by Cixin Liu
“Were you able to see these two planets back then?” Cheng Xin asked.
“No. They were impossible to see in the visible light range. Maybe those telescopes from the Solar System advance warning system could have seen them, but all I could do was deduce their existence through the data obtained via the solar gravitational lens.... I did theorize about the appearance of these two planets, and it looks like I was basically right.”
Halo had taken only fifty-two hours (by the ship’s frame of reference) to traverse the 286 light-years between the Solar System and the planetary system around DX3906, but it took eight full days to cross the sixty AU between the rim of the planetary system and Planet Blue at sub-light speeds. As Halo approached Planet Blue, Cheng Xin and AA discovered that its resemblance to the Earth was only superficial. The blue hue of this planet wasn’t the result of an ocean, but the color of the vegetation covering the continents. Planet Blue’s oceans were light yellow and took up only about a fifth of the planet’s surface. Planet Blue was a cold world; about a third of its continental surface was covered by blue vegetation, with the rest shrouded in snow. Most of the ocean was frozen, and only small patches near the equator were in liquid form.
Halo entered orbit around Planet Blue and began its descent. But the ship’s AI announced a new discovery. “An intelligent radio signal has been detected from the surface. It’s a landing beacon using communication formats dating from the start of the Crisis Era. Would you like me to follow its instructions?”
Cheng Xin and AA looked at each other excitedly. “Yes!” Cheng Xin said. “Follow its instructions to land.”
“Hypergravity will approach 4G. Please enter into secured landing positions. Landing sequence will be initiated once you’re secure.”
“Do you think it’s him?” AA asked.
Cheng Xin shook her head. In her life, moments of happiness were only gaps between mass catastrophes. She was now afraid of happiness.
Cheng Xin and AA sat in hypergravity seats, and the seats closed around them like giant palms squeezing them tight. Halo decelerated and descended, entering Planet Blue’s atmosphere after a series of powerful jolts. They could see the blue-and-white continents swinging into view in the images captured by the ship’s monitoring system.
Twenty minutes later, Halo landed near the equator. The ship’s AI suggested that Cheng Xin and AA wait ten minutes before getting out of their seats, to give their bodies a chance to adjust to Planet Blue’s gravity, which was similar to the Earth’s. Out of the porthole and on the monitoring system terminals, they could see that the yacht had landed in the middle of a blue grassland. Not too far away, they could see rolling mountains covered by snow—the landing site was near the foot of the mountain range. The sky was a light yellow, like the ocean when viewed from space. A light red sun shone in the sky. It was noon on Planet Blue, but the sky and the sun’s colors made it resemble dusk on the Earth.
Cheng Xin and AA didn’t examine the environment around them too carefully. Their attention was taken up by another small vehicle parked near Halo. It was a tiny craft, about four to five meters tall, with a dark gray surface. The profile was streamlined, but the tail fins were tiny. It didn’t seem to be an aircraft, but rather a ground-to-space shuttle.
A man stood next to the shuttle, dressed in a white jacket and dark-colored pants. The turbulence of Halo’s landing disturbed his hair.
“Is that him?” AA asked.
Cheng Xin shook her head. She knew right away that this wasn’t Yun Tianming.
The man waded through the blue sea of grass toward Halo. He moved slowly, and his posture and movements showed some exhaustion. He didn’t show any signs of surprise or excitement, as if the appearance of Halo was a perfectly normal occurrence. He stopped a few tens of meters away from the yacht and waited patiently in the grass.
“He’s good-looking,” said AA.
The man looked to be in his forties. He was East Asian in appearance, and he was indeed more handsome than Yun Tianming, with a broad forehead and wise but gentle eyes. His gaze made you believe he was always thinking, as if nothing in the universe, including Halo, could surprise him, but only cause him to think more. He lifted his hands and moved them around his head, indicating a helmet. Then he shook his head and waved one hand, indicating that they didn’t need space suits out there.
