Primordial darkness.

Cheng Xin and Guan Yifan were once again immersed in a time vacuum. The sensation was similar to when they had entered reduced lightspeed back in the shuttle. Time did not flow here, or maybe it was more accurate to say that time did not exist. They lost all sense of time, and experienced again that feeling of stepping across time, but existing outside of it.

Darkness disappeared; time began.

The sun brightened. It did so very gradually: At first it was just a disk, and then the light began to unveil the world. It was like a song that began as barely audible notes, then grew and grew into a mighty chorus. A circle of blue appeared around the sun, expanded, and turned into a blue sky. Under the blue sky, a pastoral scene slowly took shape. There was an unplanted field with black soil; next to it was an exquisite white house. There were also a few trees that brought a hint of the exotic with their broad, strangely shaped leaves. As the sun continued to brighten, the peaceful scene appeared like a welcoming embrace.

There is no appropriate expression in human language to express the moment at the start of time. To say that time began after they entered the door would be wrong because "after" required time. There was no time here, and thus no before or after. The time "after" they entered could have been shorter than a billion-billionth of a second, or longer than a billion billion years.

"That's us," said Cheng Xin.

They could see the backs of two figures standing on the horizon: a man and a woman. The man had just put down his uplifted arm.

In front of those two figures, they could see a distant white house and trees, exact duplicates of the ones nearby. They couldn't see what was at the feet of those figures due to the distance, but they could guess that it was another black field. At the end of the world was a duplicate of it, or maybe a projection.

Duplicates or projections of the world existed all around them. They looked to their sides and saw the same scene repeated. The two of them also existed in those worlds, but all they could see were the backs of those figures, who turned their heads away as Cheng Xin and Yifan turned to look at them. They looked behind them and saw the same thing -- except now they were looking at the world from the other direction.

"There are people here!" Guan Yifan pointed at the distance.

They followed a path of stepping-stones, and around them, the copies of themselves in the copies of their world walked along with them. The path was broken by a brook with no bridge over it, but the brook was so small that they could step over it. Only now did they realize that gravity was a standard 1G. They passed the copse of trees and came to the white house. The door was shut and the windows covered by blue curtains. Everything looked brand new, dustless -- as a matter of fact, they were brand new, also, as time had just begun to flow.

The entrance to the world had disappeared.

In front of the house was a pile of simple, primitive farming tools: shovels, rakes, baskets, water pails, and so on. Although some of them were shaped a bit oddly, it was easy to tell their function by appearance. What most drew their attention, though, was a row of metal columns erected next to the farming tools. They were about the height of a person, and the smooth surfaces glinted in the sunlight. Each column had four metal attachments that seemed to be folded limbs. The columns were probably robots in a resting state.

They decided to familiarize themselves with the environment before entering the house, and so they continued to walk past it. After a bit less than a kilometer, they reached the edge of the small world and faced the duplicate world before them. At first, they thought it was just a reflected image of their own world, though it was not mirrored. But after they were halfway there they decided that it couldn't be a reflection: Everything looked so real. They took a step forward and entered the duplicate world without any resistance. Looking around, Cheng Xin was struck with a hint of terror.

Everything looked the same as when they had first entered the world. They were in the same pastoral scene, with duplicates of the scene before them and to the sides, and in those copies, copies of them also existed. They turned around to look back, and they saw copies of themselves at the far end of the world they had just left, looking behind them.

"Are you familiar with the work of Charles Misner?"

Yifan let out a long sigh. "I don't think we need to go any farther. We'll never reach the end." He pointed up and then down. "I bet that without these barriers, we'd see the same scene above and below us as well."

"Who was he?"

"A physicist of the Common Era. He was the one who first came up with this concept. The world we're in is actually very simple. It's a regular cube about a kilometer on each side. You can imagine it as a room with four walls and a ceiling as well as a floor. But the room is constructed such that the ceiling is also the floor, and each wall is the same as the opposite wall. In reality, it has only two walls. If you walk through one of the walls, you'll immediately reappear at the opposite wall, and the same is true of the floor and ceiling. Thus, this is a completely enclosed world in which the end is also the beginning. The images we see all around us are the result of light returning to the starting point after crossing the world. We're still in the same world we started from, because this is the only world that exists. Every copy we see around us is just an image of this world."

"Do you know what this is?"

"So this is…"

"Yes!" Yifan swept his arm around to indicate everything. "Yun Tianming once gave you a star, and now he's given you a universe. Cheng Xin, this is an entire universe. It might be small, but it's a complete universe."

