Before arriving at Halo City, the dinghy passed by Lightspeed II. Cao Bin told Cheng Xin that Lightspeed II used to be a science city and was one of the two research centers studying how to lower the speed of light to achieve the black domain state, but it had been abandoned. Cheng Xin was very interested and wanted to stop for a visit. Reluctantly, Cao Bin turned the dinghy in that direction.

The final destination of Cheng Xin and Cao Bin was Halo City, a medium space city. Medium cities were space cities whose interior areas were below two hundred square kilometers but above fifty square kilometers. Typically, these cities were mixed within formations of large cities, but two of the medium cities, Halo City and Lightspeed II, were situated far from the Jovian city cluster, almost outside the protection of Jupiter's shadow.

"Why don't we just take a look from the outside?" Cao Bin said. "It's best not to go in. Is it dangerous?"

"Yes."

"But we went inside Pacific I, which was also dangerous."

"It's not the same. There's no one inside Lightspeed II. It's a… ghost city. At least, that's what everyone says."

As the dinghy approached, Cheng Xin realized that the city really was in ruins. It didn't spin, and the exterior appeared broken and cracked. In some places, the skin of the city had been ripped open, revealing the structural frame underneath. As she surveyed the giant ruin illuminated by the searchlights of the dinghy, Cheng Xin felt awe as well as terror. She thought of the ruin as a beached whale. It had lain there for eons, until all that was left was cracked skin and bones, and life had long ago drained away. She seemed to be looking at something even older than the Acropolis of Athens, with even more secrets.

They slowly approached a large crack, several times as wide as the body of their dinghy. The beams in the structural frame were also bent and twisted, opening up a way to the interior. The beam of the searchlight shone in so that Cheng Xin could see the distant "ground," which was completely bare. After the dinghy descended a short distance into the interior of the space city, it stopped and swept the searchlight about them. Cheng Xin saw that the "ground" was bare in every direction. Not only were there no buildings, there wasn't anything at all to indicate that people had once lived here. The crisscrossing beams forming the frame for the city were visible on the "ground."

"Is it just an empty shell?" Cheng Xin asked.

"No."

Cao Bin looked at Cheng Xin for a few seconds, as if assessing her courage. Then he reached out and shut off the searchlights.

At first, all Cheng Xin could see was darkness. Starlight spilled in from the crack in front, as though she was gazing up at the sky through a broken roof. Eventually, her eyes adjusted to the dark, and she realized that the interior of the ruined space city wasn't entirely dark, but was illuminated with a faint, flickering blue light. Cheng Xin shivered. She forced herself to calm down and looked for the source. The blue glow came from the center of the interior of the space city.

The light source blinked without pattern, like a twitching eye. The empty ground was filled with strange shadows, like a desolate wasteland illuminated by flashes of lightning on the horizon at night.

"The light is caused by space dust falling into the black hole," Cao Bin said, pointing in the direction of the light source. He was trying to relieve some of Cheng Xin's terror.

"Yes. It's about… no more than five kilometers from here. A microscopic black hole with a Schwarzschild radius of twenty nanometers and a mass equivalent to Leda, the Jovian moon."

"There's a black hole over there?"

In this phosphorescent blue glow, Cao Bin told Cheng Xin the story of Lightspeed II and Gao Way.

Lowering the speed of light through a medium wasn't particularly difficult. As early as 2008 C. E., researchers had succeeded in lowering the speed of light through a medium to an incredible seventeen meters per second in a laboratory setting. But this was fundamentally different from lowering the speed of light through vacuum. The former only required causing the atoms in the medium to absorb and re-emit the photons -- light continued to travel at its usual speed between atoms. This wasn't useful for the Black Domain Plan.

The research into lowering the speed of light through vacuum began at about the same time as the Bunker Project. As the Black Domain Plan was the second path for human survival, the international community devoted enormous resources to it, and the Bunker Project even built a large space city as a research center devoted to the subject -- that would be Lightspeed I, located in the Saturn cluster. But sixty years of extensive research yielded no breakthroughs, and not even much advancement in theoretical foundation.

The speed of light through vacuum was one of the fundamental constants of the universe. Altering it was equivalent to altering the laws of physics. Thus, lowering the speed of light required breakthroughs in fundamental physics -- and considerable serendipity. After sixty years, the only substantive result of basic research was the creation of the circumsolar particle accelerator. This, in turn, led to the success of the largest project under the Black Domain Plan: the Black Hole Project.

The head of the Black Hole Project was Gao Way. Cao Bin had worked with him for a few years. He could not hide his complex feelings about the man as he described him to Cheng Xin.

