Cosmic amnesia

Black holes emit a distinctive sound after merging, similar to a bell ringing.

This phenomenon is called ringdown.

When two black holes collide, they create ripples in spacetime called gravitational waves.

These waves travel through space at the speed of light and can be detected on Earth.

For example, in September 2015, the LIGO instruments in Washington and Louisiana detected gravitational waves from two colliding black holes 1.3 billion light years away.

The remnant black hole from such a merger rings briefly before settling into silence, much like a bell that stops vibrating after being struck.

This ringing follows predictable frequencies that help physicists test theories of gravity, including Albert Einstein’s general relativity.