A figure in white came out of the house and walked toward them. The world was so small that it was possible to clearly see anyone at any distance. They saw that the newcomer was a woman dressed in a kimono. The kimono, decorated with tiny red flowers, was like a walking flower bush that brought the feeling of spring to the universe.

Cheng Xin looked around, at a loss for words. Yifan sat down quietly on a ridge in the field and picked up a fistful of black earth, letting the soil slip from between his fingers. He sounded depressed. "He's quite a man to be able to give the woman he loved a star and a universe. But I can't give you anything."

Cheng Xin sat down as well and leaned on his shoulder. She laughed as she said, "You're the only man in this universe. I don't think you need to give me anything."

The feeling of being alone in the universe was soon shattered by the sound of a door opening.

"I know her," Yifan said. "She's the robot controlled by sophons."

Sophon bowed again. "Welcome to Universe 647. I am its manager."

"Sophon!" Cheng Xin cried out.

They walked over to meet the woman under one of the trees. Cheng Xin saw that it really was Sophon: That unparalleled beauty remained unchanged.

Sophon bowed deeply to Cheng Xin and Guan Yifan. When she straightened, she smiled at Cheng Xin. "I said that the universe is grand, but life is grander. Fate has indeed directed us to meet again."

"I couldn't have imagined," Cheng Xin said. "I'm really glad to see you. Really." Sophon brought her to the past -- more than eighteen million years ago. But that wasn't really accurate, because they were now in another time stream altogether.

"Oh no!" Sophon laughed and waved his remark away. "You are the true masters of Universe 647 and have full authority over everything here. I'm just here to serve you."

"The manager of the universe?" Yifan looked at Sophon, astounded. "What a grand title! For a cosmologist like me, that sounds like --"

To Cheng Xin and Guan Yifan, everything seemed to be a dream.

Sophon made a gesture indicating that they should follow her. They followed her to a refined parlor inside the house. The parlor was decorated in an Eastern style with a few calming brush paintings and calligraphy scrolls hung on the walls. Cheng Xin looked for artifacts taken from Pluto by Halo but didn't find any. After they sat down at an antique wooden desk, Sophon poured tea for them -- without going through the complicated steps of the Way of Tea. The tea leaves seemed to be Longjing, and they stood up at the bottom of the cups like a tiny green forest, giving off a fresh fragrance.

"I think it's meant for Cheng Xin," said Yifan.

"No. You're also one of the intended recipients. Your authorization was added to the recognition system later; otherwise you wouldn't have been allowed in. Mr. Yun hoped that you could hide in this tiny universe and avoid the collapse of the great universe -- or the big crunch -- and, after the next big bang, enter the new universe and see its Edenic Age. Right now, we exist in an independent timeline. Time is passing rapidly in the great universe, and you will certainly be able to see its end within your lifetimes. More specifically, I estimate that the great universe will collapse into a singularity after about ten years here."

Sophon spoke. "This universe is a gift. Mr. Yun Tianming gave it to the both of you."

"We'll know. We can sense the conditions in the great universe through the supermembrane."

"What are you laughing at?" Cheng Xin asked.

Sophon's words reminded Cheng Xin of what Yun Tianming and 艾 AA had carved into the rock. But Guan Yifan was reminded of something else. He noticed that Sophon spoke of the "Edenic Age" of the new universe. This was a term invented by the Galactic humans. Two possibilities presented themselves. One was that coincidentally, the Trisolarans had also picked this term. The second possibility was far more terrifying: the Trisolarans had already discovered the Galactic humans. Given how quickly Yun Tianming had arrived on Planet Blue, it was apparent that the First Trisolaran Fleet was very close to the worlds of humankind. And now, the Trisolaran civilization had developed to the point where they were capable of constructing small universes: This was a great threat to humanity.

"Myself."

"If a new big bang occurs, how will we know?" asked Yifan.

Then he laughed.

He found himself ridiculous. More than eighteen million years had passed since the day he'd departed World II to come to Planet Blue, and that was before they entered this small universe with its own time. By now, hundreds of millions of years must have passed in the great universe. He was worried about truly ancient history.

"Is anyone else's authorization in the system?"

"Why aren't Tianming and AA here?" Yifan asked. This was also the question Cheng Xin most wanted answered. She hadn't asked earlier because she was afraid of hearing an answer she didn't want to know.

Sophon shook her head. "No. I've never met him."

"The last time I saw her was on Earth."

"Have you seen Yun Tianming?" Cheng Xin asked.

Sophon shook her head again. "I don't know. The recognition system has always had Mr. Yun's authorization."

"Then how did you come to be here?"

"What about 艾 AA?"

"I don't know what Planet Blue is. If it's a planet, then Mr. Yun couldn't have brought Universe 647 to it, because this is an independent universe that does not exist within the great universe. He could only bring the entrance to the universe there."

