The Dark Forest

by Liu Cixin

  “But that’s not right, is it? The universe looks empty. We haven’t seen any other alien life apart from Trisolaris, right?”

  “That’s what we’ll talk about next. Give me a cigarette.” Luo Ji groped about in the dark for a while before taking the cigarette from Shi Qiang’s hand. When Luo Ji next spoke, Shi Qiang realized he had moved to a spot three or four meters away. “We need to increase the distance to make it feel more like outer space,” Luo Ji said. Then he lit the cigarette by twisting its filter, and Shi Qiang lit one of his own. In the dark, two tiny red planets stood in distant opposition.

  “Okay. To illustrate the problem, we now need to establish the most elementary model of cosmic civilization. These two balls of flame represent two civilized planets. The universe is made up of only these two planets, and apart from them there’s nothing else. Erase all of our surroundings. Can you locate that feeling?”

  “Yeah. That’s an easy feeling to find in a dark place like this.”

  “Let’s call these two civilized worlds your civilization and my civilization. They’re separated by a great distance, say, a hundred light-years. You can detect that I exist, but you don’t know any details. However, I’m completely ignorant of your presence.”

  “Right.”

  “Now we need to define two concepts, ‘benevolence’ and ‘malice’ between civilizations. These words themselves aren’t very rigorous in a scientific context, so we’ve got to restrict their meaning. ‘Benevolence’ means not taking the initiative to attack and eradicate other civilizations. ‘Malice’ is the opposite.”

  “That’s a low bar for benevolence.”

  “Next, consider your options for dealing with me. Please remember that the axioms of cosmic civilization should be kept in mind throughout the process, as well as the distance scale and the environment of space.”

  “I could choose to communicate with you.”

  “If you do that, you should be aware of the price you’ll pay: You’ll have exposed your existence to me.”

  “Right. In the universe, that’s no small thing.”

  “There are different degrees of exposure. The strongest form of exposure is when I know your precise interstellar coordinates. Next is when I know your general direction, and the weakest is when I only know of your existence. But even the weakest form of exposure makes it possible for me to search for you, because since you’ve detected my existence, I know that I’ll be able to find you. It’s only a matter of time, from the standpoint of technological development.”

  “But my boy, I could still take the risk to talk to you. If you’re malicious, then it’s my bad luck. But if you’re benevolent, then we could have further exchanges and ultimately be united into a benevolent civilization.”

  “Okay, Da Shi. Now we’ve come to the crux of it. Let’s return to the axioms of cosmic civilization: Even if I’m a benevolent civilization, can I determine at the start of our communication whether or not you are also benevolent?”

  “Of course not. That would violate the first axiom.”

  “So once I’ve received your message, what should I do?”

  “Naturally, you ought to determine whether I’m benevolent or malicious. Malicious, and you eradicate me. Benevolent, and we can continue communicating.”

  The flame on Luo Ji’s side rose up and moved back and forth. Evidently he had gotten up and was pacing. “That’s fine on Earth, but not out in the universe. So next we’ll introduce an important new concept: the chain of suspicion.”

  “That’s an odd term.”

  “The term is all I had at first. It wasn’t explained to me. But, later, I was able to infer its meaning from the words themselves.”

  “Who didn’t explain it?”

  “… I’ll tell you later. Let’s continue. If you think I’m benevolent, that’s not a reason to feel safe, because according to the first axiom, a benevolent civilization can’t predict that any other civilization is benevolent. You don’t know whether I think you’re benevolent or malicious. Next, even if you know that I think you’re benevolent, and I also know that you think I’m benevolent, I don’t know what you think about what I think about what you’re thinking about me. It’s convoluted, isn’t it? This is just the third level, but the logic goes on indefinitely.”

  “I get what you mean.”

  “That’s the chain of suspicion. It’s something that you don’t see on Earth. Humanity’s shared species, cultural similarities, interconnected ecosystem, and close distances means that, in this environment, the chain of suspicion will only extend a level or two before it’s resolved through communication. But in space, the chain of suspicion can be very long. Something like the Battle of Darkness will already have taken place before communication can resolve it.”

  Shi Qiang took a drag on his cigarette, and his contemplative face emerged from the darkness for a moment. “It looks now like the Battle of Darkness has a lot to teach us.”

  “That’s right. The five ships of Starship Earth formed a quasi-cosmic civilization, not a real one, because they consisted of a single species—humans—who were very close to each other. But even so, when they were dealt that dead hand, the chain of suspicion emerged. In actual cosmic civilization, the biological differences between different groups might be as high as the kingdom level, and cultural differences are even further beyond our imagining. Add to this the vast distances between them, and you have chains of suspicion that are practically indestructible.”

  “That means that the outcome is the same, regardless of whether we’re benevolent civilizations or malicious civilizations?”

  “That’s right. That’s the most important aspect of the chain of suspicion. It’s unrelated to the civilization’s own morality and social structure. It’s enough to think of every civilization as the points at the end of a chain. Regardless of whether civilizations are internally benevolent or malicious, when they enter the web formed by chains of suspicion, they’re all identical.”

