by Cixin Liu
“Let her come with you. She wants to know whether this has anything to do with Yun Tianming.”
Before departure, Yifan reminded AA to not communicate with Hunter unless it was an emergency. No one knew what other extraterrestrial monitoring equipment might be lurking in this system, and any communication could expose them to danger.
In this lonely world of only three people, even a brief separation was an occasion for anxiety. AA hugged Cheng Xin and Guan Yifan and wished them a safe journey. Before stepping onto the shuttle, Cheng Xin looked back and saw AA waving at them, lit by the watery starlight. Blue grass surged around her, and the cold wind lifted her short hair and made ripples in the grass.
The shuttle took off. In the view from the monitoring system, Cheng Xin saw the grass lit up by the flame from the thruster, and the blue grass scattering in every direction. As the shuttle rose up, the bright patch on the ground quickly dimmed, and soon, the ground sank back into starlight.
An hour later, the shuttle docked with Hunter in synchronous orbit. Hunter was tetrahedral in shape, like a tiny pyramid. The inside was very cramped and bare, and most of the space was taken up by the hibernation chamber, which had a maximum capacity for four.
Like Halo, Hunter was equipped with both a curvature engine and a fusion engine. When traveling between planets within the same system, only the fusion engine was used, because the curvature engine would have caused the ship to overshoot the target with no time for deceleration. Hunter left orbit and headed for Planet Gray, which appeared as a small spot of light. Out of consideration for Cheng Xin, Guan Yifan initially limited the acceleration to 1.5G, but Cheng Xin told him not to worry about her and just make the trip faster. He increased the acceleration, the blue flame emitted by the nozzles doubled in length, and the hypergravity increased to 3G. At this point, all they could do was to sit in the embrace of the acceleration seats. They were not able to move much, so Yifan switched the ship to surround-holographic display mode, and the ship’s hull disappeared. Suspended in space, they watched Planet Blue recede. Cheng Xin imagined the 3G gravity as coming from Planet Blue so that space separated into up and down, and they were flying up toward the galaxy.
It was possible to speak under 3G without too much trouble, so they began to converse. Cheng Xin asked Yifan why he had hibernated for so long. He told her that he had no duties during the long voyage searching for habitable worlds. After the two ships discovered the habitable World I, much of life consisted of opening up the world for settlement and constructing the basics. The first settlement resembled a small town from agrarian times, and the rough conditions did not permit any kind of scientific research. The new world’s government passed a resolution to let all scientists enter or remain in hibernation, to be awakened only when conditions permitted basic research. He was the only basic scientist aboard Gravity, although Blue Space had seven more. He was the last to be awakened of all the hibernators. Two centuries had passed since the day the two ships arrived at World I.
Cheng Xin was mesmerized by Yifan’s account of the new human worlds. But she noticed that while he discussed Worlds I, II, and IV, nothing was said about III.
“I’ve never been there. No one else has, either. Well, it’s more accurate to say that anyone who goes there cannot return. That world is sealed inside a light tomb.”
“A light tomb?”
“It’s a reduced-lightspeed black hole produced by the trails of lightspeed ships. Something happened on World III that caused them to think that their coordinates had been exposed. They had no choice but to turn their world into such a black hole.”
“We call such a place a black domain.”
“Ah, good name. As a matter of fact, the people of World III initially called it a light curtain, but outsiders referred to it as a light tomb.”
“Like a shroud?”*
“That’s right. Different people see things differently. The inhabitants of World III said it was a happy paradise—though I don’t know if they still think that way. After the light tomb was completed, it was impossible for any message from that world to reach the outside. But I think people there are pretty happy. For some people, safety is the sine qua non for happiness.”
Cheng Xin asked Yifan when the new world first produced lightspeed ships, and was told it was a century ago. Judging by this, her interpretation of Tianming’s secret message had allowed Solar System humans to achieve this stage about two centuries ahead of Galactic humans. Even taking into account the time it took to open up the worlds for settlement, Tianming had accelerated progress by at least a century.
“He’s a great man,” Yifan said after hearing Cheng Xin’s account.
But the civilization of the Solar System hadn’t been able to seize this opportunity. Thirty-five precious years had been lost, probably due to her. Her heart no longer felt pain as she thought of this; all she felt was the numbness that indicated a dead heart.
Yifan said, “Lightspeed spaceflight was a tremendous milestone for humankind. It was like another Enlightenment, another Renaissance. Lightspeed flight fundamentally transformed human thinking and changed civilization and culture.”
“I can see that. The moment I entered lightspeed, I felt myself change. I realized that I could, in my lifetime, leap across space-time and reach the edge of the cosmos and the end of the universe. Things that used to seem only philosophical suddenly became concrete and practical.”
“Yes. Things like the fate and goal of the universe used to be only ethereal concerns of philosophers, but now every ordinary person must consider them.”
“Has anyone in the new world thought of going to the end of the universe?”
“Of course. Five ultimate spaceships have already been launched.”
“Ultimate spaceships?”
