But Cheng Xin soon realized that the lights inside the cabin came from different fixtures than before -- perhaps these were backups designed specifically for reduced-lightspeed use. There were no information windows in the air. It was possible that the reduced lightspeed meant such holographic displays were no longer operable. The interface of the neural computer was limited to that flat screen, which now resembled a color bitmap display from the Common Era.

It took a long time to wake up. Cheng Xin recovered her awareness gradually. After her memory and sight came back, she knew right away that the neural computer had booted successfully. A soft light illuminated the inside of the cabin, and she could hear the machines humming reassuringly. The air was warm. The shuttle had been revived.

Guan Yifan was drifting in front of the display, tapping on it with the fingers of a gloveless hand. He turned and smiled at Cheng Xin, made a hand gesture indicating that it was okay to drink, and then handed her a bottle of water.

Since she had recovered enough to move her hands, Cheng Xin unstrapped herself and drifted next to Yifan to look at the screen with him, their space suits squeezed tightly side by side. Several windows were up on the screen, each showing rapidly scrolling numbers: diagnostics on the shuttle's various systems. Yifan told Cheng Xin that he had established contact with Hunter, whose neural computer had also apparently booted successfully.

The bottle felt warm. Cheng Xin saw that she wasn't wearing gloves, either. She realized that although she was still wearing the primitive space suit, her helmet had been removed. The temperature and pressure inside the cabin were comfortable.

At first, the universe didn't appear to have changed from what she had seen before: The ship continued to orbit around Planet Blue at reduced lightspeed; the two star clusters, blue and red, continued to change their shapes erratically at the two ends of the universe; the sun continued to dance madly between being a line and a circle; and color patches continued to whip across Planet Blue's surface. When Cheng Xin tried to match her gaze to the rapidly flowing surface of Planet Blue, she finally noticed something different: the blue and white patches had been replaced by purple ones.

Cheng Xin looked up and saw that the two portholes were still open. She drifted over. Guan Yifan dimmed the cabin lights so she could see through them without glare. They anticipated each other's needs now as though they were a single person.

"It's been sixteen days," he said.

"The fusion drive still works?" Cheng Xin asked. Before they entered hibernation, this question had weighed on her mind. She had not asked because she knew that she was likely to receive a disappointing answer, and she didn't want to give Yifan more to worry about.

"Of course not. With such a reduced lightspeed, nuclear fusion puts out too little power. We have to use the backup antimatter drive."

Yifan pointed to the screen. "The propulsion system self-diagnosis is complete. Everything's basically working. We can decelerate out of lightspeed anytime."

"Antimatter? But wouldn't the containment field be affected by the reduced lightspeed?"

"No problems there. The antimatter engine was designed specifically for reduced-lightspeed conditions. When we're on long expeditions like this, we equip all our spacecraft with reduced-lightspeed propulsion systems… Our world puts a lot of effort into developing such technologies. The goal isn't to solve the problem of accidentally entering trails left by curvature propulsion; rather, it's because we have to plan for the possibility of having to conceal ourselves inside a light tomb, or a black domain."

"We're out of lightspeed," Guan Yifan said. He pressed a button, and the screens over the two portholes retracted.

Half an hour later, the shuttle and Hunter both activated their antimatter engines and began decelerating. Cheng Xin and Guan Yifan were pressed against their seats by the hypergravity, and the porthole screens rose to block out the outside. Violent jolts seized the shuttle, but gradually subsided. The deceleration process took less than twenty minutes. Then the engines shut off, and they were again weightless.

Through the portholes, Cheng Xin saw that the blue and red star clusters were gone, and the sun was now a normal sun. But the sight of Planet Blue in the porthole on the other side surprised her: Planet Blue was now "Planet Purple." Other than the ocean, which was still a light yellow, the rest of the planet was covered by purple -- even the snow was gone. She was, however, most shocked by the appearance of space itself.

"I think they are… stars." Yifan was as amazed as she.

"What are those lines?" Cheng Xin cried out.

"We're inside the black domain?"

"I think they're stars," repeated Yifan. "A star's light must pass through two interfaces before getting to us: First, it must go through the interface between regular lightspeed and reduced lightspeed, and then through the event horizon of the black hole. That's why the stars look so strange to us now."

"That's right. We're inside the light tomb."

