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 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 $b\mathrm{/}{M}_{\mathrm{A}\mathrm{D}\mathrm{M}}$, with $M_{\mathrm{A}\mathrm{D}\mathrm{M}}$ 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 ($b\mathrm{/}{M}_{\mathrm{i}\mathrm{r}\mathrm{r}}$). 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.