"Did you know that Tianming brought this universe to Planet Blue?"

"Universe 647 was a custom order. I've been here since its completion. Remember that I am fundamentally just a collection of digital bits, and many copies of me can be made."

"No. Only the three of you."

"I'm interested," said Yifan. "I really want to see what a new universe is like before it's distorted and tampered with by life and civilization. I think it must exhibit the highest degree of harmony and beauty."

After a while, Cheng Xin said to Yifan, "AA always cared more than me about the world around her. I don't think she would have been interested in a new universe tens of billions of years later."

Cheng Xin said, "I also want to go to the new universe. The singularity and the new big bang will erase all memories of our universe. I want to bring some memory of humanity there."

Sophon nodded solemnly at Cheng Xin. "That is a great task you've set yourself. There are others doing similar work, but you're the first human from the Solar System to do this."

"You've always had higher goals in life than I," Yifan whispered to Cheng Xin. She couldn't tell if he was joking or being serious.

Sophon stood up. "Welcome to your new life in Universe 647. Why don't we go outside and take a look around?"

Yifan looked somewhat puzzled by the black soil. "I feel that soilless cultivation tanks would be more suitable here."

Outside, the spring planting was in full swing. The columnar robots were all working the fields. Some used the rakes to level and smooth out the field -- the soil was so loose already that it did not need to be plowed; some were planting seeds in the parts that had already been smoothed. The farming techniques they employed were primitive: There were no drag harrows, so the robots had to use small rakes to level the field a bit at a time; there were no planters, so the robots each carried a bag of seeds and buried the seeds in the field one at a time. The entire scene invoked a sense of ancient simplicity. Here, robots seemed somehow more natural than real farmers.

Sophon explained, "We have only enough food stored here to last two years. After that, you'll need to rely on the food you grow. These seeds are descended from the seeds Cheng Xin sent along with Mr. Yun. Of course, they've all been genetically improved."

Yifan said, "The Solar System humans spilled their last drop of blood to stay with their land -- well, save for two drops: you and AA. But what was the point? They didn't last, and neither did their land. Hundreds of millions of years have passed in the great universe, and do you think anyone still remembers them? This obsession with home and land, this permanent adolescence where you're no longer children but are afraid to leave home -- this is the fundamental reason your race was annihilated. I am sorry if I've offended you, but it's the truth."

Cheng Xin said, "Anyone from the Earth has a kind of nostalgia for soil. Remember what Scarlett's father told her in Gone With the Wind? 'Why, land is the only thing in the world worth workin' for, worth fightin' for, worth dyin' for, because it's the only thing that lasts.'"

Cheng Xin smiled at the agitated Yifan. "You haven't offended me. What you said is true. We knew that, but we couldn't help it. You probably can't help it, either. Don't forget that you and all the crew of Gravity were prisoners before becoming Galactic humans."

"That is true." Guan Yifan lost some of his fire. "I've never thought of myself as a man qualified for space."

By the standards of space, there were not too many "qualified" men -- and it was doubtful Cheng Xin would like any of them. She did think of one person who was probably qualified. His voice still echoed in her ears: We're going to advance! Advance! We'll stop at nothing to advance!

In this tiny world they were provided with all the other necessities of life. None of the objects had manufacturing marks or brand logos -- Trisolarans made them -- but they looked exactly like human products.

A year passed in Universe 647.

"Don't dwell on the past," Sophon said in her sweet voice. "Everything starts anew here."

Cheng Xin and Guan Yifan sometimes went into the fields to work alongside the robots. Sometimes they strolled about the universe -- as long as they were careful not to leave footprints, they could keep on walking indefinitely and experience the feeling of traversing countless worlds.

The wheat had been harvested twice, and Cheng Xin and Guan Yifan had now watched twice as the green seedlings gradually turned into a sea of golden stalks. The vegetable fields next to the wheat had always remained green.

But even more information existed in the databank written in the Trisolaran language. This massive ocean of data was what interested them the most. Since they couldn't find any software on the computer for translating Trisolaran writing into human languages, they had to study the Trisolaran script itself. Sophon acted as their teacher, but they soon discovered that this was an extremely difficult endeavor because Trisolaran writing was purely ideographic; unlike human scripts, which were mostly phonetic, Trisolaran writing had no connection to their speech, but expressed ideas directly. In the distant past, humans had also used ideographic scripts -- such as some hieroglyphs -- but most of these later disappeared.11 Humans read by decoding speech made visible. However, the difficulty didn't last long. The more they persisted, the easier the learning process became. After struggling for two months, they found themselves making rapid progress. Compared to phonetic scripts, the biggest advantage of an ideographic script was how fast one could read -- Cheng Xin and Guan Yifan read at least ten times as fast in Trisolaran as they did with human scripts.

