by Cixin Liu
“The light slows down?” Cheng Xin sensed a great secret was being revealed.
Luo Ji continued, “I understand that you figured out curvature propulsion from a small boat propelled by soap. Let me ask you: After the ship reached the other side of the bathtub, did you pull it back and try again?”
They hadn’t. Due to the fear of sophons, Cheng Xin had tossed the paper boat aside. But it was easy to figure out what would have happened.
“The ship would not move, or at least it would only move slowly,” Cheng Xin said. “After the first trip, the surface tension of the water in the tub had already been reduced.”
“That’s right. It’s the same principle with lightspeed ships. The very structure of space itself is changed by the trail of a curvature-propelled ship. If a second curvature-propelled ship were placed inside the trail of the first, it would hardly move. Within the trails of lightspeed ships, one must use a more powerful curvature propulsion drive. It would still be possible to use curvature propulsion to achieve the highest speed possible within such a space, but the maximum velocity is much lower than the maximum velocity of the first ship. In other words, the speed of light through vacuum is lowered within the trail of lightspeed ships.”
“How much lower?”
“Theoretically, it could be reduced to zero, but that’s not achievable in reality. But if you adjust the curvature ratio of Halo’s engine to the maximum, you can lower the speed of light in its trail down to exactly what we’ve been looking for: sixteen point seven kilometers per second.”
“Then you’d have...” AA said, staring at Luo Ji.
The black domain, Cheng Xin thought.
“The black domain,” Luo Ji said. “Of course, a single ship is insufficient to produce a black domain containing an entire star and its planetary system. We calculated that it would take more than a thousand curvature propulsion ships to accomplish such a thing. If all these ships started near the Sun and spread out in every direction at lightspeed, the trails they produced would expand and connect to each other, forming a sphere that contained the entire Solar System. The speed of light within this sphere would be sixteen point seven kilometers per second—a reduced-lightspeed black hole, or a black domain.”
“So the black domain can be a product of lightspeed ships....”
In the cosmos, the trail of a curvature propulsion drive could be a sign of danger, as well as a safety announcement. A trail far away from a world was the former; a trail that shrouded a world the latter. It was like a noose, indicating danger and aggression when held in the hand, but safety when wrapped around the holder’s own neck.
“Correct, but we found out about it too late. While studying curvature propulsion, the experimenters plowed ahead of the theoreticians. You should know that was Wade’s style. Many experimental discoveries could not be explained by theory, but without a theoretical framework, some phenomena were simply ignored. During the earliest years of research—when their biggest achievement was moving your hair—the trails produced by curvature propulsion were thin and small, and hardly anyone paid any attention, even though there were plenty of signs of something strange going on: For instance, after the trail expanded, the low speed of light caused quantum integrated circuits in nearby computers to malfunction, but no one sought to investigate. Later, after the experiments grew in scale, people finally discovered the secret of lightspeed trails. It was because of this discovery that the Federation Government agreed to collaborate with us. They did, in fact, pour all the resources they could command into the development of lightspeed spaceships, but there just wasn’t enough time.” Luo Ji shook his head and sighed.
Cheng Xin said what he couldn’t bring himself to say. “There were thirty-five years between the Halo City Incident and the completion of the Mercury base. Thirty-five precious years were lost.”
Luo Ji nodded. Cheng Xin thought the way he looked at her was no longer kind, but rather resembled the fires of the Last Judgment. His gaze seemed to say, Child, look at what you’ve done.
Cheng Xin now understood that of the three paths of survival presented to humanity—the Bunker Project, the Black Domain Plan, and lightspeed ships—only lightspeed ships were the right choice.
Yun Tianming had pointed this out, but she had blocked it.
If she hadn’t stopped Wade, Halo City might have achieved independence. Even if the independence was short-lived, they could have discovered the effects of lightspeed trails and changed the government’s attitude toward lightspeed ships. Humanity might have had time to construct a thousand lightspeed ships and build the black domain, to avoid this dimensional strike.