All the stars in space had turned into thin lines of light. Cheng Xin was actually familiar with such a sight: She had seen plenty of long-exposure photographs taken of the starry sky from Earth. Due to the Earth's rotation, the stars in the pictures all became concentric arcs of approximately the same length. But now, the stars she saw were segments of different lengths and aligned every which way. The longest few lines, in fact, took up almost a third of the sky. These lines crossed each other at different angles and made space appear far more confusing and chaotic than before.

The DX3906 solar system was now a reduced-lightspeed black hole completely sealed off from the rest of the universe. The starry sky woven by the multitude of crisscrossing silver threads was a dream that could be seen but would never be achieved.

"Let's go down to the surface," Yifan said after a long silence.

The purple continents took up most of the view from the monitoring system. They were able to confirm that the purple was due to the color of the vegetation. The change in the sun's radiation had probably caused the plants on Planet Blue to change from blue to purple as they evolved to adapt to the new light.

The shuttle decelerated further and lowered its orbit. With a series of powerful jolts, it entered the atmosphere of the planet and descended toward the surface of this world in which the two of them were doomed to spend the rest of their lives.

As a matter of fact, the very existence of the sun puzzled Cheng Xin and Guan Yifan. Since E = mc2, reduced-lightspeed nuclear fusion could produce only small amounts of energy. Perhaps the interior of the sun maintained normal lightspeed?

The shuttle's landing coordinates were set to the same spot from which it had taken off and left Halo. As they approached the surface, they saw a dense purple forest at the landing spot. Just when the shuttle was about to lift off again in search of a more open spot, the trees dashed away to escape the flames from the shuttle's thrusters. The shuttle then gently set itself down in the open space vacated by the fleeing trees.

Cheng Xin and Guan Yifan exited the shuttle and once again stepped onto the surface of Planet Blue. Warm, moist air welcomed them, and a layer of soft, bouncy humus covered the ground. The soil around them was filled with numerous holes left by the roots of the trees that had gotten out of the way. Those trees now huddled around the clearing, their broad leaves rustling in the breeze, like a crowd of whispering giants gathered around them. The clearing was completely covered by their shade. Such dense vegetation made Planet Blue a completely different world than the one they had seen before.

Cheng Xin didn't like purple. She'd always thought of it as a sick, depressing color that reminded her of the lips of invalids whose hearts did not supply them with sufficient oxygen. Yet now she was surrounded by purple everywhere she looked, and she would have to spend the rest of her life in this purple world.

The screen showed that the outside air was breathable. Compared to the last time they had been here, the oxygen content in the atmosphere was substantially higher. Moreover, the atmosphere was denser, and the atmospheric pressure was one and a half times higher than at the last landing.

Yifan returned to the shuttle and established a link with Hunter, which was still in near-ground orbit. Hunter's neural computer was very powerful, and its AI was capable of direct natural language communications. Under reduced-lightspeed conditions, the conversation from ground to space suffered a transmission delay of over ten seconds. After dropping out of lightspeed along with the shuttle, Hunter had been scanning the surface of the planet from low orbit. By now, it had completed a survey of most of the land on Planet Blue, and it had discovered no trace of humans or signs of any other intelligent life.

There was no sign of Halo, no sign of Yun Tianming's ship, no sign of any human presence.

Guan Yifan and Cheng Xin surveyed the landscape around them and realized that the geographical features were completely different from the last time. They clearly remembered that there had been rolling mountains nearby, but now the forest was growing over a plain. They went back to the shuttle to confirm that the coordinates were really correct -- they were. Then they looked even more carefully all around them, but still found no trace of any prior human visit. The site resembled virgin land -- it was as though their last visit had occurred on another planet in another space-time that had nothing to do with here.

While waiting for the test results, Cheng Xin and Guan Yifan left the shuttle and waited in the clearing. Sunlight illuminated the clearing through gaps in the canopy. Many strange, small creatures flitted through: Some were insects with spinning rotors on top like helicopters; others were like tiny, transparent balloons that drifted through the air, giving off a rainbow sheen as they passed through shafts of sunlight; but none of them had wings.

Next, Cheng Xin and Guan Yifan had to turn to a task that terrified them but was absolutely necessary: determining how much time had elapsed in this frame of reference. There was a special technique for radiometric dating under reduced-lightspeed conditions: Some elements that did not decay under normal lightspeed decayed at different rates under reduced lightspeed, which could be used to precisely tell the passage of time. Given its scientific mission, the shuttle was equipped with a device for measuring atomic decay, but the instrument required a computer for processing. Yifan had to go to some trouble to connect the instrument to the neural computer on the shuttle. They directed the instrument to test the ten rock samples taken from different parts of the planet one after another so that the results could be compared. The assay required half an hour.

