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

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 at least for charge-to-mass ratios 𝜆 =0.1, 0.4, 0.6. However, we find that as 𝜆 increases, the immediate merger and scattering thresholds defining the zoom-whirl regime move to smaller impact parameter 𝑏/𝑀ADM, with 𝑀ADM designating the binary black hole gravitational mass. This demonstrates that in the zoom whirl regime charge can leave observable imprints in key properties in high energy interactions. By contrast, charge has no significant influence in head-on collisions at the same energy scales. Additionally, we find evidence that the threshold impact parameters for immediate merger and scattering are universal, i.e., charge-independent, when we normalize 𝑏 by the sum of the initial black hole irreducible masses in the binary (𝑏/𝑀irr). This indicates that the irreducible mass, which is proportional to the black hole areal radius, may define a fundamental gauge-invariant length scale governing horizon scale scattering events in the strong-field, dynamical spacetime regime.