by Cixin Liu
“How big are the snowflakes?” AA asked.
“Most have diameters between four thousand and five thousand kilometers.” The ship’s AI, incapable of wonder, continued to speak in a serene tone.
“Bigger than the moon!” Cheng Xin said.
The AI opened a few other windows, and each showed a zoomed-in snowflake. In these images, the sense of scale was lost, and they seemed to be tiny spirits under a magnifying lens, each snowflake ready to turn into a tiny droplet as soon as it touched down on a palm.
“Oh—” Luo Ji stroked his beard again, and this time, succeeded.
“How are they formed?” AA asked.
“I don’t know,” the AI said. “I can’t find any information about the crystallization of water at astronomical scales.”
In three-dimensional space, snowflakes formed in accordance with the laws of ice-crystal growth. Theoretically, these laws did not restrict the size of snowflakes. The largest snowflake previously on record was thirty-eight centimeters in diameter.
No one knew the laws of ice crystal growth in two-dimensional space. Whatever they were, they permitted ice crystals in two dimensions to grow to five thousand kilometers.
“There’s water on Neptune and Saturn, and ammonia can also form crystals. Why didn’t we see large snowflakes there?” Cheng Xin asked.
The AI said it didn’t know.
Luo Ji squinted his eyes and enjoyed the two-dimensional version of the Earth. “The ocean looks rather nice this way, don’t you think? Only the Earth is worthy of such a lovely wreath.”
“I really want to know what the forests look like, what the grasslands look like, what the ancient cities look like,” Cheng Xin said slowly.
Grief finally struck them, and AA began to sob. Cheng Xin turned her eyes away from the snowflake ocean and made no sound as her eyes filled with tears. Luo Ji shook his head, sighed, and continued to sip his tea. Their grief was moderated to some extent by the thought that the two-dimensional space would also be their home in the end.
They would attain their eternal rest alongside Mother Earth on that plane.
* * *
The three decided to begin their third cargo trip. They exited Halo, gazed up at the sky, and saw the three two-dimensional planets. Neptune, Saturn, and the Earth had grown even larger, and the asteroid belt was wider. This was no hallucination. They asked the AI about it.
“The navigation system has detected a split in the Solar System’s navigational frame of reference. Frame of reference one continues as before. The navigational markers within this system—the Sun, Mercury, Mars, Jupiter, Uranus, Pluto, and some asteroids and Kuiper Belt objects—still satisfy the recognition criteria. Frame of reference two, however, has transformed dramatically. Neptune, Saturn, the Earth, and some asteroids have lost their characteristics as navigational markers. Frame of reference one is moving toward frame of reference two, which leads to the phenomenon you’ve observed.”
In the sky in the other direction, many moving points of light appeared before the stars—the fleet of ships seeking to escape the Solar System. Some of the glowing blue lights dragged long tails behind them. Some of the ships swept by the three of them, fairly close. The bright lights of their engines operating at maximum capacity cast moving shadows of the three observers on the ground. None of the ships tried to land on Pluto.
But it was impossible to escape from the collapsing zone. Halo’s AI was trying to say this: The three-dimensional space of the Solar System was like a large carpet that was being pulled by invisible hands into a two-dimensional abyss. These ships were nothing more than worms on the carpet inching along—they couldn’t extend their already limited allotment of time by much.
“Go ahead by yourselves,” Luo Ji said. “Just take a few more objects. I want to wait here. I don’t want to miss it.” Cheng Xin and AA understood what he meant by “it,” but they had no desire to witness the scene.
After returning to the underground hall, Cheng Xin and AA, not in the mood to pick and choose, randomly gathered a collection of artifacts. Cheng Xin wanted to take along a Neanderthal skull, but AA tossed it aside.
“You’ll have plenty of skulls on this picture,” AA said.