They spent most of their time in front of the computer, however. It was possible to invoke a terminal from anywhere in the small universe, but they didn't know where the CPU for the computer was located. The computer had a massive databank of text, image, and video from the Earth, most of which dated from before the Broadcast Era. The Trisolarans had clearly gathered the info as they studied humanity, and the material covered every field in the sciences and the humanities.

The fundamental principles underlying the small artificial universe seemed unimaginable to them; even the most basic mysteries puzzled them for the longest time. For instance, how could a complete ecological cycle function in a sealed space of only one cubit kilometer? What was the sun? What was its energy source? And most confounding: As a completely sealed system, where did the heat of the mini-universe go?

They began to read the Trisolaran material in the databank -- at first haltingly, and then faster. They had two initial goals in mind: First, they wanted to know how the Trisolarans had recorded the period of history between their civilization and Earth civilization. Second, they wanted to know how this mini-universe was constructed. For the latter, they understood that they would likely not achieve a specialist's level of understanding, but they wanted to at least understand it at the level of popular science. Sophon estimated that in order to achieve these two goals, they would need to spend one year to learn how to read Trisolaran better, then take another year to read in depth.

They asked Sophon these questions. Some she could answer; for others, she referred them to materials in the computer.

They also cared about the answer to one question in particular: Could the mini-universe communicate with the great universe? Sophon told them that there was no way for the mini-universe to transmit any information to the greater universe, but it was possible for the mini-universe to receive broadcasts from the great universe. She explained that all the universes were bubbles above a supermembrane -- this was a fundamental conceptual image from Trisolaran physics and cosmology, and she could explain it no further. The great universe had enough energy to propagate information across the supermembrane. However, this was difficult and required a great expenditure of energy -- the great universe would have to convert a Milky Way's worth of matter into pure energy. As a matter of fact, the monitoring systems in Universe 647 often received messages from other great universes on the supermembrane. Some were natural phenomena; some were messages from intelligent beings that could not be decoded -- but they had never received any message from the particular great universe they had come from.

Sometimes, they also tried to imagine life in the new universe. Sophon told them that according to cosmological theories, the new universe was certain to possess more than four macro dimensions, perhaps even more than ten macro dimensions. After the birth of the new universe, Universe 647 could automatically construct an entrance to it and examine the interior conditions. If the new universe possessed more than four dimensions, the mini-universe's exit could be moved around until a suitable habitable location was found in the great universe. Simultaneously, their mini-universe could establish communications with the refugees of other Trisolaran mini-universes, or even with Galactic human migrants. In the new universe, all the migrants coming from the old universe would practically be one race, and should be able to work together to construct a new world. Sophon emphasized that one characteristic greatly increased the probability of survival in a high-dimensional universe: Out of the many macro dimensions, it was likely that more than one dimension would belong to time.

Time flowed by day after day like the smooth, placid water in that little brook.

Cheng Xin began to write her memoir so that she could record the history she knew. She named the book A Past Outside of Time.

"Even if time were only two-dimensional, it would be a plane instead of a line," Yifan explained. "There would be an infinite number of directions, and we could simultaneously make countless choices."

"Multi-dimensional time?" Cheng Xin couldn't understand the concept at first.

One night, after the second harvest, Cheng Xin woke up to find Yifan not beside her. She got up, went outside, and saw that the sun had already turned to the moon, and the little world was immersed in the watery, cool light. She saw Yifan sitting by the brook, his posture morose.

In this world of two, each of them had grown especially sensitive to the moods of the other. Cheng Xin had already known that something was troubling Yifan. Earlier, he had been sunny and upbeat. Until a few days ago, he had regularly shared his dream that, if they could find a peaceful life in the new great universe, perhaps their children could re-create the human race. But then he had abruptly changed, frequently going off by himself to ponder something, or to calculate something at a computer terminal.

"And at least one of those choices would turn out to be right," added Sophon.

Cheng Xin sat down next to Yifan, and he pulled her into his arms. The moonlit world was very quiet, and all they heard was the babbling brook. The moon revealed a field of ripe wheat; they'd have to start the harvest tomorrow.

"Loss of mass," Yifan said.

"Yes. The Trisolarans have already constructed several hundred mini-universes. How many more have been constructed by other civilizations in the universe to escape the big crunch, or for some other purpose? Each of these mini-universes took away some matter from the great universe."

Cheng Xin said nothing. She watched the moonlight dancing in the brook, knowing that Yifan was going to explain.