The ship’s AI agreed. “Atmospheric composition: thirty-five percent oxygen, sixty-three percent nitrogen, two percent carbon dioxide, with trace amounts of inert gasses. Breathable. But the atmospheric pressure is only point five three of Earth standard. Do not engage in strenuous exercise.”
“What is that biological entity standing next to the ship?” asked AA.
“Standard human being,” the AI replied.
Cheng Xin and AA exited the ship. They hadn’t adjusted to the gravity yet, and stumbled a bit as they walked. Outside, they breathed easily, not feeling the thinness of the air. A chill breeze blew at them and brought the fragrance of grass, refreshing them. The wide-open view showed the blue-and-white mountains and earth, the light yellow sky and red sun. The whole thing resembled a false-color photograph of the Earth. Other than the strange colors, everything looked familiar. Even the blades of grass looked just like the grass on the Earth, except for their blue hue. The man came to the foot of the stairs.
“Wait a minute. The stairs are too steep. I’ll help you down.” He climbed up the stairs easily and helped Cheng Xin down. “You should have rested longer before coming out. There’s no urgency.” Cheng Xin could hear an obvious Deterrence Era accent.
His hand felt warm and strong to Cheng Xin, and his broad body shielded her from the chill wind. She had the impulse to jump into this man’s arms, the first man she had met after traveling more than two hundred light-years from the Solar System.
“Did you come from the Solar System?” the man asked.
“Yes.” She leaned against the man and descended the stairs. She felt her trust for him grow, and put more of her weight on him.
“There’s no more Solar System,” AA said as she sat down at the top of the stairs.
“I know. Did anyone else escape?”
Cheng Xin was now on the ground. She sank her feet into the soft grass and sat down on the bottom step. “Probably not.”
“Oh...” The man nodded and climbed up again to help AA. “My name is Guan Yifan. I’ve been waiting for you here.”
“How did you know we would come?” AA asked, allowing Yifan to hold her hand.
“We received your gravitational wave transmission.”
“You’re from Blue Space?”
“Ha! If you’d asked those who had just left that question, they’d think you very strange. Blue Space and Gravity are ancient history from more than four centuries ago. But I really am an ancient. I was a civilian astronomer aboard Gravity. I’ve been hibernating for four centuries, and only awakened five years ago.”
“Where are Blue Space and Gravity now?” Cheng Xin struggled to stand, pulling herself up by the railing of the stairs. Yifan continued down with AA.
“In museums.”
“Where are the museums?” AA asked. She put her arm around Yifan’s shoulder so that Yifan was practically carrying her down.
“On World I and World IV.”
“How many worlds are there?”
“Four. And two more are being opened up for settlement.”
“Where are all these worlds?”
Guan Yifan gently deposited AA on the ground and laughed. “A word of advice: In the future, no matter who you meet—human or otherwise—don’t ask for the location of their worlds. That’s a basic bit of manners in the cosmos—like how it’s impolite to ask a lady’s age.... Nonetheless, let me ask you, how old are you now?”
“We’re as old as we look,” AA said, and sat down on the grass. “She’s seven hundred and I’m five hundred.”
“Dr. Cheng looks about the same as she did four centuries ago.”
“You
know her?” AA looked up at Guan Yifan.
“I had seen pictures in transmissions from Earth. Four centuries ago.”
“How many people are on this planet?” Cheng Xin asked.
“Just the three of us.”
“That must mean that your worlds are all better than this one,” AA said.
“You mean the natural environment? Not at all. In some places, the air is barely breathable, even after a century of terraforming. This is one of the best planets we’ve seen for settlement. Although we welcome you here, Dr. Cheng Xin, we do not recognize your claim of title.”
“I’d given that up a long time ago,” Cheng Xin said. “So why haven’t people settled here?”
“It’s too dangerous. Outsiders come here often.”
“Outsiders? Extraterrestrials?” AA asked.
“Yes. This is close to the center of the Orion Arm. Two busy shipping lanes flow through here.”
