by Cixin Liu
From up close, the surface of the Ring appeared very different from images taken by the telescope. Instead of a golden yellow light, it gave off a dark copper glow. Those faint etched lines that had looked like circuitry were really lines left by micrometeoroids striking its surface. There was still no evidence of any activity, and it didn’t emit light or other radiation. Looking at the ancient surface of the Ring, all three felt a sense of familiarity. They recalled the destroyed droplets, and then tried to imagine the immense four-dimensional Ring with a mirrorlike smooth surface—it would have been a breathtaking sight.
Following the preestablished plan, Lieutenant Zhuo transmitted a message to the Ring via medium-frequency radio waves. This was a simple bitmap, a bit array that could be interpreted as six lines of dots that formed a sequence of prime numbers: 2, 3, 5, 7, 11, 13.
They weren’t expecting any answers, but an answer arrived immediately, so fast that they couldn’t believe their eyes. The information window hovering in the middle of the pinnace cabin displayed a simple bitmap similar to the one they had sent. It also consisted of six lines portraying the next six prime numbers: 17, 19, 23, 29, 31, 37.
The original plan had intended the hailing message as an experiment; there was no preparation for how to develop further communications. While the three in the pinnace debated what to do, the Ring sent a second bitmap to the pinnace: 1, 3, 5, 7, 11, 13, 1, 4, 2, 1, 5, 9.
Then a third bitmap: 1, 3, 5, 7, 11, 13, 16, 6, 10, 10, 4, 7.
A fourth: 1, 3, 5, 7, 11, 13, 19, 5, 1, 15, 4, 8.
A fifth: 1, 3, 5, 7, 11, 13, 7, 2, 16, 4, 1, 14.
The bitmaps came one after another. The first six numbers in each consisted of the six prime numbers sent by the pinnace as a greeting. As for the next six numbers in each series, both Lieutenant Zhuo and Dr. West turned to Guan Yifan, the scientist. The cosmologist stared at the scrolling numbers in the floating window and shrugged.
“I can’t see any pattern.”
“Then let’s suppose that there is no pattern.” West pointed at the window. “The first six numbers were sent by us, so it’s possible that they mean ‘you.’ The next six numbers in the transmissions show no recognizable pattern, so maybe they mean ‘all’—‘everything about you.’”
“They—or it—want to know everything about us?”
“Or at least a linguistic sample. It wants to decode it, study it, and then communicate with us further.”
“Then we should send it the Rosetta System.”
“We have to ask for authorization.”
The Rosetta System was a database developed to teach Earth languages to Trisolarans. The database included about two million characters’ worth of documents concerning the natural and human histories of Earth with numerous videos and images. There was also software to draw connections between the linguistic symbols and the images so that an alien civilization could decode and study Earth languages.
The mother ship authorized the request from the exploration team. But the pinnace didn’t have the Rosetta System in its onboard computer memory, and due to the extremely tenuous communication link between the pinnace and the mother ship, it was impossible to transmit such a large volume of data. The only solution was to have the mother ship beam the information directly at the Ring. This couldn’t be done via radio, but luckily, Gravity was equipped with a neutrino communication system. They weren’t sure if the Ring could receive neutrino signals, however.
Three minutes after Gravity transmitted the Rosetta System via a neutrino beam to the Ring, the pinnace received a new series of bitmaps from the Ring. The first one was a perfect square of sixty-four dots arranged eight by eight; the second bitmap was missing one dot in a corner, leaving sixty-three; the third bitmap was missing two, leaving sixty-two....
“It’s a countdown, or a progress bar,” said West. “I think this is to show that it has received the Rosetta System and is in the process of decoding it. We should wait.”
“Why sixty-four dots?”
“It’s a reasonably big number if you’re in base two. It’s like how we use one hundred for lots of things in base ten.”
Lieutenant Zhuo and Guan were both glad to have West with them. The psychologist did seem to have some skills when it came to establishing communications with unknown intelligences.
When the countdown reached fifty-seven, something exciting happened: The next number didn’t come in the form of a bitmap of dots. The Ring transmitted the Arabic number 56.
“Wow, fast learner,” Guan said.
The number kept on decreasing by one every ten seconds or so. A few minutes later, the number reached 0. The last message consisted of four Chinese characters:
I am a tomb.
The Rosetta System was written in a language that mixed English with Chinese. It would make sense that the Ring would use the same language to communicate with them. It just happened that this message consisted entirely of Chinese characters. Guan Yifan typed a question into the floating window, and began the conversation between humanity and the Ring.
Whose tomb is this?
The tomb of those who created it.
Is this a spaceship?
It used to be a spaceship. But now it’s dead, and so it’s a tomb.
Who are you? Who is conversing with us?
I am the tomb. It is the tomb speaking to you. I’m dead.
