Cixin Liu - Death's End

“Droplet attack!” West said to the confused Guan Yifan. “They’re both accelerating. One is headed for Blue Space, the other for us!”

Guan looked around, instinctively searching for something to grab on to in case the ship accelerated. But there was nothing around. In the end, he held on to the doctor.

West held his hands. “There won’t be enough time for any evasive maneuvers. We have only a few seconds left.”

After a brief panic, both felt an unexpected sense of relief. They were glad that death would arrive so quickly that there wasn’t even time to be terrified. Perhaps their discussion about the universe was the best preparation for death.

They both thought of the same thing, but Guan spoke it aloud first. “Looks like neither of us needs to worry about our patients anymore.”

The high-speed elevator continued to descend, and the increasing layers of earth above seemed to put all their weight on Cheng Xin’s heart.

Half a year ago, a joint session of the UN and the Solar System Fleet had elected Cheng Xin to succeed Luo Ji as the Swordholder and given her authority to control the gravitational wave deterrence system. She had received almost twice as many votes as the next candidate. She was now proceeding to the Deterrence Center in the Gobi Desert, where the deterrence authority handover ceremony was to take place.

The Deterrence Center was the deepest man-made structure ever, about forty-five kilometers beneath the surface. This location was already below the crust, past the Mohorovičić discontinuity, in the mantle of the Earth. The pressure and temperature here were both far higher than in the crust, and the stratum around her was made up mostly of solid, hard peridotite.

The elevator took almost twenty minutes to reach its terminus. Cheng Xin stepped out of the elevator and saw a black steel door. White text on the door gave the formal name for the Deterrence Center: Gravitational Wave Universal Broadcast System Control Station Zero. The insignias of the UN and the Solar System Fleet were embossed on the door.

This ultra-deep structure was quite complex. It possessed its own independent air circulation system and was not directly connected with the atmosphere above the surface—otherwise, the high air pressure generated by a depth of forty-five kilometers would cause great discomfort to the occupant. It was also equipped with a powerful cooling system to withstand the high temperature of the mantle, nearly five hundred degrees Celsius.

All Cheng Xin could see, however, was emptiness. The lobby’s walls could all apparently act as electronic displays, but they showed nothing but whiteness, as though the building wasn’t in use yet. Half a century ago, when the Deterrence Center was designed, Luo Ji had been consulted, but he had only provided one piece of input:

As simple as a tomb.

The handover ceremony was a solemn occasion, but the bulk of it had been held on the surface forty-five kilometers above. There, all the leaders of Earth International and Fleet International, representing all of humanity, had gathered, and they watched as Cheng Xin entered the elevator. Only two people would oversee the final handover: the PDC chair and the chief of staff for the Solar System Fleet, representing the two institutions directly operating the deterrence system.

The PDC chair pointed at the empty lobby and explained to Cheng Xin that they would redecorate the place based on her ideas. If she wanted, she could have a lawn, plants, a fountain, and so forth. She could also choose to have a holographic simulation of scenes from the surface.

“We don’t want you to live like him,” said the chief of staff. Perhaps because of his military uniform, Cheng Xin saw in him traces of men of the past, and his words warmed her slightly. But the heavy weight on her heart, as heavy as the forty-five kilometers of earth above her, did not lessen.

The first dark forest deterrence system consisted of more than three thousand nuclear bombs wrapped in an oil film substance deployed in orbit around the sun. After detonation, the film would cause the sun to flicker and broadcast the location of Trisolaris to the universe. Although the system was grand, it was extremely unstable. After the droplets stopped blockading electromagnetic radiation from the sun, a transmission system based on using the sun as a superantenna was immediately put in place to supplement the nuclear bomb deterrence system.

Both of these systems relied on electromagnetic radiation, including visible light, as the broadcast medium. We now know that this is the most primitive technique for interstellar communication, equivalent to smoke signals in space. Since electromagnetic waves decay and become distorted rapidly, the broadcast range is limited.

At the time of the founding of deterrence, humankind already had a basic grasp of the technology for detecting gravitational waves and neutrinos, but they lacked the ability to modulate and transmit. These were the very first technologies humans demanded from Trisolaris. Compared to quantum communications, these technologies were still primitive, since both gravitational waves and neutrinos were limited by the speed of light, but they were a whole level above electromagnetic waves.

Both of these means of transmission decayed relatively slowly and had very long broadcast ranges. Neutrinos, in particular, interacted with almost nothing else. Theoretically, a modulated beam of neutrinos could transmit information to the other end of the universe, and the accompanying decay and distortion would not affect the decoding of the information. But while neutrinos must be focused in a particular direction, gravitational waves were omnidirectional, thus gravitational waves became the main method of establishing dark forest deterrence.

The fundamental principle of gravitational wave transmission relied on the vibration of a long string of extremely dense matter. The ideal transmission antenna would involve a large number of black holes connected together to form a chain that generated gravitational waves as it vibrated. But even Trisolaris didn’t possess such a level of technology, and humankind had to resort to constructing the vibrating string out of degenerate matter. The extremely dense degenerate matter packed an enormous mass into strings mere nanometers in diameter. A single string took up only a minuscule portion of the giant antenna, the bulk of which consisted of support and protection for the ultra-dense string. Thus, the total mass of the antenna wasn’t extraordinarily large.

The degenerate matter forming the vibrating string was naturally found in white dwarves and neutron stars. Under typical conditions, this substance naturally decayed and turned into regular matter over time. Man-made vibrating strings typically had a half-life of around fifty years, beyond which the antennas lost their effectiveness. Thus, every half century, the antennas needed to be refreshed with new ones.

During the earliest stage of gravitational wave deterrence, the main strategic concern was with ensuring deterrence power. Plans were made to build a hundred broadcasting stations scattered around the continents. But gravitational wave communication suffered from a flaw: The transmission equipment could not be miniaturized. The complex, gigantic antennas were extremely costly to manufacture, and in the end only twenty-three gravitational transmitters were built. But the focus on ensuring deterrence finally faded, due to another event.

During the Deterrence Era, the ETO gradually disappeared, but another kind of extremist organization sprang up. They believed in the cause of human supremacy, and advocated for the complete annihilation of Trisolaris. The “Sons of the Earth” was one of the largest of these organizations. In Year 6 of the Deterrence Era, more than three hundred “Sons of the Earth” attacked a gravitational wave broadcasting station located on Antarctica with the aim of seizing the transmitter. Equipped with advanced weapons such as mini-infrasonic nuclear bombs, and aided by members who had infiltrated the broadcasting station ahead of time, the attackers nearly succeeded. If the defending troops stationed at the site hadn’t destroyed the antenna in time, the consequences would have been disastrous.