"I've been reading Trisolaran cosmology, and came across a proof for the elegance of the mathematics behind the great universe we all came from. The design of the total mass in the universe was precise and perfect. The Trisolarans had proved that the total mass of the universe was just enough to allow the big crunch. If the total mass were reduced even slightly, the universe would turn from being closed to open, and expand indefinitely."

"But mass has been lost," Cheng Xin said. She understood right away what he was getting at.

"But we've long been called on to think about matters that belonged to the province of God."

"We need to ask Sophon."

"I have. She told me that at the time of Universe 647's completion, the Trisolarans had not observed any influence from the loss of mass in the great universe. That universe was closed and was certain to eventually collapse."

"She had no idea, of course. She also mentioned that there was a group of intelligent beings in the universe that resembled the Zero-Homers, but they called themselves the Returners. They advocated against the construction of mini-universes, and called for the mass in completed mini-universes to be returned to the great universe… But she didn't know much else about them. All right, let's forget all this. We're not God."

Three days after the harvest, once all the wheat had been threshed and winnowed and stored away, Cheng Xin and Guan Yifan stood at the edge of the field and watched as the robots plowed the field to prepare for the next planting. The granary was now full, so there was no room for more wheat. Before, they would have debated what to plant for the next season. But now, both of them were troubled and had no interest in the topic. Throughout the entire harvest and threshing process, they had stayed in the house and discussed possible futures. They realized that even their individual life choices affected the fate of the universe, or even the fates of multiple universes. They really felt like God. The weight of responsibility made it hard to breathe, and so they left the house.

They sat by the brook until the moon turned into the sun again.

"What about after Universe 647 was constructed?"

"The broadcast has been going on for five minutes and is still continuing." Sophon pointed at the window. "In actuality, the message in the broadcast is very simple and brief, but it has lasted this long because it's in many languages. We've seen a hundred thousand languages already!"

They saw Sophon walking toward them along one of the field ridges. Sophon rarely disturbed them and only appeared when they needed her. This time, her walk was different -- she was in a hurry and did not exhibit her typical grace and dignity. Her anxious expression was also something they had not seen before.

"We've received a supermembrane broadcast from the great universe!" Sophon brought up a window and enlarged it. To make the window easier to see, she also dimmed the sun.

A torrent of symbols scrolled up the screen -- the bitmap from the supermembrane broadcast. The symbols were strange and indecipherable. Cheng Xin and Guan Yifan noticed that each row of symbols was different: They rolled past like the surface of a chaotic river.

"Absolutely. Who else would receive it? They expended so much energy that the message must be important."

"Have you seen Trisolaran or Earth languages?"

"No."

There were no Trisolaran and no Earth languages.

By now, tens of billions of years had passed in the great universe. Regardless of the content of the broadcast, if a civilization's language was listed in the broadcast, it meant that the civilization still existed or had existed once and lasted so long that it had left an indelible mark in the great universe.

The river of symbols continued to flow up the screen: two hundred thousand languages, three hundred thousand, four hundred thousand… a million. The number continued to go up.

"It doesn't matter," Cheng Xin said. "We know that we existed; we lived." She and Guan Yifan leaned against each other.

Cheng Xin and Guan Yifan realized that this message was a record of which species had survived in the great universe.

"Is the broadcast aimed at all the mini-universes?" Cheng Xin asked.

"Trisolaran!" Sophon cried out and pointed at the screen. By now, over 1.3 million languages had been broadcast, and one row, written in Trisolaran, flashed by. Cheng Xin and Guan Yifan couldn't catch it, but Sophon did.

In the eternal night of the Orion Arm of the Milky Way Galaxy, two civilizations had swept through like two shooting stars, and the universe had remembered their light.

The window now showed only the message written in Trisolaran and Earth languages. Cheng Xin and Guan Yifan couldn't even read the message because tears blurred their eyes.

On the day of the universe's Last Judgment, two humans and a robot belonging to the Earth and Trisolaran civilizations embraced each other in ecstasy.

"Earth!" Sophon cried out again a few seconds later.

They knew that languages and scripts evolved very quickly. If the two civilizations had survived for a long time or even continued to exist now, their scripts were surely very different from what was being shown on the screen. But to allow those hiding in mini-universes to understand, they had to write in ancient scripts. Compared to the total number of civilizations that had lived in the great universe, 1.57 million was a tiny number.

After 1.57 million languages, the broadcast finished.

After Cheng Xin and Guan Yifan calmed down, they read the message. The content of the message in both scripts was the same, and very simple:

A notice from the Returners: The total mass of our universe has decreased to below the critical threshold. The universe will turn from being closed to open, and die a slow death in perpetual expansion. All lives and all memories will also die. Please return the mass you have taken away and send only memories to the new universe.