Humanity could have divided into two parts: those who wanted to fly to the stars, and those who wanted to stay within the black domain and live in tranquility. Each would have gotten what they wanted.
In the end, she had committed another grave error.
Twice, she had been placed in a position of authority second only to God, and both times she had pushed the world into the abyss in the name of love. This time, no one could fix her mistake for her.
She began to hate someone: Wade. She hated that he had kept his promise. Why? Out of his masculine pride, or for her? Cheng Xin understood that Wade did not know the effects of curvature propulsion trails. His goal in researching lightspeed ships was stated eloquently by that anonymous Halo City soldier: a fight for freedom, for a chance to live as free men in the cosmos, for the billions and billions of new worlds out there. She believed that if he had known that lightspeed spaceflight was the only path to life for humanity, he would not have kept his promise.
She could not shirk her responsibility. It didn’t matter whether she really was second only to God—if she was in that position, she had to carry out her duty.
Not long ago on Pluto, Cheng Xin had experienced one of the most relaxed moments of her life. Indeed, it was easy to face the end of the world: All responsibilities were gone, as were all worries and anxieties. Life was as simple and pure as the moment when one first emerged from the mother’s womb. Cheng Xin just had to wait in peace for her poetic, artistic end, for her moment to join the giant painting of the Solar System.
But now, everything had been turned upside down. Early cosmology had presented a paradox: If the universe was infinite, then every spot in the universe would feel the cumulative effects of the infinite gravity exerted by an infinity of celestial bodies. Cheng Xin really did feel an infinite gravity now. The power came from every corner of the universe, ruthlessly tearing at her soul. The horror of her last moments as the Swordholder 127 years ago resurfaced as four billion years of history pressed down on her and suffocated her. The sky was full of eyes staring at her: the eyes of dinosaurs, trilobites, ants, birds, butterflies, bacteria... just the number of men and women who had lived on the Earth possessed a hundred billion pairs of eyes.
Cheng Xin saw AA’s eyes, and understood the words in her gaze: You’ve finally experienced something worse than death.
Cheng Xin knew that she had no choice but to live on. She and AA were the last two survivors of human civilization. Her death would mean the death of half of all that was left of humanity. Living on was the appropriate punishment for her mistake.
But the course ahead was a blank. In her heart, space was no longer black, but colorless. What was the point of going anywhere?
“Where should we go?” Cheng Xin muttered.
“Go find them,” Luo Ji said. His image was even more blurred and now only black and white.
His words illuminated Cheng Xin’s dark thoughts like lightning. She and AA looked at each other and immediately understood who “them” meant.
Luo Ji continued, “They’re still alive. The Bunker World received a gravitational wave transmission from them five years ago. It was a short message, and didn’t explain where they were. Halo will periodically hail them with gravitational waves. Maybe you’ll find them; maybe they’ll find you.”
Luo Ji’s black-and-white image disapp
eared as well, but they could still hear his voice. He said one last thing, “Ah, it’s time for me to go into the picture. Safe travels, children.”
The transmission from Pluto was cut off.
On the monitor, they could see Pluto light up and expand in two dimensions. The part of Pluto containing the museum was the first to touch the plane.
The Doppler effect of Halo’s speed was now visible. The light from the stars ahead shifted to bluish, while the light from the stars behind shifted to reddish. The color shift was apparent in the two-dimensional Solar System.
Outside, no other fleeing spaceships could be seen; Halo had passed them all. All the fleeing spaceships were now falling onto the two-dimensional space like drops of rain against glass.
Very few transmissions could now be received from the direction of the Solar System. Due to the Doppler effect, the brief bursts of voices sounded strange, like singing.
“We’re very close! Are you behind us?”...“Don’t do this! No!”...“There’s no pain. I’m telling you, it’ll be over in a flash.”...“You still don’t believe me, after all this? Fine, don’t believe me.”...“Yes, sweetie, we’ll become very thin.”...“Come here! We should be together.”
