A huge pond, with ripples spreading from two heavy stones circling each other. The stones draw closer, the ripples become more frequent and stronger, and soon the stones will collide. Gravitational waves are like those ripples, but not on water — in the very fabric of spacetime. They are emitted by pairs of massive black holes as they merge into one. Space-based detectors, like the future TianQin, will catch this quiver to hear the 'ring' of mergers across the universe.
Hearing the ring is not enough. Many such mergers are accompanied by a flash of light — an electromagnetic signal that ordinary telescopes can see. But to point a telescope, you need to know where to look, and you need to do it fast — before the flash fades. Previously, processing the signal took hours or days. The authors of this work created a program that does it in a minute. It takes the recording of the spacetime quiver and immediately gives the likely position of the source on the sky, as if you could instantly tell where a stone fell just from the ripples on the water.
This result shows for the first time: you can get accurate enough coordinates before the merger itself. Astronomers now have a real chance to point their telescopes in advance and see the entire event — from the final orbits to the bright flash. Such joint observation of gravitational waves and light will open a new chapter in studying how the most massive objects in the universe grow and interact. And then the ripples on the water will become not just a quiver, but the full story of the falling stones.
The TianQin detector will consist of three satellites orbiting Earth about 100 000 kilometers apart. They form a triangle and measure the distance between them to within a fraction of an atom to catch a passing gravitational wave.
In Liu Cixin's novel 'The Three-Body Problem,' aliens use gravitational waves for communication, and in reality, we are learning to catch them to peek into the heart of galaxies.