Half an hour later, they climbed back into the shuttle to face facts. The screen on the control panel showed the test results from the ten samples. Many elements had been tested and the charts were complicated. All the samples yielded similar results. Underneath, the average of the results was listed simply:

"Maybe several tens of thousands of years have passed," Cheng Xin muttered.

Then they said no more, but sat on the stairs outside the shuttle, leaning against each other and taking comfort in their heartbeats.

"Or even longer," said Guan Yifan, looking deep into the woods. "In our current state, tens of thousands of years aren't very different from hundreds of thousands of years."

Average atomic decay dating results (error range: 0.4%): Stellar time periods lapsed: 6,177,906; Earth years lapsed: 18,903,729.

Cheng Xin counted the digits in the last number three times, turned around, and quietly exited the shuttle. She descended the stairs and returned to this purple world. Tall purple trees surrounded her, a beam of sunlight cast a tiny circle of brightness next to her feet, moist wind lifted her hair, living balloons drifted overhead, and almost nineteen million years followed her.

Yifan came to her. They locked gazes, and their souls embraced.

"Cheng Xin, we missed them."

More than eighteen million years after the DX3906 system turned into a reduced-lightspeed black hole, seventeen billion years after the birth of the universe, a man and a woman held each other tightly.

Cheng Xin sobbed her heart out over Yifan's shoulder. In her memories, she had cried like this only once before, when Tianming's brain had been taken out of his body. That was… 18,903,729 years plus six centuries ago, and those six centuries were but a rounding error at such geologic timescales. This time, she cried not only for Tianming. She cried out of a sense of surrender. She finally understood how she was but a mote of dust in a grand wind, a small leaf drifting over a broad river. She surrendered completely and allowed the wind to pass through her, allowed the sunlight to pierce her soul.

They sat on the yielding humus and continued to hold each other, letting time flow by. The dappled sunlight gently shifted around them as the planet continued to rotate. Sometimes Cheng Xin asked herself, Has another ten million years passed by? A small, rational part of her mind strangely whispered to her that such a thing was possible: There really were worlds where one could step through a thousand years at will. Consider the death lines: If they ruptured and expanded just a bit, the speed of light within would rise from zero to an extremely small number, like the rate at which continents drifted over the ocean: a centimeter for every ten thousand years. In such a world, if you got up from your lover and walked a few steps away, you would be separated from him by ten million years.

Letting the past go, she allowed her growing esteem for Guan Yifan to take over her heart.

"What are you looking for?"

Yifan shook his head. "I understand your desire, but --"

"艾 AA would know to carve words into stone," Cheng Xin muttered.

"I don't understand…"

"Words. I want to see if there are words."

Yifan nodded, but obviously only to appease her. They returned to the shuttle. Although this was a walk of only a few steps, they leaned against each other as if afraid time would divide them if they were physically separated. Yifan contacted Hunter and directed it to do a deep scan of the area within a circle centered on this coordinate with a radius of three kilometers. The depth of the scan was set to be between five and ten meters, focusing on human writing or other significant markings.

Yifan looked at Cheng Xin, confused.

After they knew not how long, Yifan asked her softly, "What should we do?"

Cheng Xin didn't explain; instead, she grabbed Yifan by the shoulders: "Can you have Hunter do a deep scan of this area and see if there's anything under the surface?"

"There really won't be anything. Eighteen million years will erase everything: Time is the cruelest force of all."

"I want to look more. There must be some sign."

"Carving words into stone."

They'd missed each other.

"To better last through the eons, the words ought to be large."

Hunter passed overhead fifteen minutes later and sent back the results about ten minutes after that: nothing.

Guan Yifan ordered the ship to do another scan at a depth range between ten and twenty meters. This took another hour, the bulk of which was spent waiting for the ship to pass overhead. Still nothing. At that depth there was no more soil, only bedrock.

Guan Yifan adjusted the scanning range to between twenty and thirty meters. "This is the last time," he said to Cheng Xin. "The sensors can't go deeper than that."

They waited for the ship to orbit Planet Blue another time. The sun was setting and the sky was full of lovely, fiery clouds, while the purple woods were limned with a golden glow.

