This is the third and last book of the Three Body Problem series; the previous book is The Dark Forest.

The UN had just created The Stars Our Destiny program to raise money, where one could buy rights to a star. That day, a friend of Yun Tianming, a shy astrophysicst, gave him a large sum of money because of Yun Tianming’s help in starting his business. Yun Tianming had decided to die, since he had nothing in life to live for. The only light in his life was a classmate, Cheng Xin, a girl who had befriended him on occasion. Since he had no use for the money, he bought her a star (one of the very few to purchase one), anonymously.

It was the Crisis Era, shortly after the Trisolaran fleet had been confirmed, and Cheng Xin was part of an international team under the direction of a acerbic American named Thomas Wade. Their job was to brainstorm ideas to save Earth. Cheng Xin eventually came up with the only usable idea they had: send a person to the Trisolaran fleet who would work from the inside to save Earth. Despite knowing the mission (due to the sophons’ ever-present monitoring), the Trisolarans would want to investigate an actual example of humanity. The only problem was that in order to intercept the Trisolaran fleet’s flight path in a usable timeframe, the ship would need to accelerate to nearly light speed, which was not possible. Cheng Xin’s idea was that they could send just a brain, in hibernation, so that the ship was very light. If they had humanity’s nuclear bombs pre-placed, they could detonate after the ship passed and accelerate it. Cheng Xin found out about Yun Tianming’s suicide just in time to rescue him, and he agreed to be loyal to humanity and be sent as a brain to the Trisolaran fleet. Cheng Xin sent some seeds along with him, but Wade’s price for the extra weight was that she hibernate until contact was made, because humanity would need someone who knew him well. Unfortunately, one of the bombs at the end did not detonate quite accurately and the course of the ship veered away from the fleet. In the end, the Staircase Project was another one of the Crisis Era projects, like the Wallfacer Project, that ended up not working out well, and by the time Cheng Xin was woken up, it had been forgotten.

She was woken up over 200 years later, during the Deterrence Era, after Luo Ji had discovered the dark forest deterrence: the risk of Earth transmitting the coordinates of Trisolaris (which would assure destruction, just in case, by other races in the galaxy) was enough to ensure Trisolaran cooperation. Cheng Xin was woken up because a woman named 艾AA (it was cool to have an English-Chinese name in this period) had discovered to planets orbiting Cheng Xin’s start, one of which was earth-like, and the UN wanted to buy the star from her. She refused, and eventually they reached a compromise: she felt it her duty to humanity to allow the use of the planets (and energy from the star), but she wanted to keep Yun Tianming’s gift, so she kept ownership of the star. The price was somewhat less, but still astronomical.

In the Deterrence Age, everything was feminized, so much so that AA had to show Cheng Xin what men looked like, since they seemed to her to be indistinguishable from women. This called into question how deterrence worked. At present, Luo Ji was the Swordholder, the person who could press the button to send out the signal (which now was sendable through gravity-wave transmission). But he was over 100 years old, and there was a search for a new Swordholder. Wade had been woken up, too, and was in the running for Swordholder, and shot Cheng Xin with a Common Era projectile gun, since he felt that she would be the top-running candidate. He was correct, and Cheng Xin was chosen as Swordholder.

Being Swordholder meant living in a underground bunker, facing a white wall and being ready to press the button if the wall lit up with information about a Trisolaran attack. Luo Ji had done it for over fifty years. But fifteen minutes after Cheng Xin, who had a motherly streak, took over, the Trisolaran droplets started moving on an intercept course to destroy the gravity wave transmitters. Cheng Xin refused to press the button, because she could not destroy a world, even if it were an enemy world.

The Trisolaran heavier-than-matter droplets penetrated the crust and easily destroyed the gravity-wave transmitters. Humanity, which had lauded her as the pinnacle of civilization now hated her. And the Trisolarans, who had been transmitting knowledge to humans and had even (they claimed) made changes to their society after what they saw in humanity, promptly ordered all humans to relocate to Australia within a year. The Trisolaran mouthpiece was a elegant, expressive, feminine robot named Sophon. Sophon organized an army to enforce this. Then, after humanity (except for a small number of rebels) complied, she had the army destroy electrical generating capability on Australia, ensuring that much of humanity would die (since Australia’s carrying capacity without electricity was hardly 4.5 billion).

