Did you know that a black hole can ring?
Just like a bell or drum, if you “hit” a black hole, it rings with a characteristic sound. Except the noise it makes is in gravitational waves - oscillations in the spacetime metric as predicted by Einstein’s general relativity.
But how do you hit a black hole? One way to do this is by colliding and merging two black holes together - the final black hole will initially be highly distorted, and it will radiate gravitational waves as it settles down into its stable, equilibrium state.
The full gravitational wave signal looks something like this. The two orbiting black holes produce a sinusoidal signal, which peaks in amplitude when they merge. Then, the ringdown has the form of a damped sinusoid which is shown in green.
Looking closer at the ringdown waveform, we see that it consists of a sum of modes with different frequencies.
The spectrum of ringdown frequencies is completely determined by the final black hole’s mass and angular momentum, which is one of the reasons the ringdown is so valuable.
If we can measure these frequencies in the ringdown, we can then infer the properties of the final black hole.
This is like “hearing the shape of a drum”, and detecting these frequencies is a key goal in gravitational wave astronomy.