“Some call them doomsday ships. These lightspeed ships have no destination at all. They turn their curvature engines to maximum and accelerate like crazy, infinitely approaching the speed of light. Their goal is to leap across time using relativity until they reach the heat death of the universe. By their calculations, ten years within their frame of reference would equal fifty billion years in ours. As a matter of fact, you don’t even need to plan for it. If some malfunction occurs after a ship has accelerated to lightspeed, preventing the ship from decelerating, then you’d also reach the end of the universe within your lifetime.”
“I pity Solar System humans,” said Cheng Xin. “Even at the very end, most of them lived lives confined to a tiny portion of space-time, like those old men and women who never left their home villages during the Common Era. The universe remained a mystery to them until the end.”
Yifan lifted his head to gaze at Cheng Xin. Under 3G, this was a very strenuous exercise. But he persisted for some time.
“You don’t need to pity them. Really, let me tell you: don’t. The reality of the universe is not something to envy.”
“Why?”
Yifan lifted a hand and pointed at the stars of the galaxy. Then he let the 3G force pull his arm back to this chest.
“Darkness. Only darkness.”
“You mean the dark forest state?”
Guan Yifan shook his head, a gesture that appeared to be a struggle in hypergravity. “For us, the dark forest state is all-important, but it’s just a detail of the cosmos. If you think of the cosmos as a great battlefield, dark forest strikes are nothing more than snipers shooting at the careless—messengers, mess men, etc. In the grand scheme of the battle, they are nothing. You have not seen what a true interstellar war is like.”
“Have you?”
“We’ve caught a few glimpses. But most things we know are just guesses.... Do you really want to know? The more you possess of this kind of knowledge, the less light remains in your heart.”
“My heart is already completely dark. I want to know.”
And so, more than six centuries after Luo Ji had fallen through ice into that lake, another dark veil hiding the truth about the universe was lif
ted before the gaze of one of the only survivors of Earth civilization.
Yifan asked, “Why don’t you tell me what the most powerful weapon for a civilization possessing almost infinite technological prowess is? Don’t think of this as a technical question. Think philosophy.”
Cheng Xin pondered for a while and then struggled to shake her head. “I don’t know.”
“Your experiences should give you a hint.”
What had she experienced? She had seen how a cruel attacker could lower the dimensions of space by one and destroy a solar system. What are dimensions?
“The universal laws of physics,” Cheng Xin said.
“That’s right. The universal laws of physics are the most terrifying weapons, and also the most effective defenses. Whether it’s by the Milky Way or the Andromeda Galaxy, at the scale of the local galactic group or the Virgo Supercluster, those warring civilizations possessing godlike technology will not hesitate to use the universal laws of physics as weapons. There are many laws that can be manipulated into weapons, but most commonly, the focus is on spatial dimensions and the speed of light. Typically, lowering spatial dimensions is a technique for attack, and lowering the speed of light is a technique for defense. Thus, the dimensional strike on the Solar System was an advanced attack method. A dimensional strike is a sign of respect. In this universe, respect is not easy to earn. I guess you could consider it an honor for Earth civilization.”
“I thought of something I wanted to ask you. When will the collapse of space in the vicinity of the Solar System into two dimensions cease?”
“It will never cease.”
Cheng Xin shuddered.
“You are scared? Do you think that in this galaxy, in this universe, only the Solar System is collapsing into two dimensions? Haha...”
Guan Yifan’s bitter laughter caused Cheng Xin’s heart to seize up. She said, “What you’re saying makes no sense. At least, it doesn’t make sense to lower spatial dimensions as a weapon. In the long run, that’s the sort of attack that would kill the attacker as well as the target. Eventually, the side that initiated attack would also see their own space fall into the two-dimensional abyss they created.”
Nothing but silence. After a long while, Cheng Xin called out, “Dr. Guan?”
“You’re too... kind-hearted,” Guan Yifan said softly.
“I don’t understand—”
“There’s a way for the attacker to avoid death. Think about it.”
Cheng Xin pondered and then said, “I can’t figure it out.”
“I know you can’t. Because you’re too kind. It’s very simple. The attacker must first transform themselves into life forms that can survive in a low-dimensional universe. For instance, a four-dimensional species can transform itself into three-dimensional creatures, or a three-dimensional species can transform itself into two-dimensional life. After the entire civilization has entered a lower dimension, they can initiate a dimensional strike against the enemy without concern for the consequences.”
Cheng Xin was silent again.
“Are you reminded of anything?” Yifan asked.
Cheng Xin was thinking of more than four hundred years ago, when Blue Space and Gravity had stumbled into the four-dimensional fragment. Yifan had been a member of the small expedition that conversed with the Ring.
Did you build this four-dimensional fragment?
You told me that you came from the sea. Did you build the sea?
Are you saying that for you, or at least for your creators, this four-dimensional space is like the sea for us?
More like a puddle. The sea has gone dry.
Why are so many ships, or tombs, gathered in such a small space?
When the sea is drying, the fish have to gather into a puddle. The puddle is also drying, and all the fish are going to disappear.
Are all the fish here?
The fish responsible for drying the sea are not here.
We’re sorry. What you said is really hard to understand.
The fish that dried out the sea went onto land before they did this. They moved from one dark forest to another dark forest.