  “But if you’re much weaker than I am, you’re not a threat to me. So I could always communicate with you, right?”

  “That won’t work, either. Here we need to introduce a second important concept: the technological explosion. I didn’t get a full explanation for this, either, but it was far easier to infer than the chain of suspicion. Human civilization has five thousand years of history, and life on Earth might be as much as a few billion years old. But modern technology was developed over the course of three hundred years. On the scale of the universe, that’s not development. It’s an explosion! The potential for technological leaps is the explosive buried within every civilization, and if it’s lit by some internal or external factor, it goes off with a bang. On Earth it took three hundred years, but there’s no reason why humanity should be the fastest of all cosmic civilizations. Maybe there are others whose technological explosions were even more sudden. I’m weaker than you, but once I’ve received your message and know of your existence, the chain of suspicion is established between us. If at any time I experience a technological explosion that suddenly puts me far ahead of you, then I’m stronger than you. On the scale of the universe, several hundred years is the snap of a finger. And it might be that my knowledge of your existence and the information I received from our communication was the perfect spark to set off that explosion. That means that even though I’m just a newborn or growing civilization, I’m still a big danger to you.”

  Shi Qiang watched Luo Ji’s flame in the darkness as he thought for a few seconds, then looked at his own cigarette. “So I have to keep quiet.”

  “Do you think that will work?”

  They smoked. The balls of flame brightened and their faces emerged from the darkness like the gods of this simple universe, deep in thought.

  Shi Qiang said, “No, it won’t. If you’re stronger than me, then since I was able to find you, one day you’ll be able to find me. And then there will be a chain of suspicion between us. If you’re weaker than me, you could experienc
e a technological explosion at any time, and that would take us back to the first case. To sum up: one, letting you know I exist, and two, letting you continue to exist, are both dangerous to me and violate the first axiom.”

  “Da Shi, you’ve really got a clear mind.”

  “My brain can keep up with yours so far, but we’re only getting started.”

  Luo Ji was silent in the dark for a long time. His face emerged in the weak light of the ball of flame two or three times before he said, “Da Shi, this isn’t a start. Our reasoning has already reached a conclusion.”

  “Conclusion? We haven’t figured anything out! Where’s the picture of cosmic civilization you promised?”

  “If neither communication nor silence will work once you learn of my existence, you’re left with just one option.”

  In the long silence that followed, the two flames went out. There was no wind, and the dark silence turned thick as asphalt, connecting sky and desert into a murky whole. At last Shi Qiang uttered one word in the darkness: “Fuck!”

  “Extrapolate that option out to the billions upon billions of stars and hundreds of millions of civilizations, and there’s your picture,” Luo Ji said, nodding in the darkness.

  “That’s … that’s really dark.”

  “The real universe is just that black.” Luo Ji waved a hand, feeling the darkness as if stroking velvet. “The universe 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. An eternal threat that any life that exposes its own existence will be swiftly wiped out. This is the picture of cosmic civilization. It’s the explanation for the Fermi Paradox.”

  Shi Qiang lit another cigarette, if only to have a bit of light.

  “But in this dark forest, there’s a stupid child called humanity, who has built a bonfire and is standing beside it shouting, ‘Here I am! Here I am!’” Luo Ji said.

  “Has anyone heard it?”

  “That’s guaranteed. But those shouts alone can’t be used to determine the child’s location. Humanity has not yet transmitted information about the exact position of Earth and the Solar System into the universe. From the information that has been sent out, all that can be learned is the distance between Earth and Trisolaris, and their general heading in the Milky Way. The precise location of the two worlds is still a mystery. Since we’re located in the wilderness of the periphery of the galaxy, we’re a little safer.”

  “So what’s the deal with the spell?”

  “Using the sun, I transmitted three images to the cosmos. Each one consisted of thirty points representing the planar projection of a three-dimensional coordinate system containing the position of thirty stars. Combining the three images into three-dimensional coordinates forms a cubic space populated by those thirty points. That represents the relative positions of 187J3X1 and its twenty-nine surrounding stars. There’s also a label pointing out 187J3X1.

  “Think about it carefully and you’ll get it. A hunter in a dark forest, stalking with bated breath, suddenly notices that a piece of bark has been stripped from a tree in front of him. On the eye-catching bit of white wood that’s been revealed is a position in the forest, written in characters all hunters can recognize. What will he think about that location? He’s certainly not going to imagine that someone has laid out supplies for him there. Out of all the possibilities, the most likely one is that the blaze is informing everyone that there’s live prey at that location that needs to be eliminated. It doesn’t matter what motivated someone to leave the mark. What’s important is that the dead hand has stretched the nerves of the dark forest to the breaking point, and it’s the most sensitive nerve that’s most liable to make a move. Suppose there are a million hunters in the forest—the number of civilizations on the billions upon billions of stars in the Milky Way could be thousands of times that. Perhaps nine hundred thousand of them will disregard the marking. Of the remaining one hundred thousand, maybe ninety thousand of them will probe the location, and, after confirming that it has no life, disregard it. But one of the remaining ten thousand hunters will surely make a choice to fire on that position, because for civilizations at a certain level of technological development, attacking may be safer and less of a hassle than probing. If there’s really nothing at that location, then it’s no loss.

