How to Catch a Black Hole's Ring
When two black holes merge, a new black hole is born. At first, it quivers like a struck bell, and this quivering spreads through space as waves. Detectors on Earth catch these waves — from them, we can learn the mass and spin of the black hole.
But the signal is weak and full of noise. If you take different chunks of the recording, they might give slightly different answers.