Excerpt from A Past Outside of Time Space Sentries: The Solar System Advance Warning System

The Earth had observed photoids only twice in the past: the destruction of 187J3X1 and of the Trisolaran system. Knowledge about the phenomenon was thus limited. All that was known was that a photoid moved at close to the speed of light, but there was no data concerning its volume, rest mass, or relativistic mass as it approached the speed of light. A photoid was certainly the most primitive weapon capable of attacking a star, since it relied only on the enormous kinetic energy generated by its high relativistic mass to damage the target. Once a civilization possessed the technology to accelerate an object to near the speed of light, a “bullet” with very little mass possessed immense destructive power. This was indeed “economical.”
The most valuable data concerning photoids was obtained right before the annihilation of the Trisolaran system. Scientists were able to make an important discovery: Due to a photoid’s ultrahigh velocity, powerful radiation ranging from visible light to gamma rays was emitted as it collided with the few atoms scattered in space and interstellar dust. The radiation had distinctive characteristics. Since the photoids were extremely small, direct observation of them was impossible. But the characteristic radiation could be detected.
At first blush, it seemed impossible to give advance warning for photoids, because they moved at close to the speed of light. This meant that they moved almost as fast as the radiation they generated, and reached their target almost simultaneously. In other words, the observer was outside the event’s light cone.
But reality was a bit more complicated. Any object with rest mass could not achieve lightspeed. Although a photoid’s speed approached lightspeed, it was still slightly slower than true lightspeed. This difference meant that the radiation from the photoid moved just a bit faster than the photoid itself. If the photoid had to travel a long distance, this difference was magnified. Also, a photoid’s trajectory to the target wasn’t an absolute straight line. Since it wasn’t massless, it couldn’t avoid the gravitational attractions of nearby celestial bodies, and its path typically ended up being slightly curved. The curvature was much greater than the curvature of light through the same gravitational field. For the photoid to strike the target, its trajectory had to take this effect into account. This meant that the path traveled by the photoid was longer than the path taken by its radiation.
For these two reasons, the radiation from the photoid would reach the Solar System before the photoid itself. The twenty-four-hour estimated warning period was calculated based on the maximum distance at which photoid emissions could be observed. By the time the radiation reached the Earth, the photoid itself would still be about 180 AU away.
But that was merely the ideal scenario. If the photoid were launched from a nearby spaceship, there would
 
be almost no warning—like what had happened to Trisolaris.
Thirty-five observation units were planned for the Solar System advance warning system. These would monitor the sky in every direction for photoid emissions.
 

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