High-energy interactions of charged black holes in full general relativity I: Zoom-whirl orbits and universality with the irreducible mass

TL;DR

This paper investigates high-energy charged black hole collisions in full general relativity, revealing charge effects on zoom-whirl orbits and demonstrating the universality of impact parameter thresholds when normalized by irreducible mass.

Contribution

It is the first to show that the irreducible mass sets a universal, gauge-invariant scale for horizon-scale scattering in charged black hole interactions.

Findings

Charge influences the impact parameter thresholds for zoom-whirl orbits.

Threshold impact parameters normalized by irreducible mass are charge-independent.

Electrodynamics does not suppress zoom-whirl orbits at high energies.

Abstract

We simulate high-energy scattering of equal-mass, nonspinning black holes endowed with like charges in full general relativity while varying the impact parameter $b$. We show that electrodynamics does not suppress zoom-whirl orbits for at least charge-to-mass ratios $\lambda = 0.1, 0.4, 0.6$. However, we find that as $\lambda$ increases, the immediate merger and scattering thresholds defining the zoom-whirl regime move to smaller impact parameter $b/M_{\rm ADM}$, with $M_{\rm ADM}$ designating the binary black hole gravitational mass. This demonstrates that charge leaves observable imprints in key properties at energy scales where charge has negligible influence in head-on collisions. Additionally, we find that these threshold impact parameters become universal, i.e., charge-independent, when we normalize $b$ by the sum of the initial BH irreducible masses in the binary ($b/M_{\rm irr}$). This is the first explicit demonstration that the irreducible mass, which is proportional to the black hole areal radius, defines a fundamental gauge-invariant length scale governing horizon scale scattering events in the strong-field, dynamical spacetime regime.