Scientists had tried all kinds of extreme physical techniques in their efforts to alter the speed of light. Once, the strongest artificial magnetic field had been used. But the best way to influence light in vacuum was through a gravity field. Since it was extremely difficult to generate a local gravity field in a laboratory setting, the only path forward seemed to be a black hole. The circumsolar particle accelerator was capable of creating microscopic black holes.

"The man suffered from severe autism -- no, I'm not talking about some kind of lonely genius choosing to isolate himself, but a real mental condition. He was extremely closed off and had trouble communicating with anyone, and he had never even touched a woman. His extraordinary professional success would only be possible in this age, but despite his accomplishments, most of his supervisors and colleagues thought of him as merely a high-powered intelligence battery. He was tortured by his illness and tried to change himself, and in this, he was different from other geniuses."

"Starting from, oh, I think the eighth year of the Broadcast Era, he dedicated himself to the theoretical study of lowering the speed of light. Over time, I think he began to develop a strange identification between the speed of light and his own personality -- if he could change the speed of light, then it was the same as changing himself."

"But the speed of light through vacuum really was the most stable thing in the cosmos. Research into lowering the speed of light resembled torturing light without regard for consequences. People tried to do everything with light: strike it, twist it, break it, dissect it, stretch it, crush it, even destroy it -- but the result was, at most, a change in its frequency in vacuum. But the speed of light remained unchanged, like an unscalable wall. After all these decades, theoreticians and experimenters alike were in despair. There was a saying: If there really were a Creator, the only thing He welded shut in all Creation was the speed of light."

"The Black Hole Project began in Year 1 of the Bunker Era and lasted eleven years. The planners did not invest much hope in it. Both theoretical calculations and astronomical observations had indicated that even black holes could not change the speed of light. These demons of the universe could only use their gravity fields to change the path of light and its frequency, not affecting the speed of light through vacuum one iota. However, to continue the research for the Black Domain Plan, it was necessary to create experimental conditions with superpowerful gravity fields, which depended on black holes. In addition, since a black domain is in essence a large-scale reduced-lightspeed black hole, perhaps close-range observation of a microscopic regular-lightspeed black hole would yield unexpected insights."

"For Gao Way, the despair had yet another layer. By the time I went into hibernation, he was almost fifty. He had still never been with a woman, and he thought of his own fate as being as resistant as the speed of light; he became even more withdrawn and solitary."

"After the microscopic black hole was injected into Leda, it began to absorb mass and rapidly grow. At the same time, the intense radiation generated by material falling into the black hole melted the surrounding rock. Soon, the eight-kilometer-radius Leda melted entirely, and the potato-shaped rock turned into a red-glowing ball of lava. The lava ball shrank slowly, but glowed brighter and brighter, until it finally disappeared with a blinding flash. Observation showed that other than a small amount of material ejected by the radiation, most of the mass of Leda had been absorbed by the black hole. The black hole remained stable, and its Schwarzschild radius, or event horizon radius, had grown from the size of a fundamental particle to twenty-one nanometers."

"Leda was Jupiter's smallest moon, with a mean radius of only eight kilometers. It was nothing more than a large rock. Before making the black hole, they had lowered the moon from its high orbit and turned it into a body orbiting the Sun in parallel with Jupiter, like the city cluster. However, unlike the city cluster, it was located at the Sun-Jupiter L2 Lagrangian point, which is where we are now. This allowed it to maintain a stable distance from Jupiter without having to constantly adjust its position. At the time, this was the most massive body humans had managed to move through space until then."

"The circumsolar particle accelerator was capable of producing microscopic black holes rapidly, but these tiny black holes also evaporated very quickly. To produce a stable black hole, a microscopic black hole was guided out of the accelerator as soon as it was produced, and then injected into Leda."

"Anyone with some basic scientific background knows that the probability that Gao was 'sucked' into the black hole was practically nonexistent. The reason that black holes are traps from which even light cannot escape isn't because their overall gravitational power is overwhelming -- though a large black hole formed by the collapse of a star does possess immense overall gravity -- but due to the density of their gravitational fields. From a distance, the total gravity of a black hole is no different from the gravity of a quantity of normal matter of equivalent mass. If the Sun collapsed into a black hole, the Earth and the other planets would still continue on in their orbits without being sucked in. It's only when you got very close to the black hole that its gravity displayed strange behavior."

"They constructed a space city around the black hole -- that's Lightspeed II. The black hole was suspended in the middle of the space city, which was empty, didn't spin, and whose interior was a vacuum connected to space. It was, in essence, a giant container for the black hole. Personnel and equipment could be brought into the city to study the black hole."