Now, over fifty years prior, humanity had sent a transmission to Bronze Age and Blue Space, the two surviving ships from the defeat by the droplets, inviting them to return. Only Bronze Age did, and they were arrested for crimes against humanity (killing their companion ship, in a miniature dark forest situation). But when the captain was taken to Bronze Age to return control of the ship, he sent a brief message to Blue Space not to return, as he was shot. Having a ship that knew the location of the Solar System and Trisolaris was a risk to both worlds, so humanity sent Gravity (now with faster engines) to intercept it. Gravity had a gravity-wave transmitter, so the the Trisolarans insisted on sending a droplet, just in case.

Some distance outside the solar system, the quantum link with the sophons was lost. Blue Space allowed itself to be captured, but they had discovered that they were in a place where four-dimensional space was sometimes accessible. Since four-dimensional space allows the inside of three-dimensional space to be visible (just like we can see both dimensions on a sheet of paper at once), they were able to disable the droplets. Thus, when they received the transmissions from earth a year after the Trisolaran takeover, they were able to prevent destruction by the droplets. The crew of Gravity voted to send out the transmission. As soon as the Trisolarans received the gravity-wave transmissions, they promptly vacated the solar system. (Although the coordinates of the solar system were not transmitted, there was enough communication between Trisolaris and Earth that once the location of one was known, the other was essentially known as well.) The destruction of Trisolaris was quick, after about six years, and although the Trisolarans had evactuated their planet, the ships were not far enough and were destroyed, leaving only the former attack fleet as the surviving Trisolarans. The sun’s destruction was almost sure, but it was unknown when.

Eventually the Trisolarans evacuated their sophons and even Sophon herself. But she allowed Luo Ji to ask one question, as a gesture of the high respect they held him in. He asked if there was a way to broadcast to the universe that a star was safe, and she said yes, but gave no details. Before Sophon left, the Trisolarans arranged a virtual meeting between Cheng Xin and Yun Tianming, on the condition that they could not exchange any strategic information. The Trisolarans had intercepted Yun Tianming’s ship, and cloned him. He was a handsome, confident man now, and grateful for the seeds that Cheng Xin had sent him. They were prevented from even hinting at anything useful, so Yun Tianming suggested that they tell each other stories, like they used to as children. (In fact, they had not known each other until college, although they grew up in the same city, but Cheng Xin was alert and went along with it.) Yun Tianming had written up a number of the tales and published them, and they were popular among Trisolarans. He chose three to tell, which blended in with each other.

There was a kingdom with a beautiful princess. However, the king was old, and had designated the princess as his heir, instead of the eldest brother. The eldest brother was angry, but he pretended to be okay with it. Then he went out to a kingdom known for its high quality goods and brought back a painter who could paint so life-like that the subject of the portrait would be sucked into the painting. All that was required was for him to glance quickly at the subject. The eldest son introduced the painter to the court and promised beautiful paintings. That night, the painter took some special paper from where he was from, and painted each member of the family. But while he was still painting the princess’ portrait, an old man twirling a strange umbrella arrived. He was the artists’ teacher, and he twirled the umbrella because it was old and broken (although it was made of dragon bones) only the centrifugal motion kept the umbrella up, and only if he was under it was he protected from being sucked into the painting his student had made of him some time prior. He gave the umbrella to the princess and vanished into his painting (but he was content—he knew his student was excellent and it would be a perfect likeness).

So the princess’ nurse and a guard quickly left the palace and went to find the third son, who had gone to Tomb Island in the middle of the Glutton’s Sea, some years ago and had not returned. Now the kingdom was an island, and some years ago Glutton Fish had been released into the sea, which cut off all access to the outside world, because the Glutton Fish would eat anything in the water, including a wooden boat. A long time ago, before the Glutton Fish, the nurse had come into possession of some bars of soap from the kingdom of high quality goods. True soap from this kingdom had no weight, because it was made of pure bubbles, and on contact with water it would produce bubbles, which produced bubbles. She had used the soap to bathe the princess over the years, but she still had some bars left. It turned out that the bubbles made the Glutton Fish feel so happy that they just enjoyed the feeling and did not eat anything. So they put the bar of soap in front of a boat that was on the beach, and rowed safely across to Tomb Island, which they could see in the distance.

