## Details

Complementing the spectacular breakthroughs in gravitational wave and multi-messenger observations of merging black holes and neutron stars, advancements in our theoretical understanding and modelling of strong gravity are essential to elucidating the nonlinear dynamics of spacetime. These are motivated by the goal of unlocking the full potential of the observations, as well as that of deepening our understanding of gravitational theory. I will illustrate some of the new physics that might be probed by the dynamics of black holes and neutron stars, and describe some recent theoretical developments that allow for uncovering novel phenomena and making detailed predictions in this regime, namely testing general relativity and signatures of new particles that arise through black hole instability. I will also discuss how such instabilities and modifications to gravity can take black holes to their theoretical extreme, and what this teaches us about general relativity.