Cheng Xin and AA listened. The voices became fewer and fewer, and separated by longer gaps. After thirty minutes, they heard the last voice coming out of the Solar System:
“Ahhhhhhhhh—”
The voice was cut off. The giant painting called the Solar System was complete.
Halo continued to fall toward the plane. The speed it had already achieved was slowing down its fall, but the ship still hadn’t achieved escape velocity. By now, Halo was the only man-made three-dimensional object in the Solar System, and Cheng Xin and AA were the only people not in the painting. Halo was very close to the plane, and from this angle, looking at the two-dimensional Sun was like looking at the sea from shore: the dim, dark red surface stretched into the distance without bounds. The freshly flattened Pluto was now very large, and still expanded at a rate that was visible to the naked eye. Cheng Xin examined the exquisite “tree rings” of Pluto and tried to find traces of the museum, but she couldn’t see anything—it was too small. The giant waterfall that was three-dimensional space tumbling into the flat plane seemed inexorable. Cheng Xin began to doubt whether the curvature propulsion engine really was capable of propelling the ship to lightspeed. She hoped for everything to be over.
But then, the ship’s AI spoke.
“Halo will enter lightspeed in one hundred and eighty seconds. Please select a destination.”
“We don’t know where to go,” said AA.
“You can select a destination after we’ve entered lightspeed. However, you won’t subjectively be spending much time in lightspeed, and it’s easy to overshoot your destination. It’s best if you select it now.”
“We don’t know where to find them,” Cheng Xin said. Their existence gave the future some light, but she still felt lost.
AA clutched Cheng Xin’s hands. “Have you forgotten? Other than them, he also exists in the universe.”
Yes, he still exists. Cheng Xin was overwhelmed by heartache. She had never yearned to see anyone as much as him.
“You have a date,” AA said.
“Yes, we have a date,” Cheng Xin repeated mechanically. The torrents of emotion left her numb.
“Then let’s go to your star.”
“Yes, let’s go to our star!” Cheng Xin turned to the ship’s AI. “Can you find DX3906? That was the assigned number back at the beginning of the Crisis Era.”
“Yes. The star is now numbered S74390E2. Please confirm.”
A large holographic star map appeared before them. It showed everything within five hundred light-years of the Solar System. One of the stars glowed bright red, and a white arrow pointed at it. Cheng Xin was very familiar with it.
“That’s the one. Let’s go there.”
“Course set and confirmed. Halo will enter lightspeed in fifty seconds.”
The holographic star map disappeared. In fact, the ship’s entire hull disappeared, and Cheng Xin and AA seemed to be floating in space itself. The AI had never employed this display mode before. In front of them was the starry sea that was the Milky Way, which was now pure blue, reminding them of the real sea. Behind them was the two-dimensional Solar System, suffused with a bloody red.
The universe shuddered and transformed. All the stars in front of them shot straight ahead, as though that half of the universe had transformed into a black bowl and all the stars were falling into the bottom. They clustered ahead of the ship and fused into a single glow, like a giant sapphire in which it was not possible to distinguish individual stars. From time to time, individual stars shot out of the sapphire and swept past the pure black space to fall behind the ship, changing color the whole way: from blue to green, then yellow, and turning red once they were behind the ship. Looking back from the ship, the two-dimensional Solar System and the stars fused into a red ball like a campfire at the end of the universe.
Halo flew at the speed of light toward the star that Yun Tianming had given Cheng Xin.
PART VI
Galaxy Era, Year 409 Our Star
Halo shut off the curvature engine and coasted at lightspeed.
During the voyage, AA tried to comfort Cheng Xin, even though she knew this was a hopeless task.
“It’s ridiculous for you to blame yourself for the destruction of the Solar System. Who do you think you are? Do you think if you stand on your hands, you’ve lifted the Earth? Even if you hadn’t stopped Wade, the outcome of that war would have been hard to predict.