Cheng Xin acknowledged that she was right. The earliest Neanderthals had lived no more than a few hundred thousand years ago. Optimistically, the flattened Solar System would not have visitors until a few hundred thousand years from now. In their eyes, Neanderthals and modern humans would appear to be the same species. Cheng Xin looked around at the other artifacts, and none excited her. For themselves in the present, and for those unimaginable observers in the far future, nothing here mattered as much as the world that was dying outside.
They took a last look at the dim hall and left with the artifacts. Mona Lisa watched them leave, smiling sinisterly and eerily.
On the surface, they saw that yet another two-dimensional planet had appeared in the sky: Mercury (Venus was on the other side of the Sun at this moment). It looked smaller than the two-dimensional Earth, but the light generated by its recent collapse into two dimensions made it very bright.
After they packed the artifacts in the hold, Cheng Xin and AA came out of Halo. Luo Ji, who was waiting outside, leaning on his cane, said, “All right. I think that’s enough. It’s meaningless to carry more, anyway.”
The women agreed. They stood together with Luo Ji on the Plutonian ground and waited for the most magnificent scene of the play: the flattening of the Sun.
At this moment, Pluto was forty-five AU from the Sun. Earlier, since both Pluto and the Sun were in the same region of three-dimensional space, the distance between them hadn’t changed. But when the Sun came into contact with the plane, it ceased to move, while Pluto continued to fall toward it, along with the space around it, causing the distance between them to shrink rapidly.
When the Sun began to two-dimensionalize, the naked eye could only see that its brightness and size appeared to increase suddenly. The latter was due to the rapid expansion of the flattened portion of the Sun on the plane, but from a distance it appeared as though the Sun itself was growing. Halo’s AI projected a large information window outside the ship to show a holographic feed from a telescopic lens, but as Pluto pulled closer to the Sun, even the naked eye could see the grand spectacle of a star collapsing into two dimensions.
As soon as the Sun began to two-dimensionalize, a circle expanded on the plane. Soon, the planar Sun’s diameter exceeded the diameter of the remaining part of the Sun. This process took only thirty seconds. Based on the mean solar radius of seven hundred thousand kilometers, the rim of the two-dimensional Sun grew at the rate of twenty thousand kilometers per second. The planar Sun continued to grow, forming a sea of fire on the plane, and the three-dimensional Sun sank slowly into this blood-red sea of fire.
Four centuries ago, Ye Wenjie had stood on the peak of Red Coast Base and watched such a sunset during the last moments of her life. Her heart had struggled to beat like a zither string about to break, and a black fog had begun to cloud her eyes. On the western horizon, the Sun that was falling into the sea of clouds seemed to melt, and the Sun’s blood seeped into the clouds and the sky, creating a large crimson swath. She had called it humanity’s sunset.
And now, the Sun really was melting, its blood seeping into the deadly plane. This was the last sunset.
In the distance, white fog rose from the ground outside the landing field. Pluto’s solid nitrogen and ammonia sublimated, and the fresh, thin atmosphere began to scatter the sunlight. The sky no longer appeared pure black, but showed hints of purple.
While the three-dimensional Sun was setting, the two-dimensional Sun was rising. A flat star could still radiate its light inside the plane, so the two-dimensional Solar System received its first sunlight. The sides of the four two-dimensional planets facing the sun—Neptune, Saturn, the Earth, and Mercury—all took on a golden glow, though the light only fell along a one-dimensional curved edge. The giant snowflakes tha
t surrounded the Earth melted and turned into white vapor, which was blown by two-dimensional solar wind into two-dimensional space. Some of the vapor soaked up the golden sunlight and appeared as if the Earth had hair that drifted with the wind.
An hour later, the Sun had completely collapsed into two dimensions.