Cheng Xin and Guan Yifan locked gazes. In each other's eyes, they saw the dark future for the great universe. In perpetual expansion, all the galaxies would move farther away from each other until none were visible from any other. By then, standing at any point in the universe, all one would see was darkness in every direction. The stars would go out one by one, and all celestial bodies would turn into thin dust clouds. Coldness and darkness would reign over all, and the universe would become a vast, empty tomb. All civilizations and all memories would be buried in that endless tomb for eternity. Death would be eternal.

Yifan shook his head slowly. "I study a grand universe whose diameter is sixteen billion light-years. I don't want to spend the rest of my life in this universe that's only a kilometer in each direction. Let's go back."

The only way to prevent this future was to return the matter locked up in all the mini-universes constructed by all the civilizations. But such a decision meant that the mini-universes would not survive, and all the refugees in the mini-universes had to return to the great universe. That was the meaning of the name of the Returners' movement.

The two said everything they needed to say to each other with their eyes and made their decision wordlessly. But Cheng Xin still spoke aloud. "I want to go back. But if you want to stay here, I'll stay with you."

"I must advise against that," said Sophon. "We can't precisely determine how fast time is passing in the great universe, but I can be certain that at least ten billion years have passed there since the time you came here. Planet Blue has long since vanished, and the star Mr. Yun gave you was extinguished a long time ago. We know nothing of the conditions in the great universe, and it's possible that it's not even three-dimensional anymore."

"I thought you could move the exit of the mini-universe at lightspeed," Yifan said. "Can't you move it around to find a habitable location?"

"If everyone in every mini-universe thinks that way," said Cheng Xin, "then they will have doomed the great universe."

"If you insist, I will try. But I still think staying here is the best choice. There are two possible futures if you remain: If the Returners succeed in their mission, the great universe will collapse into a singularity and lead to a new big bang so that we can go to the new universe. But if the Returners fail and the great universe dies, you can live out the rest of your lives in this mini-universe. This isn't too bad."

Sophon gazed at Cheng Xin wordlessly. Given the speed of Sophon's thought, perhaps this period of time felt as long as several centuries to her. It was hard to imagine that software and algorithms could produce such a complex expression. Perhaps Sophon's AI software had brought up all the memories accumulated across almost twenty million years since she had met Cheng Xin. All these memories seemed to precipitate in her gaze: sorrow, admiration, surprise, reproach, regret… so many complicated feelings mixed together.

[Excerpt from A Past Outside of Time, The Stairs of Responsibility]

"You're still living for your responsibility," Sophon said.

Later, in high school and college, the responsibility to study hard continued, but there was also the added obligation to make myself useful rather than a drain on society.

When I was little, my only duty was to study hard and obey my parents.

All my life has been spent climbing up a flight of stairs made of responsibility.

By the time I started to work toward my doctorate, my responsibilities became more concrete. I needed to contribute to the development of chemical rockets, to build more powerful, more reliable rockets so that more materials and a few men and women could be sent into Earth orbit.

Later, I joined the PIA, and my responsibility was to send a probe into space a light-year away to meet the invading Trisolaran Fleet. This was a distance about ten billion times greater than the distance I had worked with as a rocket engineer.

And then, I received a star. During the new era, it brought me previously unimaginable responsibilities. I became the Swordholder, whose duty was to maintain dark forest deterrence. Looking back on it now, perhaps it was a bit of an exaggeration to claim that I held the fate of humankind; but I really did control the direction of development for two civilizations.

I want to tell all those who believe in God that I am not the Chosen One. I also want to tell all the atheists that I am not a history-maker. I am but an ordinary person. Unfortunately, I have not been able to walk the ordinary person's path. My path is, in reality, the journey of a civilization.

Later, my responsibilities became more complicated: I wanted to endow humans with lightspeed wings, but I also had to thwart that goal to prevent a war.

I don't know how much those catastrophes and the final destruction of the Solar System had to do with me. Those are questions that could never be answered definitively. But I'm certain they had something to do with me, with my responsibilities.

And now, I've climbed to the apex of responsibility: I am responsible for the fate of the universe. Of course this responsibility doesn't belong only to me and Guan Yifan, but we own a share of the responsibility, a share of something that I never could have imagined.

And now we know that this is the journey that must be made by every civilization: awakening inside a cramped cradle, toddling out of it, taking flight, flying faster and farther, and, finally, merging with the fate of the universe as one.