This time, the shuttle's screen showed the images transmitted back by the ship. After processing by enhancement software, they could see a few fragments of white words embedded in the dark rock: "e," "liv," "a," "life," "you," "little," "side," "Go." The white color was to indicate that the words were carved into the bedrock; each character was about a meter square, and they were arranged into four rows. The words were twenty-three to twenty-eight meters below them, carved into a forty-degree incline.

WE GIVE YOU A LITTLE SURVIVE THE COLLAPSE INSIDE GO TO THE NEW

Hunter's AI invoked the geological expert system to interpret the results. They found out that the giant characters had initially been carved into the surface of a large sedimentary rock formation on the side of a mountain. The original surface was about 130 square meters. Over the eons, the mountain on which the rock had been located sank, and that was how the carved rock ended up below them. More than four lines of text had been carved into it, but the lower portion of the rock had been broken up during the geological transformations and all the text there lost. The surviving text was incomplete as well -- the last three lines all had missing characters at the end.

WE LIVED A HAPPY LIFE TOGETHER

Cheng Xin and Guan Yifan embraced again. They cried tears of joy at the news concerning 艾 AA and Yun Tianming, and shared the happiness that they had enjoyed more than one hundred eighty thousand centuries ago. Their despairing hearts grew peaceful.

"Anything was possible," said Yifan.

"I wonder what their life here was like?" Cheng Xin asked, tears glistening in her eyes.

Cheng Xin knew that was indeed possible. But even if that civilization had lasted ten million years, the eight million-plus years that had come after would have erased all traces of it.

"Did they have children?"

"Anything was possible. They could have even founded a civilization here."

Something strange interrupted their meditation: a rectangle limned by faint lines of light, about a man's height, hovered over the clearing like the dashed selection lines marked out by dragging a mouse. It moved through the air, but did not go far before returning to its original position. It was possible that it had been there all along, but the outline was so faint and thin that it was invisible during daytime. Whether it was made by a force field or actual substance, there was no doubt it was the creation of intelligence. The lines making up the rectangle seemed to evoke the line-shaped stars in the sky.

Time really was the cruelest force of all.

"Seems hard to believe. How could it have survived more than eighteen million years?"

But he was wrong. The object had indeed survived eighteen million years. And, if necessary, it could survive until the end of the universe, because it existed outside of time.

"Do you think this is the… gift they left for us?" Cheng Xin asked.

The door remembered that, initially, it had been placed next to the rock carved with text, and it had a real metal frame. But the metal had eroded away after only five hundred thousand years, though the object had always remained brand new. It had no fear of time because its own time had not yet started. It had been thirty meters underground, next to the carved rock, but it had detected the presence of humans and risen to the surface. During the process, it did not interact with the crust, moving like a ghost. It now confirmed that these two were indeed the ones it had been expecting.

"I think it looks like a door," Cheng Xin said.

Yifan picked up a small branch and tossed it at the rectangle. The branch passed through it and landed on the other side. They saw a luminescent flock of little balloon creatures drift over. A few passed through the rectangle and one even crossed the glowing outline.

"Your hand… It didn't go through." Cheng Xin pointed to the other side of the rectangle.

Yifan reached out and touched the frame. The light and his finger passed through each other, and he felt nothing. Without even thinking, he extended his hand into the space outlined by the rectangle. Cheng Xin screamed. Yifan pulled his hand back, and everything looked unharmed.

Yifan tried again. His hand and forearm disappeared as they entered the plane of the rectangle and did not go through. From the other side, Cheng Xin saw the cross section of his forearm, like the surface of a window. All his bones, muscles, and blood vessels were clearly visible. He pulled his hand back and tried again with a branch. It went through the frame without problems. Right after, two insects with spinning rotors passed through the rectangle as well.

"It allowed you in."

"Probably you, as well."

Cheng Xin gingerly tried, and her arm also disappeared in the "door." Yifan observed the cross section of her arm from the other side, and had a moment of déjà vu.

"It really is a door -- a smart door that recognizes what's going through it," Yifan said.

"No, you wait here."

"We should go together," Cheng Xin said resolutely.

Yifan stared back into her eyes for a long moment, and finally nodded. "Perhaps we should bring some things with us."

"Wait for me here," Yifan said. "I'll go investigate."

Cheng Xin grabbed him by both shoulders and turned him to face her. She looked into his eyes. "Do you really want us to also be separated by eighteen million years?"

Ten minutes later, they passed through the door, hand in hand.