From Pluto, the Sun appeared as a giant oval. The two-dimensional planets were tiny fragments compared to it. Unlike the planets, the Sun did not display clear “tree rings” but was separated into three concentric sections around a core. The center was very bright, and no details could be seen—probably corresponding to the core of the original Sun. The wide ring outside the core probably corresponded to the original radiation zone—a boiling, two-dimensional, bright red ocean where countless cell-like structures rapidly formed, split, combined, and disappeared in a manner that seemed chaotic and agitated when viewed locally, but followed grand patterns and order when viewed as a whole. Outside that was the original Sun’s convection zone. Like in the original Sun, currents of solar material transferred heat into space. But unlike the chaotic radiation zone, the new convection zone revealed clear structure, as many ring-shaped convection loops, similar in shape and size, arranged themselves side by side in neat order. The outermost layer was the solar atmosphere. Golden currents leapt away from the circular rim and formed a large number of two-dimensional prominences, resembling graceful dancers cavorting wantonly around the Sun. Some of the “dancers” even escaped the Sun and drifted far into the two-dimensional universe.
“Is the Sun still alive in two dimensions?” asked AA. She spoke for the hope of all three. They all wished for the Sun to continue to give light and heat to the planar Solar System, even if there was no more life in it.
But her hope was soon dashed.
The flattened Sun began to dim. The light from the core diminished rapidly and soon it was possible to see fine annular structures within. The radiation zone was also quieting, and the boiling calmed down, turning into a viscous peristalsis. The loops in the convection zone distorted, broke apart, and soon disappeared. The golden dancers around the rim of the Sun wilted like dried leaves and lost their vivaciousness. Now it was possible to tell that at least gravity continued to function in the two-dimensional universe. The dancing solar prominences lost the support of solar radiation and began to be dragged back to the edge of the Sun by its gravity. Finally, the dancers yielded to gravity and fell lethargically, until the Sun’s atmosphere was no more than a thin, smooth ring wrapped around the Sun. As the Sun went out, the golden arcs at the edges of the planets also dimmed, and the Earth’s two-dimensional hair, formed from the sublimated ocean, lost its golden glow.
Everything in the three-dimensional world died after collapsing into two dimensions. Nothing survived in a painting with no thickness.
Perhaps a two-dimensional universe could possess its own sun, planets, and life, but they would have to be created and operate under completely different principles.
* * *
While the three were focused on the flattening Sun, Venus and Mars collapsed into the plane as well. Compared to the Sun, however, the two-dimensionalization of these two terrestrial planets was rather unremarkable. The flattened Mars and Venus were very similar to the Earth in terms of their “tree ring” structure. There were many hollow areas near the rim of Mars, places in the Martian crust that contained water, suggesting that Mars had possessed far more water than people thought. After a while, the water also turned an opaque white, but no giant snowflakes appeared. There were giant snowflakes around the flattened Venus, but they weren’t anywhere as numerous as the ones near the Earth, and the Venusian snowflakes were yellow in hue, indicating that they were not water crystals. A while later, the asteroids on that side of the Sun were also flattened, completing the other half of the Solar System necklace.
Tiny snowflakes—three-dimensional ones—now fell from the light purple Plutonian sky. These were the nitrogen and ammonia that had sublimated in the burst of energy during the Sun’s flattening, and which were now freezing into snow as the temperature plummeted following the Sun’s extinguishment. The snow fell more heavily, and soon accumulated a thick layer over the monolith and Halo. Although there were no clouds, the heavy snow blurred Pluto’s sky, and the two-dimensional Sun and the planets turned hazy behind a curtain of snow. The world looked smaller.
“Don’t you feel at home?” AA lifted both hands and spun in the snow.
“I was just thinking the same thing,” Cheng Xin said, and nodded. She had also thought of snow as something unique to the Earth, and the giant snowflakes around the flattened Earth had confirmed this feeling. The snow falling on this cold, dark world on the edge of the Solar System surprisingly provided her a trace of the warmth of home.
Luo Ji watched as AA and Cheng Xin tried to catch the snow. “Hey, you two! Don’t even think about taking off your gloves!”
Cheng Xin did feel an impulse to take off her gloves and catch the snow with her bare hands. She wanted to feel the slight chill, and watch the crystalline snowflakes melt with her own body heat.… but of course she had enough presence of mind to not indulge the impulse. The nitrogen-ammonia snowflakes were at a temperature of minus-210-degrees Celsius. If she really took off her gloves, her hand would turn as fragile and hard as glass and the feeling of being on Earth would disappear instantaneously.
