s second paradox, and one of its dominant themes. That it turns out to be inextricably linked to Fermi’s paradox is a testament not only to Liu’s skills as a storyteller, but also to the considerable thought he has devoted to the ideas that inspire his fiction.
Unable to flee the Trisolaran threat, humanity must stand and fight. After this decision is made, The Dark Forest spans two centuries of preparations for the Doomsday Battle, involving not just a race to develop a militarized space fleet but also a desperate attempt to keep at least some part of Earth’s defense strategy secret from the Trisolarans. Four individuals, dubbed “Wallfacers” in honor of ancient Eastern meditation practices, are selected to formulate defense plans in the secrecy of their own minds. Since human thought is the only refuge from sophon surveillance, the Wallfacers must not communicate the true intent of their plans to anyone. In theory this will preserve an element of surprise, possibly giving the human space fleet one much-needed advantage in the Doomsday Battle.
Luo Ji finds himself selected to be a Wallfacer, despite his objections and for no reason he can understand. By his own admission he is neither ambitious nor unusually intelligent. He confesses to not feeling much empathy for his fellow humans, and he wants no part of the fight against the Trisolarans, which he considers futile. His only desire is to live his life in peace and die before the crisis consumes every pleasure of the late Golden Age of humanity. He is also the only Wallfacer the Trisolarans are trying to kill.
If you’re thinking this all points back to cosmic sociology, then you will appreciate how deftly Liu weaves Ye Wenjie’s axioms and concepts into the plot in ways both subtle and explicit, especially the concept of chains of suspicion. The dramatic culmination of these ideas is a haunting set piece that plays out against the darkness of deep space, a darkness so complete and unrelenting that it holds sway over life itself:
Darkness abounds in The Dark Forest, but not as a cause for despair. Instead, Liu invites us to not be afraid, and to find beauty, hope, and indeed love in the fleeting moments of light we call life.
Cixin Liu, The Dark Forest, English translation by Joel Martinsen. New York: Tor, 2015.