What happens to apparent horizons when black holes collide?

The common picture of a binary black hole merger is the “pair of pants” diagram for the event horizon, which has been understood for over half a century. However, in many circumstances, such as those encountered in numerical simulations, the event horizon may not provide the most convenient description. In such cases, it is more practical to work with quasi-local definitions of black hole boundaries, such as marginally outer trapped surfaces (MOTS). The analog of the pair of pants diagram for the apparent horizons has only recently been understood. In this talk, I will discuss the rich structure that emerges for, and principles that govern the evolution of, the apparent horizons during a binary black hole merger. Based on: 2104.10265