Through Universe 647's control system, Sophon managed to move the mini-universe's exit inside the great universe. The door moved quickly through the great universe, searching for a habitable world. The amount of information that the door could transmit to the mini-universe was very limited, and no images or videos were possible. All that could be sent back was a rough analysis of the environment. This was a number between negative ten and ten, indicating the habitability of the environment. Humans could survive only if the number were greater than zero.

The ultimate fate of all intelligent beings has always been to become as grand as their thoughts.

[Outside of Time, Our Universe]

The door jumped tens of thousands of times in the great universe. After three months, only once did they discover a habitable planet, with a rating of three. Sophon had to concede that this was probably the best result they could get.

"We're not afraid," said a resolute Cheng Xin. Yifan nodded. "Let's go there."

"A rating of three indicates a dangerous and inhospitable world," Sophon warned.

Next, it was time to return matter from the mini-universe to the great universe.

Sophon had explained that the mini-universe had no matter of its own. All of its mass had come from material brought out of the great universe. Of the several hundred mini-universes constructed by the Trisolarans, Universe 647 was one of the smallest. In total, it required about five hundred thousand metric tons of matter from the great universe, which was about the carrying capacity of a large oil tanker. It was practically nothing at the scale of the universe.

The door appeared in Universe 647. Like the door Cheng Xin and Guan Yifan had seen on Planet Blue, it was also a rectangle limned by glowing lines. But this door was much bigger, perhaps to make it easier to transport material through it. Initially, the door was not connected to the great universe, and anything could pass through it without leaving the mini-universe. Sophon adjusted its parameters so that anything moving through it would disappear and reappear in the great universe.

They began with the soil. After the last harvest, the field had been left fallow. The robots used a wheelbarrow to cart the moist earth; at the door, two of the robots lifted the wheelbarrow to dump the soil through the door; and the soil disappeared. It happened very quickly. Three days later, all the soil in the mini-universe was gone. Even the trees around the house had been returned through the door.

With all the soil removed, they saw the metallic floor of the mini-universe. The floor was pieced together with smooth metal tiles that reflected the sun like a mirror. The robots took off the metal tiles one by one and sent them through the door as well.

Underneath the floor was a small spaceship. Although the ship was less than twenty meters long, it contained the most advanced technologies of the Trisolarans. Designed with human occupants in mind, it could seat three, and was equipped with both a nuclear fusion drive and a curvature drive. There was a miniature ecological cycling system aboard suitable for human needs as well as equipment for hibernation. Like Halo, it was capable of landing and taking off from planetary surfaces. It had a slender, streamlined profile, perhaps to make it easier to go through the mini-universe's door. It had been intended for the inhabitants of Universe 647 to enter the new great universe after the next big bang. It could serve as a living base for a considerable amount of time, until they found a suitable location in the new universe. But now, they would use it to return to the old great universe.

As the rest of the metal floor tiles were removed, they also revealed more machinery beneath. These were the first objects Cheng Xin and Guan Yifan had seen in the mini-universe that bore obvious signs of being of Trisolaran origin. Like Cheng Xin had suspected, the design of these machines evinced an aesthetic completely different from human ideals. Cheng Xin and Guan Yifan couldn't even tell at first that they were looking at machinery; rather, the objects resembled strange sculptures or natural geologic formations. The robots began to disassemble the machinery and send the pieces through the door.

After some machine under the floor was shut off, gravity disappeared from the mini-universe. The white house began to float in air.

Cheng Xin and Sophon busied themselves in a room and wouldn't let Yifan in. They said that they were working on a "women's project" and would surprise him later.

The weightless robots disassembled the sky, which was a thin membrane capable of displaying a blue sky and white clouds. Finally, the remnants of the floor below the machinery were also disassembled and sent away.

Disassembling the machines also meant turning off the ecological cycling system. Cheng Xin and Yifan had to put on space suits.

The water in the mini-universe had evaporated and fog was everywhere. The sun shone from behind a veil of clouds and a spectacular rainbow appeared that crossed from one end of the universe to the other. Whatever liquid water was left in the mini-universe formed spheres of various sizes and drifted around the rainbow, reflecting and refracting sunlight.

Sophon adjusted the parameters in the door again to allow gas through. A low rumble shook the mini-universe, caused by air escaping through the door. Below the rainbow, the white fog cloud formed a great maelstrom around the door, like a view of a typhoon from space. And then, the whirling fog turned into a tornado, and let out a high-pitched howl. The drifting balls of water were sucked into the twister, torn apart, and disappeared through the door. Countless small objects drifting in the air were also swallowed up by the cyclone. The sun, the house, the spaceship, and other large objects also drifted in the direction of the door, but robots equipped with thrusters quickly secured them back in place.

