Doomsday Arrives
In the final period of the Bunker Era, humanity's carefully prepared Bunker Project met with total failure. The predicted form of attack was a photoid strike — using a high-velocity particle to destroy the sun and trigger a solar-system-scale catastrophe. Based on this prediction, humanity built numerous space cities behind gas giants like Jupiter and Saturn as shelters, believing the giant planets could block the shockwave from a solar explosion.
However, the attack that actually arrived was far more terrible than a photoid strike. A two-dimensional foil — a minuscule fragment of two-dimensional space — was launched into the solar system. When the foil contacted matter in three-dimensional space, it began irreversibly converting surrounding space from three dimensions to two. This dimensional reduction attack had no directionality, no blind spots — whether hiding behind a gas giant or anywhere in the solar system, there was no escape from two-dimensionalization. The Bunker Project's foundational assumption was fundamentally wrong.
The two-dimensionalization began spreading from near Pluto's orbit. Everything in three-dimensional space — stars, planets, space cities, human bodies — was unfolded onto an infinitely extending plane upon contact with the dimensional boundary. The sun, once two-dimensionalized, became an enormous circular pattern, planets' internal structures spread out in concentric layers like tree rings, and human bodies were flattened into anatomical diagrams. This was a mode of death beyond human imagination — not explosion, not combustion, but the reduction of an entire dimension of existence.
The Only Escape Route
In this ultimate catastrophe, the only way to escape was to reach light speed — only objects traveling at light speed could outrun the constantly expanding boundary of the two-dimensionalized region. The only type of vehicle capable of reaching light speed was a lightspeed ship equipped with a curvature drive engine.
The development of curvature drive technology had a tortuous history. After the Deterrence Era, Wade — the cold and decisive former PDC intelligence director — had secretly led a research team to develop lightspeed ship technology. However, when Cheng Xin learned that this research might trigger social panic and ethical controversy, she used her authority to shut down Wade's project. Wade had desperately pleaded with Cheng Xin before surrendering control: "Lose humanity, and you lose much; lose your animal nature, and you lose everything." But Cheng Xin ultimately decided to halt the research.
Ironically, lightspeed ship technology eventually achieved a breakthrough through the secret efforts of some researchers. A critically important clue came from Yun Tianming — the human brain sent into the Trisolaran fleet by the Staircase Program. After being revived by the Trisolaran civilization, Yun Tianming seized an opportunity during a conversation with Cheng Xin to transmit intelligence about the Trisolaran civilization's core technologies, using three carefully crafted fairy tales as the vehicle. Hidden within these three fairy tales was key information about curvature drive, lightspeed ships, and the cosmic safety notice concept. After extensive decryption, human scientists eventually extracted the technical principles for building lightspeed ships from these tales.
The Moment of Escape
When two-dimensionalization began spreading, only one lightspeed ship in the solar system was operational — the "Halo." Cheng Xin and her friend Ai AA boarded this ship at the last possible moment. The instant the curvature drive engaged, the ship reached light speed, and space behind them compressed into an infinitely thin line.
Looking back at the solar system from the lightspeed ship, Cheng Xin and Ai AA witnessed the final destruction of human civilization. The once-vast three-dimensional solar system was transforming into an immense two-dimensional scroll — the sun's concentric circle pattern, the planets' cross-sectional patterns, countless space cities and human figures all unfolded onto this continuously expanding plane. This scroll flickered with its final light against the universe's black backdrop, like a magnificent yet tragic cosmic elegy.
After escaping the solar system, Cheng Xin and Ai AA faced endless cosmic void. They were among the very few surviving humans from the solar system — carrying memories and knowledge accumulated across thousands of years of Earth civilization, drifting through interstellar space. The lightspeed ship enabled interstellar travel, but they no longer had a home to return to.
The Cost of the Curvature Wake
The lightspeed ship's escape was not without cost. When operating, the curvature drive engine left a permanent "wake" in the space through which the ship traveled — a spatial corridor with reduced light speed. This wake was like a scar in the universe, one that would never heal on its own.
For civilizations that understood the universe's rules, a curvature wake was a clear signal: a civilization that had mastered lightspeed travel once existed here. In the dark forest universe, such a signal might attract attention and strikes from other civilizations. In fact, the "safety notice" — a way of declaring one's harmlessness to the universe — was achieved precisely by actively reducing light speed around a star system, essentially wrapping one's stellar system in a "dark domain" using curvature wakes, announcing to the universe: "We have sealed ourselves and have neither the capability nor the intention to expand outward."
Cheng Xin and Ai AA's escape left a curvature wake traversing the solar system. But by then the solar system was already perishing in two-dimensionalization, and the wake's existence no longer had practical security implications. However, the concepts of curvature drive technology and the safety notice became key knowledge for understanding cosmic civilizations' survival strategies — had humanity mastered this knowledge earlier, the solar system's fate might have been entirely different.