by Cixin Liu
“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 lifted 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 stri
kes 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.”
“Yes, it is enough, thank you.” Cheng Xin held Yifan’s hand.
* * *
Cheng Xin and Guan Yifan spent the rest of the journey in the forced slumber of the sleep-aid machine. The trip lasted four days. By the time they awakened in the hypergravity of deceleration, Planet Gray took up most of their field of view.
Planet Gray was a small planet. It visually resembled the moon, a barren rock, but instead of craters, much of Planet Gray’s surface was taken up by desolate plains. Hunter entered orbit around Planet Gray. Due to the lack of an atmosphere, the orbit was very low. The ship approached the coordinates provided by the monitoring satellite, where the five unidentified spacecraft had landed and then taken off. Yifan had planned to land the shuttle there and investigate the traces left by the spacecraft, but he and Cheng Xin had not anticipated that the mysterious visitors would leave behind such large signs that they were visible from space.
“What is that?” Cheng Xin cried out.
“Death lines.” Yifan recognized them right away. “Don’t get too close,” he said to the AI.
He was referring to five black lines. One end of each line was connected to the surface of the planet, and the other end extended into space, like five black hairs growing out of Planet Gray. Each line stretched higher than Hunter’s orbit.
“What are they?”
“Trails left by curvature propulsion. Those lines are the result of extreme curvature manipulation. The speed of light within the trails is zero.”
On the next orbit, Guan Yifan and Cheng Xin entered the shuttle and descended toward the surface. Due to the low orbit and the lack of an atmosphere, the descent was smooth and fast. The shuttle landed about three kilometers from the death lines.
They leapt across the surface under 0.2G. A thin layer of dust covered the surface of Planet Gray, along with gravel of various sizes. Due to the lack of atmospheric scattering of sunlight, shadows and lit areas were sharply delineated. When they were about a hundred meters from the death lines, Yifan waved Cheng Xin to a stop. Each death line was about twenty or thirty meters in diameter, and from here, they resembled death columns.
“These are probably the darkest things in the universe,” Cheng Xin said. The death lines showed no details except an exceptional blackness showing the boundaries of the zero-lightspeed region, with no real surface. Looking up, the lines showed up clearly even against the dark backdrop of space.
“These are the deadest things in the universe as well,” said Guan Yifan. “Zero-lightspeed means absolute, one hundred percent death. Inside it, every fundamental particle, every quark is dead. There is no vibration. Even without a source of gravity inside, each death line is a black hole. A zero-gravity black hole. Anything that falls in cannot reemerge.”
Yifan picked up a rock and tossed it toward one of the death lines. The rock disappeared inside the absolute darkness.
“Can your lightspeed ships produce death lines?” Cheng Xin asked.
“Far from it.”
“So you’ve seen these before, then?”
“Yes, but only rarely.”
Cheng Xin gazed up at the giant black columns reaching into space. They lifted up the domed sky and seemed to turn the universe into a Palace of Death. Is this the ultimate end for everything?
In the sky, Cheng Xin could see the end of the columns.
She pointed in that direction. “So the ships entered lightspeed at the end?”
“That’s right. These are only about a hundred kilometers high. We’ve seen columns even shorter than these, presumably left by ships that entered lightspeed almost instantaneously.”
“Are these the most advanced lightspeed ships?”
“Maybe. But this is a rarely seen technique. Death lines are usually the products of Zero-Homers.”
“Zero-Homers?”
“They’re also called Resetters. Maybe they’re a group of intelligent individuals, or a civilization, or a group of civilizations. We don’t know exactly who they are, but we’ve confirmed their existence. The Zero-Homers want to reset the universe and return it to the Garden of Eden.”
“How?”
“By moving the hour hand of the clock past twelve. Take spatial dimensions as an example. It’s practically impossible to drag a universe in lower dimensions back into higher dimensions, so maybe it’s better to work forward in the other direction. If the universe can be lowered into zero dimensions and then beyond, the clock might be reset and everything returned to the beginning. The universe might possess ten macroscopic dimensions again.”
“Zero dimensions! Have you seen such a thing done?”
“No. We’ve only witnessed two-dimensionalization. We’ve never even seen one-dimensionalization. But somewhere, some Zero-Homers must be trying. No one knows if they’ve ever succeeded. Comparatively, it’s easier to lower the speed of light to zero, so we’ve seen more evidence of such attempts to lower the speed of light past zero and return it to infinity.”
“Is that even theoretically possible?”
“We don’t know. Maybe the Zero-Homers have theories that say yes, but I don’t think so. Zero-lightspeed is an impassable wall. Zero-lightspeed is absolute death for all existence, the cessation of all motion. Under such conditions, the subjective cannot influence the objective in any way, so how can the ‘hour hand’ be shifted past it? I think the Zero-Homers are practicing a kind of religion, a kind of performance art.”
Cheng Xin stared at the death lines, her terror mixed with awe. “If these are trails, why don’t they spread?”