How can black hole mergers happen if we cannot see anything fall into a black hole?

Question

If we cannot observe anything fall into a black hole as an external observer, how can we observe a black hole merger?

Answer 1

It is true that we cannot observe anything beyond the event horizon of a black hole. However, we can certainly observe matter that is in the process of falling into a black hole, but is still outside of the event horizon. This matter forms an accretion disk around the black hole, and we can see light and shorter wavelength radiation emitted by the matter in the accretion disk.

Observations of black hole mergers come from detecting the gravitational waves from the merger (and from its immediate aftermath, known as "ringdown") rather than observing electromagnetic radiation. So what is detected is minute changes in the structure of space-time itself. I don't think anyone has (so far) observed a black hole merger in visible light, although gamma ray bursts that seem to be associated with black hole mergers detected by gravitational wave observatories have been detected.