“Then what are you doing here? Just waiting for us?”
“No. I came with an exploratory expedition. They’ve already left, but I stayed to wait for you.”
About a dozen hours later, the three welcomed night on Planet Blue. There was no moon, but compared to the Earth, the stars here were far brighter. The Milky Way was like a sea of silver fire that cast their shadows on the ground. This place wasn’t much closer to the center of the galaxy than the Solar System. However, the space between here and the Sun was filled with interstellar dust, making the Milky Way appear much dimmer from the Solar System.
In the bright starlight, they could see the grass around them moving. At first, Cheng Xin and AA thought it was an illusion produced by the wind, but then they realized that the grass underfoot was writhing as well, and making a rustling noise. Yifan told them that the blue grass really did move. The roots of the grass were also feet, and as the seasons changed, the grass migrated across the latitudes, mainly at night. As soon as AA heard that, she tossed away the stalks of grass she was playing with in her hands. Yifan explained that the blades of grass really were plants, and relied on photosynthesis, possessing only a basic sense of touch. The other plants in this world were also capable of moving. He pointed to the mountains and they saw the forests moving in the starlight. The trees moved far faster than the grass, and resembled armies marching at night.
Yifan pointed at a spot in the sky where the stars were slightly less dense. “A few days ago we could see the Sun in that direction, much more clearly than you could see this star from the Earth. Of course, what we saw was the Sun of two hundred eighty-seven years ago. The Sun went out on the day the expedition left me here.”
“The Sun is no longer emitting light, but its area is huge. Perhaps you can still see it through telescopes,” AA said.
“No, you won’t be able to see anything.” Yifan shook his head and pointed at that patch of sky again. “Even if you go back there now, you wouldn’t be able to see anything. That part of space is empty. The two-dimensional Sun and planets you saw were actually just the result of the release of energy when three-dimensional material collapsed into two dimensions. What you saw wasn’t two-dimensional material, only the refraction of electromagnetic radiation at the interface between two-dimensional and three-dimensional space. After the energy was released, nothing would be visible. The two-dimensional Solar Space has no contact with three-dimensional space.”
“How can that be?” Cheng Xin asked. “It’s possible to see the three-dimensional world from four-dimensional space.”
“True. I personally got to see three-dimensional space from four-dimensional space, but it’s not possible to see the two-dimensional world from three dimensions. This is because three-dimensional space has thickness, meaning that there is a dimension that could stop and scatter the light from four-dimensional space, making it visible from four dimensions. But two-dimensional space has no thickness, so light from three-dimensional space passes through it without hindrance. The two-dimensional world is completely transparent and cannot be seen.”
“There’s no way at all?” AA asked.
“No. In theory, nothing allows it.”
Cheng Xin and AA were silent for a while. The Solar System had disappeared completely. The only hope they had held out for the mother world was gone. But Guan Yifan did bring them a bit of comfort.
“There’s only one way to detect the presence of the two-dimensional Solar System from three-dimensional space: gravity. The gravity of the Solar System still has an effect, so, in that empty space ought to be detectable as an invisible source of gravity.”
Cheng Xin and AA looked at each other thoughtfully.
“Sounds like dark matter, doesn’t it?” Yifan laughed. Then he changed the subject. “Why don’t we talk about the date you came for?”
“You know Yun Tianming?” AA asked.
“No.”
“What about the Trisolaran Fleet?” Cheng Xin asked.
“We don’t know much. The First and Second Trisolaran Fleets never joined together. More than sixty years ago, there was a large-scale space battle near Taurus. It was brutal, and the resulting wreckage formed a new interstellar dust cloud. We know that one of the sides in the battle was the Second Trisolaran Fleet, but we don’t know who they were fighting against. We also don’t know how the battle ended.”
“What happened to the First Trisolaran Fleet?” Cheng Xin asked. Her eyes flickered in the starlight.