“Could Halo City really have achieved independence? Even Wade couldn’t be certain of that. Could the Federation Government and Fleet really have been scared of a few antimatter bullets? Maybe Halo City could have destroyed a few warships, or even a space city, but ultimately, Halo City would have been exterminated by the Federation Fleet. And in that version of history, there would be no Mercury base, no second chance.
“Even if Halo City had managed to achieve independence, continued to research curvature propulsion, discovered the slowing effects of the trails, and finally collaborated with the Federation Government to build more than a thousand lightspeed ships in time, do you think people would have agreed to build the black domain? Remember how confident people were that the Bunker World would survive a dark forest strike—why would they have agreed to isolate themselves in the black domain?”
AA’s words slid across Cheng Xin’s thoughts like drops of water across a lily pad, leaving no trace. Cheng Xin’s only thought was to find Yun Tianming and tell him everything. In her mind, a journey of 287 light-years would take a long time, but the ship’s AI informed her that the trip would only take fifty-two hours in the ship’s frame of reference. Everything felt unreal to Cheng Xin, as though she had already died and gone to another world.
Cheng Xin spent a long time gazing out of the portholes at space. She understood that each time a star leapt out of the blue cluster in front, swept past the ship, and joined the red cluster behind the ship, it meant that Halo had passed it. She counted the stars and watched as they turned from blue to red—the sight was hypnotic. Eventually, she fell asleep.
By the time Cheng Xin awakened, Halo was close to its destination. It turned 180 degrees and activated the curvature engine for deceleration—in fact, the ship was pushing against its own trail. As the ship decelerated, the blue and red clusters began to spread out like two clusters of exploding fireworks, and soon evolved into a sea of stars distributed evenly around the ship. The slowing down of the ship also gradually erased the red and blue shifts. Cheng Xin and AA saw that the Milky Way ahead of them still looked about the same, but behind them, none of the stars looked familiar. The Solar System was long gone.
“We’re now two hundred eighty-six point five light-years from the Solar System,” said the ship’s AI.
“So two hundred eighty-six years has already pass
ed back there?” AA asked. She looked as if she had just awakened from a dream.
“Yes, if you are using their frame of reference.”
Cheng Xin sighed. For the Solar System in its current condition, was there a difference between 286 years and 2.86 million years? But she thought of something.
“When did the collapse into two dimensions stop?”
The question made AA speechless, as well. Right: When—if ever—did it stop? Was there an instruction within that small, packaged two-dimensional foil that would eventually stop it? Cheng Xin and AA had no theoretical understanding of how three-dimensional space collapsed into two dimensions, but they instinctively thought the idea of an instruction embedded into two-dimensional space to halt its infinite expansion was too magical, the kind of magic that seemed impossible.
Would the collapse never stop?
It was best to not think about it too much.
The star called DX3906 was about the Sun’s size. As Halo began decelerating, it still looked like an ordinary star, but by the time the curvature engine shut off, the star appeared as a disk whose light seemed redder than the Sun’s.
Halo engaged the fusion reactor, and the silence on the ship was broken. The humming of the engine filled the ship, and every surface vibrated slightly. The ship’s AI analyzed the data obtained by the monitoring system and confirmed the basic facts about this solar system: DX3906 had two planets, both of them solid. The one farther from the star was about the size of Mars, but it had no atmosphere and appeared gray in color—so Cheng Xin and AA decided to call it Planet Gray. The other planet, closer to the star, was about the size of the Earth, and its surface resembled the Earth’s: an atmosphere containing oxygen and many signs of life, but without evidence of agriculture or industry. Since it was blue, like the Earth, they decided to call it Planet Blue.
AA was very happy that her research had been confirmed. More than four hundred years ago, she had discovered the star’s planetary system. Before then, people had thought it was a bare star without any planets. Through that work, AA had gotten to know Cheng Xin. Without that coincidence, her life would have turned out completely differently. Fate was such an odd thing: Four centuries ago, when she had gazed at this distant world through the telescope, she could never have imagined that she’d come here one day.