  “Now,” Luo Ji concluded, “that hunter has appeared.”

  “That spell of yours can’t be sent anymore, right?”

  “That’s right, Da Shi. The spell needs to be broadcast to the entire galaxy, but the sun’s been locked down, so it can’t be sent anymore.”

  “Was humanity just a step late?” Shi Qiang flicked aside his cigarette end. The flame drew an arc through the darkness as it fell, momentarily illuminating a small circle of sandy ground.

  “No, no. Think about it: If the sun hadn’t been sealed off, and I had threatened Trisolaris with sending out a spell against them, what would have happened?”

  “You would’ve been stoned to death like Rey Diaz. And then they would have enacted legislation to prohibit anyone else from thinking along those lines.”

  “That’s right, Da Shi. Because we’ve already revealed the distance between the Solar System and Trisolaris as well as our general heading in the Milky Way, exposing the location of Trisolaris is tantamount to exposing the location of the Solar System. That’s a death strategy. Maybe we’re a step late, but it’s a step that humanity would never be able to take.”

  “You should have threatened Trisolaris back then.”

  “Things were too weird. I wasn’t certain about the idea at the time, so I needed to confirm it. After all, there was plenty of time. But the real reason was that, deep in my heart, I really didn’t have the mental strength. I don’t think anyone else would, either.”

  “Thinking about it now, we shouldn’t have gone to see the mayor today. This situation—if the world learns about it, then it’s even more hopeless. Think about how the first two Wallfacers ended up.”

  “You’re right. The same thing would happen to me, so I hope that neither of us says anything. But you still can, if you want. Like someone once told me: Either way, I’ve fulfilled my duty.”

  “Don’t worry, my boy. I won’t say anything.”

  “Regardless, there’s no longer any hope.”

  They walked along the embankment and to the highway, where it was slightly less dark. The sparse lights of the residential area in the distance were enough to blind them.

  “There’s one more thing. That person you mentioned?”

  Luo Ji hesitated. “Forget it. All you need to know is that the axioms of cosmic civilization and the theory of the dark forest were not my invention.”

  “Tomorrow I’m going into the city to work with the government. If you need any help in the future, just say the word.”

  “Da Shi, you’ve helped me more than enough. I’ll head into the city tomorrow, too, to the Hibernation Immigration Bureau, to take care of waking up my family.”

  * * *

  Contrary to Luo Ji’s expectations, the Hibernation Immigration Bureau said that reawakening Zhuang Yan and Xia Xia was still blocked, and the bureau’s director made it clear to him that his Wallfacer powers were ineffective in this regard. He consulted Hines and Jonathan, who were unacquainted with the details of the situation, but they told him that the revised Wallfacer Act contained a provision stating that the UN and Wallfacer Project Commission could take all necessary steps to ensure that the Wallfacer remained focused on his work. Which meant that, after two centuries, the UN was once again using Luo Ji’s situation as a tool to coerce and contro
l him.

  Luo Ji requested that his hibernator settlement remain in its current state and be kept free of outside harassment. This request was faithfully executed. The news media and the masses of pilgrims were kept at a distance, and after calm was restored throughout New Life Village #5, it was like nothing at all had happened.

  Two days later, Luo Ji attended the first hearing of the revived Wallfacer Project. He didn’t go to the underground UN headquarters in North America, but attended by video link from his spartan residence in New Life Village #5, where scenes from the assembly appeared on the ordinary television in his room.

  “Wallfacer Luo Ji, we were prepared to face your anger,” the commission chair said.

  “My heart has been burnt to ash. It no longer has the capacity for anger,” Luo Ji said, reclining lazily on the sofa.

  The chair nodded. “That’s a wonderful attitude. However, the commission feels you ought to leave your village. That little place isn’t a worthy command center for the defense of the Solar System.”

  “Do you know about Xibaipo? It’s an even smaller village not far from here. Over two centuries ago, that’s where our nation’s founders commanded one of the largest offensives in history.”26

  The chair shook his head. “Clearly you haven’t changed at all. Very well. The commission respects your habits and choices. You should get to work. It’s not going to be like back then, with you claiming that you’re always at work, is it?”

  “I can’t work. The conditions for my work no longer exist. Can you harness stellar power to broadcast my spell into the universe?”

  The representative of the Asian Fleet said, “You know that’s impossible. The droplet’s radio suppression of the sun is continuous. And we don’t anticipate that it will stop for the next two or three years, by which time the nine other droplets will have reached the Solar System.”

  “Then there’s nothing I can do.”

  The chair said, “No, Wallfacer Luo Ji. There’s one important thing you haven’t done. You haven’t disclosed the secret of the spell to the UN and SFJC. How did you use it to destroy a star?”