“We haven’t received any information about them.... In any event, you shouldn’t stay here too long. This is not a safe place. Why don’t you come with me to our world? The terraforming there is over, and life is getting better.”
“I agree!” AA said. Then she held Cheng Xin by the arm. “Let’s go with him. Even if you wait here for the rest of your life, you most likely won’t hear anything. Life shouldn’t be a lifetime of waiting.”
Cheng Xin nodded silently. She knew that she was chasing a dream.
They decided to wait one more day on Planet Blue before departing.
Guan Yifan had a small spaceship waiting in synchronous orbit. The ship was tiny and didn’t have a name, only a number. But Yifan called it Hunter, and explained that the name was to honor the memory of a friend who’d lived on Gravity more than four hundred years ago. Hunter was not equipped with an ecological cycling system, and for long voyages passengers had to enter hibernation. Although Hunter was only a few percent of Halo’s volume, it was also a lightspeed ship equipped with a curvature engine. They decided to have Yifan ride on Halo as well and control Hunter as a drone. Cheng Xin and AA didn’t ask about the course they would take, and Yifan even refused to answer questions about the duration of the anticipated voyage. He was extremely cautious when it came to information about the location of human worlds.
For the day, the three took short hikes in the vicinity of Halo. This was a day of many firsts for Cheng Xin, AA, and all the Solar System humans who had disappeared along with the home world: the first trip to an extrasolar planetary system; the first steps on the surface of an exoplanet; the first voyage to a world with life outside the Solar System.
Compared with the Earth, the ecology of Planet Blue was relatively simple. Other than the mobile blue vegetation, there was not much life to be found, except for a few species of fish in the ocean. There were no complex animals on land, only simple insects. The world resembled a simplified Earth. It was possible for Earth plants to survive here, so humans could live here even without advanced technology.
Guan Yifan was filled with admiration for Halo’s design. He said that for Galactic humans, people who had made their home in the Milky Way, there was one quality about Solar System humans that they did not inherit and could not learn: enjoyment of life. He spent much time in the lovely courtyards, and indulged himself with holographic projections of grand sights from ancient Earth. He still looked as thoughtful as ever, but his eyes were moist.
During this time, 艾 AA cast Yifan frequent amorous glances. The relationship between them gradually cha
nged as the day went on. AA thought up all kinds of excuses to be close to Yifan, and listened intently while he spoke, nodding from time to time and smiling. She had never behaved like this in front of any other man. During the centuries Cheng Xin had known her, AA had countless lovers, often dating two or more at the same time, but Cheng Xin knew that AA had never really been in love. However, she was clearly smitten with this cosmologist from the Deterrence Era. Cheng Xin was happy to see this. AA deserved a happy new life in this new world.
As for Cheng Xin, she knew that she was spiritually dead. The only hope that had sustained her was finding Tianming, and now this hope seemed like an impossible dream. Truthfully, she had always known that a date made for four centuries later and 286 light-years away was an impossible dream. She would continue to keep her body alive, but it was just a matter of fulfilling her duty of preventing the death of half of the population to survive the destruction of Earth civilization.
Night fell again. They decided to sleep aboard Halo and leave in the morning.
At midnight, Guan Yifan was awakened by his wrist communicator. It was a call from Hunter in synchronous orbit. Hunter passed on the information gathered by the three small monitoring satellites left by the expedition—two of which orbited around Planet Blue and the last around Planet Gray. The alert had come from the one around Planet Gray.
Thirty-five minutes ago, five unidentified spacecraft had landed on Planet Gray. Twelve minutes later, the spacecraft had lifted off and disappeared without even entering planetary orbit. There was strong interference with the satellite, and the images it transmitted were blurry.
Yifan’s expedition was responsible for seeking out and studying traces left in this planetary system by other civilizations. After receiving the alert from the satellite, he immediately decided to take the shuttle up to Hunter to investigate. Cheng Xin insisted on coming with him. Yifan initially refused, but agreed after AA spoke to him.