Now the princesses’ other brother, Prince Deep Water, had a unique property, that he always remained the same size, no matter how far away he was. The evil painter had not been taught how to paint in this style, so he could not paint the prince. There was still half a bar of soap left, and the prince was glad to be able to return and confront the eldest brother. The two brothers were equally skilled swordsmen, but because the prince did not obey the laws of perspective, it was hard to judge his distance, and he killed his brother. Then the captain of the guard (who had traveled with the princess) rushed to the room of where the paintings were. There was no one there, although there was a stunning self-portrait of the painter, still a little went. The captain found the painting of the princess, which was no less beautiful than in real life, and burned it. When the princess came in, she let the umbrella slow down (and it emitted a louder and louder cry in warning), but she stayed alive. Then she said she wanted to leave the island kingdom and explore the world, and left it in the hands of her brother, the prince. He was more suited to rule, anyway, she said. The captain said he would accompany her, to protect her. The princess took out the remaining two bars of soap, with which they crossed the Glutton Fish and explored the outside world.

After the stories, their time was up, and they promised to meet again at Cheng Xin’s star.

Cheng Xin had been alone, with no recording devices, so she was promptly debriefed by the government, in a special room with electrified walls that was impervious to sophons (in case they had not actually left). Over several sessions they tried to understand what Yun Tianming was trying to tell them. Eventually AA, who had been CEO of Cheng Xin’s corporation the Halo Group, said she wanted a bath with real Common Era soap and demanded Cheng Xin buy her some. So they bought a bar in a museum for an exorbitant price, and then AA demanded a paper boat. She placed a bit of the soap in the back of the boat and it zoomed across the water: the soap lowered the water tension in the back of the boat, causing the water tension in the front to pull the boat. One thing Yun Tianming was telling them that curvature propulsion drives, which could travel at close to the speed of light, could exist. Cheng Xin directed AA to have the Halo Group research curvature propulsion.

Yun Tianming’s stories generally had a two part interpretation. The story created a metaphor, which then pointed to a physical solution. The meaning of the umbrella was deciphered in two parts: it clearly represented a flywheel governor and related to the speed of light. Then, someone realized that the name of the kingdom with the magic products was the name of two Norweigan towns joined together. This was the location of the Maelstrom, which was sort of a water black-hole: below a certain speed, a boat could not leave the Maelstrom once it went over the lip. From this it was clear that the story was saying that protection could be had by lowering the speed of light to the escape velocity of the solar system (about 16 km/sec). Any incoming bullet of destruction would instantly slow down upon entry (because nothing can go faster than the speed of light), causing an internal shockwave which would destroy it. At the same time, it would be clear to a technologically advanced observer than nothing within the star’s system could ever escape, and thus the star was completely safe.

Humanity came up with three plans of protection. One was the Bunker Project: build space cities behind the giant planets, so they would be shielded from the death of the star in the event on an attack. Since they would already be used to surviving in darkness, the loss of the sun would not be a problem. The second was to create a “black domain” around the solar system by lowering the speed of light; this was deemed completely infeasible. The third was to create a lightspeed fleet. Shortly afterwards, a false alarm of a “photoid” (a star-destroying bullet travelling near lightspeed) by the attack system early warning observatory caused a massive panic and the UN outlawed research on curvature propulsion. (AA wanted to escape in the Halo Group’s ship, but true to character, Cheng Xin refused to leave when everyone else could not, and also because liftoff pad was blocked and they would have to engage their engines where they were parked, which would kill the crowds of people around. Other people rich enough to have a private rocket did not have such scruples.)