The house was disassembled. The last room to be taken apart was the parlor decorated in an Eastern style in which Sophon had welcomed them. All the scrolls, the tea table, and the pieces of the house were sent out the door by the robots.

As the air thinned, the rainbow disappeared, and the fog dissipated. The air became more transparent, and gradually, the mini-universe's space appeared. Like space in the great universe, it was also dark and deep, but there were no stars. Only three objects floated in space: the sun, the house, and the spaceship, along with about a dozen weightless robots. In Cheng Xin's eyes, this simplified world resembled the naïve, clumsy pictures she had drawn in her childhood.

Cheng Xin and Guan Yifan activated the thrusters on their space suits and flew toward the depths of space. After a kilometer, they reached the end of the universe and, in a flash, found themselves back where they had started. They could see the projected images of every floating object repeated endlessly in every direction. Like two mirrors placed against each other, the images extended in rows into infinity.

The sun finally went out. It was a metal sphere where one hemisphere, the part that had emitted light, was transparent. Three robots pushed it through the door. Only lamps now illuminated the mini-universe, and the vacuum that was space soon cooled. What was left of the water and air soon turned into ice fragments that sparkled in the lamplight.

Sophon directed the robots to line up and go through the door, one after the other.

Sophon held a metal box. This box would be left behind in the mini-universe, a message in a bottle for the new universe that would be born after the next big bang. The box contained a miniature computer whose quantum memory held all the information in the mini-universe's computer -- this was practically the entire memory of the Trisolaran and Earth civilizations. After the birth of the new universe, the metal box would receive a signal from the door, and it would go through the door using its own tiny thrusters and enter the new universe. It would drift through the high-dimensional space of the new universe until the day it was picked up and read. At the same time, it would continuously broadcast its message using neutrinos -- assuming the new universe also had neutrinos.

Finally, only the slender ship was left in the mini-universe, along with three figures drifting near it.

"Can we keep another five kilograms here?" Cheng Xin asked. She was on the other side of the ship and dressed in her space suit. In her hand, she held a glowing, transparent sphere. The sphere was about half a meter in diameter, and a few balls of water drifted inside it. Inside some of the water spheres were tiny fish, along with green algae. There were also two miniature drifting continents covered with green grass. The light came from the top of the transparent sphere, where there was a tiny glowing emitter, the sun of this miniature world. This was a completely sealed ecological sphere, the result of more than ten days of work by Cheng Xin and Sophon. As long as the tiny sun inside the sphere continued to give off light, this miniature ecological system would persist. As long as it remained here, Universe 647 would not be a lifeless, dark world.

Cheng Xin and Guan Yifan believed that in the other mini-universes, at least those mini-universes that heeded that call of the Returners, the same things were being done. If the new universe were really born, it would contain many bottles containing messages drifting through it. It was believable that a considerable number of bottles contained storage mechanisms that held the memories and thoughts of every individual of that civilization, as well as their complete biological details -- maybe the records would be sufficient for a new civilization in the new universe to revive that old civilization.

"Of course," said Guan Yifan. "The great universe isn't going to fail to collapse because it misses five kilograms." He had another thought that he did not voice: Perhaps the great universe really would fail to collapse because it lacked a single atom's mass. The precision of Nature can sometimes exceed the imagination. For instance, life itself required the precise collaboration of various universal constants within a billion-billionth of a certain range. But Cheng Xin could still leave behind her ecological sphere. Out of all the countless mini-universes created by the countless civilizations, it was certain that some number of them would not heed the call of the Returners. Ultimately, the great universe was certain to lose at least a few hundred million tons of matter, or perhaps even a million billion billion tons.

Hopefully, the great universe could ignore such a loss.

Cheng Xin and Guan Yifan entered the spaceship, and Sophon came in last. She had long ceased wearing her magnificent kimono and turned once again into that lean and nimble warrior dressed in camouflage. She had all sorts of weapons and survival gear strapped to her body, the most prominent being the katana on her back.

The fusion drive activated and the thrusters emitted a dim blue light. The spaceship slowly went through the door of the universe.

The message in a bottle and the ecological sphere were the only things left in the mini-universe. The bottle faded into the darkness so, in this one-cubic-kilometer universe, only the little sun inside the ecological sphere gave off any light. In this minuscule world of life, a few clear watery spheres drifted serenely in weightlessness. One tiny fish leapt out of a watery sphere and entered another, where it effortlessly swam between the green algae. On a blade of grass on one of the miniature continents, a drop of dew took off from the tip of the grass blade, rose spiraling into the air, and refracted a clear ray of sunlight into space.

"Don't worry," she said to her two human friends. "As long as I'm alive, no harm will come to you."