You mean you’re a ship whose crew died? In other words, you’re the control system for the ship?
(There was no reply to this.)
We can see many other objects in this region of space. Are they also tombs?
Most of them are tombs. The others will be tombs soon. I don’t know them all.
Are you from far away? Or have you always been here?
I’m from far away; they’re also from far away, from different places far away.
Where?
(There was no answer.)
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 who dried the sea went onto land before they did this. They moved from one dark forest to another dark forest.
The last sentence was like a thunderclap. The three inside the pinnace cabin and everyone in the distant two mother ships hearing the exchange via a faint link all shuddered.
Dark forest... what do you mean?
The same thing you mean.
Are you going to attack us?
I’m a tomb; I’m dead; I won’t attack anyone. There is no dark forest state between spaces of different dimensions. The lower-dimensional space cannot threaten the higher-dimensional space, and the resources of the lower-dimensional space are of no use to the higher-dimensional space. But the dark forest exists everywhere between those sharing the same dimensions.
Can you give us any suggestions?
Leave this puddle immediately. You are thin pictures. You’re fragile. If you stay in the puddle, you’ll turn into tombs in no time.... Wait, there seem to be fish on your pinnace.
Guan sat there, stunned for a few seconds, and then realized that the pinnace really did have some fish. He always carried an ecological sphere with him, about the size of a fist. Inside the glass sphere was water, a tiny fish, and some seaweed; together, everything formed a carefully designed miniature enclosed ecosystem. This was Guan’s favorite possession, so he had taken it with him on this adventure. If he couldn’t return, this would ha
ve accompanied him into the afterlife.
I like fish. Can I have it?
How do we give it to you?
Toss it over.
The three put on the helmets for their space suits and opened the pinnace’s hatch. Guan lifted the ecological sphere to his eyes. Carefully, since he was in four-dimensional space, he held the sphere by its three-dimensional edge, and gave it a final glance. From this four-dimensional perspective, every detail in the sphere was revealed, and this tiny world of life seemed even more rich, varied, and colorful. Guan swung his arm and tossed the sphere in the direction of the Ring. He watched as the small, transparent sphere disappeared in space. Then he closed the hatch and continued the conversation.
Is this the only puddle in the universe?
There was no answer. After that, the Ring remained silent and responded no more to attempts at communication.
Gravity informed them that more micrometeoroids had struck Blue Space. An increasing number of drifting objects, including some small-scale four-dimensional objects, possibly debris from ships or other artifacts, surrounded both ships. Captain Chu ordered their immediate return. The plan to board the Ring had to be scrapped.
Since they now knew their distance from the mother ship, the return trip could be covered twice as fast. In two hours, they were back in the vicinity of Blue Space, and successfully located a warped point to return home.
The explorers were treated as heroes and given a celebratory welcome—even if their discoveries appeared to yield no practical applications for the future of the two ships.
Captain Chu asked, “Dr. Guan, what do you think is the answer to the last question you asked the Ring?”
“I go back to the analogy I made before. The probability that we managed to stumble into the only soap bubble with a diameter of forty to fifty astronomical units on the surface of a sheet of paper sixteen billion light-years across is so minuscule that it might as well be zero. I’m certain that there are other soap bubbles, probably many more.”
“Do you think we’ll encounter more in the future?”
“I think there’s an even more interesting question: Have we encountered them before? Think about the Earth: It’s been careening through space for several billion years. Is it not possible that it had entered a four-dimensional fragment in the past?”
“That would have been an astonishing sight. I find it hard to imagine that humanity had experienced it.... But I wonder if dinosaurs could have located warped points...”
“Why are there bubbles at all? Why are there so many four-dimensional fragments in three-dimensional space?”
“It’s a great mystery.”
“Captain, I think it’s likely a dark secret.”
Blue Space and Gravity began to back out of the fragment. As the ships accelerated, gravity pulled them toward the ships’ sterns. Guan Yifan and the science officers from both ships tried to cram as much research about the fragment as possible into the next few days and spent almost all their time in four-dimensional space. This was only in part due to the requirements of their research—they also found the confinement and claustrophobia of being in three-dimensional space unbearable.
On the fifth day after beginning acceleration, all those in four-dimensional space found themselves back in three-dimensional space in a flash without having passed through a warped point. The electromagnetic sensors on the two ships indicated that there were no more warped points aboard either ship.
Blue Space and Gravity were outside the fragment.
This surprised them. Based on their calculations, they should still be cruising through the fragment for another twenty hours. Their early exit was probably due to one of two reasons: one, the fragment had sped up in a direction opposite to the ships’ current heading. Two, the fragment was shrinking. The crews believed reason number two was more likely. Other than the data, they also remembered the answer from the Ring:
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.
The two ships stopped accelerating and began to decelerate at full power. Finally, they stopped at the vicinity of the boundary of the four-dimensional fragment, where it was safe.