Lacking the ability to change fundamental constants of nature, humanity proceeded along the Bunker Plan. But Wade, who had just been release from prison for the attempted murder of Cheng Xin, found her, because he knew that they both wanted curvature propulsion drives. But she lacked the ruthlessness to do whatever it took to push it to completion, and they both knew that he did. He asked her to turn over the company to him. She agreed, on the condition that if the research had the possibility of harming the human race that she be awakened, and if she said to stop that he would stop. He agreed, and both she and AA went into hibernation.

Sixty-two years later they were awakened. Unlike AA’s Deterrence Age culture of femininity and information-saturation, the people seemed more like Common Era people, especially the men. Halo Group had built a giant particle accelerator near the orbit of Jupiter (since space is essentially a vacuum, there was no complete structure, just accelerator stations around the orbit. They discovered that the Trisolarans had given them false information during the information transfer period. The Bunker Project, however, was a success, and city-sized habitats of various shapes orbited all the giant planets. She toured all the different varieties before arriving at the renegade habitat of Halo City.

Halo Group was a prestigious corporation with its own research habitat, which was now being blockaded by the Federation for researching curvature propulsion. Wade demonstrated curvature propulsion to her, and said that they had been creating antimatter in the particle accelerator for the past four years, and now they had a group of soldiers armed with guns that shot bullets with antimatter suspended in them. One shot could take out a ship. Wade expected that once they explained this to the Federation, they would see the uselessness of a war, and Halo Group would be permitted to continue developing the curvature propulsion drive. But Cheng Xin saw only ruthless men who would stop at nothing to achieve their goal, and of easily ruptured habitats filled of thousands of people, and she told him to stop. A little to her surprise, he did, and Halo Group surrendered to the Federation, and Wade was sentenced to death and executed for crimes against humanity. Halo Group was dissolved, although the Federation returned to Cheng Xin an amount roughly equivalent in value to what it had been before Wade took over. Cheng Xin and AA went back into hibernation.

The experience of Blue Space in the four dimensional world, where they observed many alien habitats, revealed that life was common. One species was flying relatively near the solar system. There was a civil war between the homeworld and a colony world, and it seemed that the homeworld had converted to two-dimensional life to avoid being destroyed by the colony world—survival at all costs. They noticed the transmissions between Earth and Trisolaris, observed that markings around Trisolaris that indicated it was a danger and that it had been quickly cleansed. But the other system that communicated with them (the Earth system) had not cloaked themselves, despite enough years elapsing to have developed the ability. If they did not want to cloak themselves (by slowing light speed), then they were dangerous. The system could not be destroyed with a mass device, though, there were corners of safety in amonst the planets, so a different device, a sort of sheet of paper was launched instead.

Cheng Xin and AA were awakened when the “paper” arrived in the Solar System, just a few years after they went to sleep. They were taken to Halo, an upgraded version of the shuttle that Cheng Xin owned, and told to go to the repository of human art on Pluto, take some artifacts, and attempt to leave the system. The “paper” was a two-dimensional space, and sucked the three-dimensional normal space into it. Now Cheng Xin finally understand Yun Tianming’s meaning behind the paintings. As it expanded everything became flattened into two dimensions, so that, again, everything was visible at once, and in vivid detail, but since it three dimensions projected onto two, the shapes were not always their expected ones. Unfortunately, the Solar System was doomed—the required speed to escape was light speed.

They went to the archive on Pluto, where humanity’s extensive research showed that the best way of preserving information for a billion years was to carve it into rock. There they met Luo Ji, who helped them load up art onto the shuttle. Then, as the “paper” expanded towards them, he instructed Halo‘s computer to accelerate to light speed. The Federation, had decided to research curvature propulsion after all, and developed a surreptitious partnership with Halo Group. They had just completed a prototype, which was installed on Halo. Luo Ji had no desire to come, he was old, and was content to go into the painting. Eventually the painting consumed the entire solar system, including the sun, and the light vanished. Cheng Xin set the course to her star.

It took 286 years to travel there, but only about a two days of their time. When they arrived, they found two planets, as AA had discovered, although the plant life was strangely colored (partly because the star was much dimmer than the sun), and could walk, although it was still plants. There was a radio beacon from a human ship on the planet, which turned out to be a small craft piloted by Guan Yifan, a member of Gravity‘s crew. Yun Tianming did not appear to be there, to Cheng Xin’s great disappointment. AA and Guan Yifan, however, were quickly falling in love.

