Remembrance of Earth's Past, #3
Death's End
by Cixin Liu
Contents
Excerpt from A Past Outside of Time Space Sentries: The Solar System Advance Warning System
Overview
This excerpt explains the scientific basis and limits of humanity's Solar System advance warning system against photoid attacks. Photoids cannot be seen directly, but their distinctive radiation may arrive ahead of them because they travel slightly slower than light and follow gravity-curved paths.
The proposed network of thirty-five observation units offers only conditional protection: a distant launch could provide roughly twenty-four hours of warning, while a nearby launch might leave almost none. The chapter underscores how fragile the Solar System remains despite humanity's attempt to prepare for a dark forest strike.
Summary
The excerpt explains that humanity has observed photoids only twice: during the destruction of 187J3X1 and during the annihilation of the Trisolaran system. Because of this limited experience, scientists know only that photoids travel near lightspeed and use enormous relativistic kinetic energy to attack stars, making them a crude but highly economical stellar weapon.
Data gathered just before Trisolaris was destroyed revealed the key to detection. As a photoid passes through sparse atoms and dust in space, collisions produce distinctive radiation ranging from visible light to gamma rays. Since the photoid itself is too small to observe directly, this radiation becomes the only practical warning sign.
The excerpt clarifies why advance warning is possible despite the photoid's near-lightspeed speed. A photoid has rest mass, so it cannot travel at true lightspeed, and its radiation moves slightly faster. Its path is also bent by gravity more than light is, making the photoid travel a longer route than the radiation it emits.
These effects mean the radiation can reach the Solar System before the weapon, potentially giving about twenty-four hours of warning if the launch comes from far enough away. In that ideal case, the photoid would still be roughly 180 AU away when its emissions were detected from Earth.
The system has a major limitation: if a photoid is launched from a nearby spaceship, warning time may be nearly nonexistent, as happened to Trisolaris. Humanity therefore plans thirty-five observation units to watch the entire sky for photoid emissions as part of the Solar System advance warning system.