"The research continued for many years. This was the first time that humans could study a black hole specimen in laboratory conditions, and many discoveries were made that helped with the development of theoretical physics and fundamental cosmology. But none of these results helped with the task of lowering the speed of light in vacuum."

"Six years after the commencement of studies on the black hole specimen, Gao Way died. According to the official account of the World Academy of Sciences, he was accidentally 'sucked into the black hole' during an experiment."

"Inside Lightspeed II, there was a protective net around the black hole with a radius of five thousand meters. Research personnel were forbidden to enter. Since the radius of Leda was originally only eight thousand meters, the black hole's gravity at this distance was not much greater than the gravity on the surface of the original Leda. It's not a very powerful pull -- a person standing there was essentially weightless, and could easily escape using the thrusters on their space suit. Thus, Gao couldn't have been 'sucked' in."

"Ever since the stable black hole specimen was obtained, Gao Way was infatuated with it. After struggling against the speed of light for so many years and not being able to alter even a single one of the many digits in this constant that came close to three hundred thousand, Gao was agitated and filled with a sense of failure. As the constancy of the speed of light was one of the fundamental laws of nature, he had come to despise the laws of nature, as well as being afraid of them. But now, in front of his eyes, was something that had compressed Leda into twenty-one nanometers. Within its event horizon, in that space-time singularity, known laws of nature had no effect."

"What happened after Gao Way was sucked in was almost too strange to describe. Scientists observed the black hole via remote-controlled microscopes, and discovered that at the black hole's event horizon -- that's the surface of that tiny sphere with a diameter of twenty-one nanometers -- there was the figure of a person. It was Gao Way passing through the event horizon."

"No one saw the process of Gao's disappearance, and if there was a recording, it's never been released. He was one of the Black Hole Project's principal physicists, and he had the password to open the protective net. I'm certain that he went in and drifted toward the black hole until he was too close to return… He probably wanted to get a close-up look at the object of his infatuation, or perhaps he wanted to enter into that singularity where the laws of nature no longer mattered, so that he could escape all this."

"Gao Way often hung against the protective net and stared for hours at the black hole five kilometers away. He watched its luminescence -- like we're doing now -- and sometimes claimed that the black hole was talking to him, that he could decipher the message of its flickering light."

"But within Gao Way's own frame of reference, he had already passed through the event horizon."

"Under general relativity, a distant observer would see a clock near the event horizon slow down, and the process of Gao Way falling toward the event horizon would also slow down and stretch into infinity."

"And thus the life insurance company refused to pay out, although Gao Way had passed through the event horizon in his frame of reference, and should now be dead. But the insurance contract was made within the frame of reference of our world, and from this perspective, it is impossible to prove that Gao Way is dead. It's not even possible to begin the settlement process. Insurance claims settlement can only occur after the conclusion of an accident, but as Gao Way is still falling toward the black hole, the accident isn't over, and will never be over."

"Even more oddly, the figure's proportions were normal. Perhaps it was because the black hole was so small, but tidal forces did not seem to be at work. He had been compressed into the nanometer range, but space there was also extremely curved. More than one physicist believed that the body structure of Gao Way wasn't harmed at the event horizon. In other words, he's probably still alive at this moment."

"A woman then sued the World Academy of Sciences and demanded that the academy cease all further research on this black hole specimen. By that point, distant observation was unlikely to yield any further results. In order to be useful, future research would have to manipulate the black hole in some way, such as sending experimental objects into the black hole, which would generate massive amounts of radiation, and might disturb space-time in the vicinity of the event horizon. If Gao Way were still alive, these experiments might endanger his life. The woman didn't win her suit, but for a variety of reasons, research on this black hole stopped, and Lightspeed II was abandoned. Now we can only wait for this black hole to evaporate, which is estimated to take another half century."

"However, we now know that at least one woman did love Gao Way, though he never knew it. Later, that woman still came here regularly and tried to send radio or neutrino messages at the black hole. She even wrote her love in big letters and posted it against the protective net, hoping that the falling Gao Way could see it. But based on his own frame of reference, he had already passed through the event horizon into the singularity… It's a complicated matter."

Cheng Xin now felt the flickering black hole was really sending out a message, even more like someone blinking. She pulled her gaze back, feeling as empty in her heart as this ruin in space. Softly, she said to Cao Bin, "Let's go to Halo City."

Cheng Xin stared at the blue phosphorescence far away in the darkness. She now knew that there was a man there, a man who was falling forever, at the event horizon where time stopped. Such a man was still alive when viewed from this world, but had already died in his own world… So many strange fates, and so many unimaginable lives…