The edge of the four-dimensional fragment was invisible. The space before them was empty, placid like the surface of a deep pool. The sea of stars that was the Milky Way shone brightly, giving no hint that a great secret was hidden close by.
But soon they did notice a strange and spectacular sight: From time to time, luminous lines appeared in the space before them. Those lines were very thin, and very straight on first appearance. They seemed to have no thickness to the naked eye, and stretched in length between five thousand and thirty thousand kilometers. The lines appeared suddenly. At first, they gave off a blue glow. Then the color gradually shifted to red, and the straight lines curved and broke into many pieces, until they finally vanished. Observation showed that these lines manifested at the edge of the fragment, as though a giant pen constantly marked out the boundary.
They launched an unmanned probe toward the region of space where the lines appeared, and by luck, the probe managed to observe one of the lines when it flashed into existence at close range. The probe was about a hundred kilometers away, rushing toward the line at full speed. By the time it arrived, the line had curled, broken up, and disappeared. The probe detected massive quantities of hydrogen and helium in the vicinity, and also some dust from heavy elements, mainly iron and silicon.
After analyzing the data, Guan and the science officers concluded that the lines were created by four-dimensional matter entering three-dimensional space. As the fragment shrank, four-dimensional matter dropped into three-dimensional space and decayed instantly. Although these bits of matter took up very little volume in four-dimensional space, their decay into three dimensions flattened the fourth dimension, causing their volume to increase greatly and expand into the form of straight lines. By their calculations, a few tens of grams of four-dimensional matter could form a line stretching almost ten thousand kilometers in three dimensions.
Based on the rate at which the boundary of the fragment was receding, in twenty days or so, the Ring would also enter three-dimensional space. The two ships decided to wait to observe such a wonder of the universe—they had plenty of time, after all. Using the glowing decay-lines as markers, the two ships cautiously proceeded, maintaining the same speed as the receding edge of the fragment.
During the next dozen or so days, Guan Yifan was absorbed in deep thought and calculations, and the science officers engaged in vigorous debates. Finally, they reached the consensus that, based on current theoretical physics, they could not do too much theoretical analysis of the four-dimensional fragment. But the theories that had been developed in the past three centuries could at least make some predictions that were confirmed by observations: A higher dimension existing in macro form decayed toward lower dimensions as inevitably as water fell over a cliff. The decay of four-dimensional space into three dimensions was the root cause of the fragment’s shrinking.
But the lost dimension wasn’t truly lost: It simply curled up from the macroscopic to the microscopic and became one of the seven dimensions folded up within the quantum realm.
They could again see the Ring with the naked eye. The existence of this self-declared tomb would soon end in three-dimensional space.
Both Blue Space and Gravity stopped advancing and backed off three hundred thousand kilometers. As the Ring entered three-dimensional space, the decay process would release tremendous amounts of energy—this was why the lines that appeared earlier had given off so much light.
Twenty-two days later, the edge of the fragment receded past the Ring. The moment the Ring entered three-dimensional space, the universe seemed to be cut in half. The cut surface glowed with a blinding light, as though a star had been pulled into a line in an instant. From the spaceships it was impossible to see the end points, but it was as if God had held
a T-square against the plan for the universe and sketched a line straight across from left to right. Careful observation by instruments revealed that the line was close to one AU in length, or about 130 million kilometers, almost long enough to connect the Earth with the Sun. In contrast to other lines that had been observed, this one had a thickness that was visible even from several hundred thousand kilometers away. The light emitted by the line turned from a hot bluish-white to a merely warm red, and then gradually dimmed. The line itself also became twisted and loose, breaking into a belt of dust. It no longer glowed by itself, but seemed suffused with the light of the stars, a serene silver. Observers from both ships shared a strange impression: The dust belt resembled the Milky Way in the background. What had happened seemed to be the flash of a giant camera that took a picture of the galaxy. Afterwards, the photograph slowly developed in space.
Guan felt a trace of sorrow in the face of such a majestic sight. He was thinking of the ecological sphere he had given to the Ring. It wasn’t able to enjoy the gift for long. As it decayed into three-dimensional space, the Ring’s internal four-dimensional structures were annihilated instantaneously. Those other dead or dying ships within the fragment would not be able to escape a similar fate in the end. In this vast universe, they could only persist for a time in the tiny four-dimensional corner.
A vast and dark secret.
Blue Space and Gravity sent out many probes to the dust belt. Other than scientific investigation, they also wanted to see if they could gather some useful resources. The Ring had decayed into common elements in three-dimensional space: mostly hydrogen and helium. These could be collected as fuel for nuclear fusion. However, since these elements existed mostly in the form of gas in the dust belt, they dissipated quickly, and in the end very little was collected. There were also some heavy elements, however, and they were able to gather some useful metals.
