Trapping, Irregular Waveforms, and Efficient Radiation in Ultra-relativistic Black Hole Encounters

Abstract

We demonstrate that ultra-relativistic black hole encounters reveal a new regime of the two-body interaction in general relativity. Evolving equal-mass, nonspinning black holes with initial center-of-mass Lorentz factors up to $\gamma\approx 5.1$ using numerical relativity, we find that the resulting waveforms defy the standard expectation of a post-Newtonian description followed by a smooth transition to a prompt Kerr ringdown. Instead, at nonzero impact parameter, the system can exhibit prolonged, highly irregular emission and significant horizon absorption, even without coalescence. We show these phenomena are driven by transient null trapping and repeated lensing of radiation in the binary interaction region. Furthermore, our simulations indicate that over $65\%$ of the initial ADM energy can be radiated as gravitational waves at $\gamma\approx 5.1$, which is substantially larger than previously estimated by extrapolating from lower boost data.