“Is it worth it to pay such a price for victory in war?” Cheng Xin asked. She could not imagine how it was possible to live in a world of one fewer dimension. In two-dimensional space, the visible world consisted of a few line segments of different lengths. Could anyone who was born in three-dimensional space willingly live in a thin sheet of paper with no thickness? Living in three dimensions must be equally confining and unimaginable for those born to a four-dimensional world.
“It’s better than death,” said Yifan.
While Cheng Xin was still recovering from the shock, Yifan continued, “The speed of light is also frequently used as a weapon. I’m not talking about building light tombs—or, as you call them, black domains. Those are just defensive mechanisms employed by weak worms like us. The gods do not stoop so low. In war, it’s possible to make reduced-lightspeed black holes to seal the enemy inside. But more commonly, the technique is used to construct the equivalents of pits and city walls. Some reduced-lightspeed belts are large enough to traverse an entire arm of a galaxy. In places where the stars are dense, many reduced-lightspeed black holes can be connected together into chains that stretch for tens of millions of light-years. That’s a Great Wall at the scale of the universe. Even the most powerful fleets, once trapped, would not be able to escape. Those barriers are very difficult to cross.”
“What is the ultimate result of all this manipulation of space-time?”
“Dimensional strikes will eventually cause more and more of the universe to become two-dimensional, until one day the entire universe is two-dimensional. Similarly, the construction of fortifications will eventually cause all the reduced-lightspeed areas to connect, until the different lowered lightspeeds all average out: This new average will be the new c for the universe.
“At that time, any scientist from a baby civilization—like us—would think that the speed of light through vacuum is barely a dozen kilometers per second, and this is an ironclad universal constant, just like we now think the same of three hundred thousand kilometers per second.
“Of course, I’ve only brought up two examples. Other universal laws of physics have been used as weapons as well, though we don’t know all of them. It’s very possible that every law of physics has been weaponized. It’s possible that in some parts of the universe, even... Forget it, I don’t even believe that.”
“What were you going to say?”
“The foundation of mathematics.”
Cheng Xin tried to imagine it, but it was simply impossible. “That’s... madness.” Then she asked, “Will the universe turn into a war ruin? Or, maybe it’s more accurate to ask: Will the laws of physics turn into war ruins?”
“Maybe they already are.... The physicists and cosmologists of the new world are focused on trying to recover the original appearance of the universe before the wars more than ten billion years ago. They’ve already constructed a fairly clear theoretical model describing the pre-war universe. That was a really lovely time, when the universe itself was a Garden of Eden. Of course, the beauty could only be described mathematically. We can’t picture it: Our brains don’t have enough dimensions.”
Cheng Xin thought back to the conversation with the Ring again.
Did you build this four-dimensional fragment?
You told me that you came from the sea. Did you build the sea?
“You are saying that the universe of the Edenic Age was four-dimensional, and that the speed of light was much higher?”
“No, not at all. The universe of the Edenic Age was ten-dimensional. The speed of light back then wasn’t only much higher—rather, it was close to infinity. Light back then was capable of action at a distance, and could go from one end of the cosmos to the other within a Planck time.... If you had been to four-dimensional space, you would have some vague hint of how beautiful that ten-dimensional Garden must have been.”
“You’re saying—”
“I’m not saying anything.” Yifan seemed to have awakened from a dream. “We’ve only seen small hints; everything else is just guessing. You should treat it as a guess, just a dark myth we’ve made up.”
But Cheng Xin continued to follow the course of the discussion taken so far. “—that during the wars after the Edenic Age, one dimension after another was imprisoned from the macroscopic into the microscopic, and the speed of light was reduced again and again.... ”
“As I said, I’m not saying anything, just guessing.” Yifan’s voice grew softer. “But no one knows if the truth is even darker than our guesses.... We are certain of only one thing: The universe is dying.”
The ship stopped accelerating, and weightlessness returned. Before Cheng Xin’s eyes, space and the stars appeared more and more hallucinatory, more and more like a nightmare. Only the 3G hypergravity had brought some sense of solidity. She had welcomed the powerful embrace of those arms, an embrace that had provided some protection against the terror and frigidity of the dark myths of the universe. But now the hypergravity was gone, and only nightmare remained. The Milky Way appeared as a patch of ice hiding bloody remains, and DX3906 nearby appeared as a cremator burning over an abyss.
“Can you turn off the holographic display?” Cheng Xin asked.
Yifan turned it off, and Cheng Xin returned from the vastness of space to the cramped eggshell interior of the cabin. Here, she recovered a trace of the security she craved.
“I shouldn’t have told you all that,” Yifan said. His sorrow was sincere.
“I would have found out sooner or later,” Cheng Xin said.
“Let me repeat: They are just guesses. There’s no real scientific proof. Don’t think about it too much. Focus on what’s before your eyes; focus on the life you must live.” Yifan put a hand over hers. “Even if what I told you is true, those events are measured at the scale of hundreds of millions of years. Come with me to our world, which is now also your world. Live out your life and stop skipping across the surface of time. As long as you live your life within a hundred thousand years and a thousand light-years, none of those things need concern you. That ought to be enough for anyone.”