A tiny coin circles a heavy bowling ball. The coin is a small black hole or a neutron star, and the ball is a supermassive black hole at the center of a galaxy. Such pairs are called extreme mass ratio inspirals: the small object gradually spirals inward, emitting ripples in spacetime — gravitational waves.
Three future space detectors are gearing up to catch these ripples: LISA, TAIJI, and TianQin. They are expected to start operating around 2035. Each instrument consists of three satellites flying in a triangle, measuring the slightest changes in the distances between them. The researchers asked: how much better will they perform together than alone? It turns out that a network of three detectors in one month of observations achieves the same accuracy as a single LISA over a full year. The difference is especially striking when pinpointing the source's location on the sky: the error shrinks by up to a factor of 100.
This result matters not only for astronomy. The more precisely we measure such signals, the better we can test how gravity behaves in the strongest fields — where spacetime curvature becomes extreme. The detector network turns a short "snapshot" into a detailed map, allowing us to peer into the vicinity of a black hole as never before. And then the little coin by the bowling ball will tell us much more about itself.
For the network of three detectors, the error in localizing the source on the sky after one month of observations can be a hundred times smaller than that of a single LISA.