From ``Death's End'', by Cixin Liu, Translated by Ken Liu.

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. 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 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 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 a 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 degenerated 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, ever 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. Bu 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 5 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.

This incident terrified both worlds. People began to realize the great danger posed by gravitational wave transmitters. Simultaneously, Trisolaris pressured Earth until gravitational wave transmission technology was strictly controlled, and the twenty-three broadcasting stations were cut down to four. Three of the stations were terrestrial, located in Asia, North America, and Europe, and the last one was the spaceship Gravity.

All the transmitters relied on an active trigger, because the dead hand trigger technique used in the circumsolar nuclear bomb system was no longer necessary. Luo Ji had established deterrence by himself, but now, if the Swordholder were killed, others would be able to take over.

Initially, the gigantic gravitational wave antennas had to be built on the surface. As technology advanced, in Year 12 of the Deterrence Era, the three terrestrial antennas and their support systems moved deep underground. Everyone understood, however, that burying the transmitters and the control center beneath the Earth's surface primarily guarded against a threat originating from humankind itself, but was meaningless against an attack by Trisolaris. For droplets constructed from strong-interaction material, tens of kilometers of rock were not much different from mere liquid, barely able to impede their progress.