“There’s no more home,” Luo Ji said, shaking his head and leaning against his cane. “Home is now just a picture.”
The nitrogen-ammonia snow didn’t last long. The snowflakes thinned out and the purple haze from the nitrogen-ammonia atmosphere faded. The sky was once again perfectly transparent and dark. They saw that the Sun and the planets had grown even bigger, indicating that Pluto had moved even closer to that two-dimensional abyss.
When the snow stopped, a bright glowing light appeared near the horizon. The intensity of the light grew rapidly, and soon overwhelmed the fading two-dimensional Sun. Although they couldn’t see the details, they knew that it was Jupiter, the Solar System’s largest planet, falling into the plane. Pluto spun slowly, and part of the flattened Solar System had fallen below the horizon, so they thought they wouldn’t get to witness Jupiter’s collapse, but it appeared that the rate of fall into two dimensions was accelerating.
They asked Halo’s AI to look for transmissions from Jupiter. Very few images and videos were being transmitted now, and most were indecipherable. Almost all of the messages they got were audio only. Every communication channel was filled with noise, mostly human voices, as though all the remaining space in the Solar System had been filled with a frenzied sea of people. The voices cried, screamed, sobbed, laughed hysterically … and some even sang. The chaotic background noise made it impossible to tell what they were singing, only that it was many voices singing in harmony. The music was solemn, slow, like a hymn. Cheng Xin asked the AI whether it was possible to receive any official broadcasts from the Federation Government. The AI said that all official communications from the government had terminated when the Earth was flattened. The Federation Government couldn’t fulfill the promise to carry out its duties until the end of the Solar System after all.
Ships trying to escape continued to stream by the vicinity of Pluto.
“Children, it’s time to go,” said Luo Ji.
“Let’s go together,” said Cheng Xin.
“What’s the point?” Luo Ji shook his head and smiled. He pointed at the monolith with his cane. “I’m more comfortable over there.”
“All right. We’ll wait until Uranus is flattened so that we get to spend more time with you,” AA said. There really didn’t seem to be any point in insisting. Even if Luo Ji got on Halo, it would only delay the inevitable by another hour. He didn’t need that bit of time. Indeed, if Cheng Xin and AA didn’t have a mission to carry out, they wouldn’t care for that bit of time either.
“No. You must go now!” Luo Ji said. He struck the ground with his cane forcefully, which made him float
up under the low gravity. “No one knows how much faster the collapse is happening now. Carry out your mission! We can stay in contact, and that’s no different from being together.”
Cheng Xin hesitated for a moment, then nodded. “All right. We’ll leave. Stay in contact!”
“Of course.” Luo Ji lifted his cane in farewell and turned to walk toward the monolith. With the light gravity, he almost floated over the snow on the ground and had to use the cane to slow himself. Cheng Xin and AA watched until the aged figure of this Wallfacer, Swordholder, and humanity’s final grave keeper disappeared behind the door of the monolith.
Cheng Xin and AA went back inside Halo. The yacht took off right away, its thrusters tossing up snow everywhere. Soon, the ship achieved Pluto’s escape velocity—just a hair above one kilometer per second—and reached orbit. From the porthole and the monitor they could see that swaths of white now joined the blue and black patches of the Plutonian surface. The giant words “Earth Civilization,” written in multiple scripts and languages, had been covered by the snow and were almost illegible. Halo passed through the gap between Pluto and Charon as though flying through a canyon, the two celestial bodies were so close.
In this “canyon” there were now many other moving stars—the escaping spaceships. They all moved far faster than Halo. One ship swept past Halo at a distance of no more than a hundred kilometers, and the glow from its nozzles lit up Charon’s smooth surface. They could clearly see its triangular hull and the nearly ten-kilometer-long blue flame shooting out of its nozzles.
