Appears in collection : Balzan Conference on Open Issues in Gravitation

The collision of black holes at ultra-relativistic speeds represents one of the most extreme physical regimes accessible to theoretical study, offering a unique testing ground for General Relativity, the Cosmic Censorship Conjecture, and high-energy astrophysics. Based on extensive numerical simulations detailed in recent studies (Healy et al., 2023, 2025), this essay explores the fundamental upper bounds of these cataclysmic events. We synthesize findings on the maximum possible gravitational recoil—approaching 10% of the speed of light—and the maximum radiated gravitational energy, estimated at 32% of the system's total mass. Expanding on the related bibliography cited within these works, we contextualize these results against historical limits derived from head-on collisions and discuss their profound implications for the gauge-gravity duality, holography, and the dynamics of primordial black holes in the early Universe.