How the Ringing of Black Holes Reveals Their Secrets ✎ express
After a merger, the newborn black hole emits a fading tremble — gravitational waves, similar to a ringing bell. By analyzing the overtones of this signal, spectroscopy can determine its mass and spin rate.
Typically, astronomers analyze only the ringdown itself, discarding data on how the holes approached. But the force and direction of the "impact" (the pair's orbit) are equally important. The new SPRING tool links both phases for the first time, using pre-merger observations to clean up gravitational-wave signals from extraneous noise.
Previously, the ringdown was thought to consist of a single pure tone, but in the GW250114 signal, SPRING distinguished two, as if the bell has a hidden undertone. This paves the way for true black hole spectroscopy, where we study spacetime curvature without being bound by theory.
The ringdown fades within a couple of milliseconds, but its analysis yields the hole’s mass with an accuracy of a few percent, like weighing a mountain by listening to a pebble fall.
In the film Interstellar, the black hole Gargantua was recreated using real equations for light bending. Today, those same calculations help us hear black holes in gravitational-wave data — instead of light, we capture the gravitational echo.