A ship landed on the other planet, and then took off again, and Cheng Xin insisted on visiting to see if it was Yun Tianming. She went with Guan Yifan on his ship. There was no sign of anyone on the planet, and but there were some black pillars, which Guan Yifan explained were regions where the speed of light was almost zero, created when a ship used curvature propulsion to accelerate to light speed almost instantaneously. Curvature propulsion produced regions of slow light speed behind it, just as the soap-boat produced low-surface tension behind it. Thus, a symmetrical fleet of ships accelerating to light speed could create a black domain. Her choice to save humanity from the (possible) destruction of war by ceasing research into curvature propulsion had, in fact, doomed it to destruction by two dimensions.

Guan Yifan explained to her that almost all physical constants could be weaponized in some form like this. Life in the universe kept the location of its worlds a closely guarded secret. He explained that the average speed of light in the universe was gradually slowing down due to the use of curvature propulsion, just like soap boats eventually come to a stop because the soap eventually spreads out over the entire surface of the water make the entire area low-tension. Perhaps the primordial universe had the full ten dimensions, instead of seven of them being wrapped into a tiny space, but weaponization of dimensions had reduced them. However, Cheng Xin was glad to know that both humanity and the Trisolarans had not died out and still had civilizations.

They were traveling back to the first planet, when Yun Tianming arrived. His arrival disturbed the fragile black pillars, which quickly expanded to engulf the entire system. However, because Cheng Xin and Guan Yifan were traveling faster than the new light speed, they ended up at light speed, causing severe time dilation. Also, the computer on their ship stopped working, because the speed of light was now dozens of kilometers per second, which was too slow for computation. Guan Yifan’s ship had been built with precautions, and he was able to boot up a neural computer specially built for the purpose, but it would take twelve days. Although they would only experience two days, Yun Tianming and AA would experience much longer; they would certainly be dead. Fortunately, Cheng Xin was falling in love with Guan Yifan due to his help in getting her into a life-support suit when life-support failed as a result of the computer failing.

When they finally made it to the planet and landed, they discovered that that almost 19 million years had passed. There was no sign of humans or a shuttle, or anything. A deep scan of the planet revealed a partially extant message carved into the rock in large letters, saying that Yun Tianming and AA had had a happy life, and recommending that they “survive the collapse inside”. Perhaps they had children, perhaps even an entire civilization; perhaps not, it was impossible to know. A strange door appeared, and they went in.

Inside they found a house, and Sophon, who told them that Yun Tianming had commissioned an extra-dimensional space for her, which existed outside the universe. From its safety they could survive the collapse of the universe, which since time passed differently, they would live to experience. They learned to read Trisolaran and studied the principles of their universe (both being scientists). They wondered what the new universe born after the collapse of the current “great universe” would be like. Perhaps time would be multi-dimensional—one could make multiple decisions simultaneously, and certainly one of those choices would be right. After a two years (their time), they received a broadcast transmission from the great universe, broadcasting to all the mini-universes, in 1.57 million languages (including Earth and Trisolaran, indicating that some of each race had survived in their arks). The message was from the Returners, a race that wanted the universe to collapse so it could be reborn fresh again. They had discovered that the great universe had very close to the mass necessary to collapse, but the mini universes had removed enough mass that it would no longer collapse. The message requested that the mini universe return the mass back to the great universe.

Cheng Xin insisted that they return their mass, even though enough time had passed that Sophon could only find a barely habitable planet. Cheng Xin, as she always had, that concern for others is what makes them human. So they deconstructed their universe. The only thing Cheng Xin left to survive into the new universe was her memoirs, and a closed terrarium of two fishes and some algae.

Death’s End is indeed the exploration of ideas that I had hoped The Dark Forest would be. Liu Cixin describes multiple time periods with various situations and extrapolates how humanity reacts. These range from the failure of deterrence due to humanity’s concern for other life, to the beginnings of Trisolaran colonization and how humanity deals with lack of resources, to the success and flourishing of the Bunker Plan and its space habitations, to the failure of the plan and the destruction of humanity, and even to a plan for surviving the death of the universe. These different ages are well fleshed-out, and are a realistic and interesting exploration of the ideas.

Although the book is largely exploration (in a similar vein as Diaspora, although with much different ideas), there are a few consistent themes. One theme is that life prioritizes survival, and that the dark forest scenario plays out not only cosmically, but also in any situation where resources are limited. As such, humanity quickly turns against each other when the electricity is destroyed in Australia. A secondary theme is Cheng Xin living out true humanity, which cares for others, even though that is not a very compatible strategic plan in a dark forest situation.

Like the rest of the series, this book has some elements that feel very unusual, which I assume can be attributed to the fact that the author is Chinese, not American or British, like most science fiction an English speaker is likely to read. The idea of subtly hiding messages in stories is interesting, and not something one usually encounters. I wonder if the idea is much more obvious to a Chinese writer, since CCP censorship requires puns and riddles to safely speak politically. Also, while science fiction, particularly from the Bradbury era, is frequently pessimistic about humanity, nothing I have read has really even suggested that the universe is fundamentally unsafe. In software development one eventually develops a habit of attempting to break each line of code as you write it (otherwise someone else will end up breaking it and you will have a bug to fix); the dark forest scenario is sort of like Liu Cixin trying to break civilization. The Western mindset readily assumes that we will “succeed”, because for the past 500 years the West has largely succeeded at what it attempted. But the CCP has created a culture where even your neighbors can be weaponized against you, and since the laws are intentionally written vaguely, your life can be destroyed at any time. The Chinese world is not a fundamentally safe one, so it may be easier to expand that to another level and with that possibility, explore how species interact in the universe. On the other hand, the dark forest scenario is similar to political scientist John Mearsheimer’s realist school: relationships between nations are governed by the fact that there is one above us to enforce anything; fundamentally nations cannot trust each other. Perhaps the exploration of the dark forest is simply galactic realpolitik and not informed by culture at all. Perhaps political scientists rarely write science fiction.

Another unusual characteristic is that there is not really a resolution. Humanity goes from fear of destruction, to confidence in safety, to destruction, to outliving the death of the universe, and finally the last two humans (that we know of) choose to return to mortality. Part of the lack of resolution is probably due to the fact that China does not have a Judea-Christian mindset with a God who has a plan for the world, which is why Western literature generally does have a resolution. However, another part is due to the theme and plot being at least partially at odds. The plot is humanity’s struggle for survival in a hostile universe, as sort of man-vs-nature plot. The theme is that the essence of humanity is that we do not take the dark forest realpolitik strategy, that the essence of being human is to care for others. The book ends with the theme, but most of the plot has been on a completely different theme.

Despite living in China, the book assumes that the trajectory is self-governance via democracy when we are being human, and totalitarianism when we regress to focusing on survival. Given that the story has a message-in-a-story, is it reasonable to think that the book is also a subtle critique of Chinese government? Perhaps it is even a critique of broader Chinese society, that it is so survival focused that it acts totalitarian and is not following the essence of humanity shown by Cheng Xin? On the other hand, does Liu Cixin actually think that self-governing democracy is really the arc of history (perhaps with the caveat that democracy is not appropriate for modern China because of its survival-focus), or is that simply how the science fiction genre is supposed to work?

Liu Cixin has written an expansive exploration of life in a dark forest universe. He travels through hundreds of years of history and explores multiple societies with a result that feels very natural, even inevitable. The world-building is some of the best I’ve read, with societies believably based around certain social values and realities. The technology (and even the dark forest idea) are based on actual scientific speculations, although there is a judicious use of technomagic in the form of the dimensional attack. He even works through what consequences of using curvature propulsion and dimensional attacks might be (namely reduction in the average speed of light, and reduction in the number of usable dimensions, respectively). The plot is a little more strained, although the dark forest attack does keep a coherent framework. Characterization is the weakest part, as the characters largely remain unchanged. But, since the book is about exploring ideas, that is not a huge failing. This is the best book of the series, with creative ideas that rewrite the rules of the game in every section.