Event horizon r₊
Ergosphere r_ergo(θ)
ISCO (pro / retro)
Photon sphere (pro / retro)
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A spinning black hole is not the same object as the textbook non-rotating Schwarzschild hole: general relativity's exact Kerr solution splits the event horizon from a second, oblate surface called the ergosphere, inside which spacetime is dragged around so violently that nothing — not even light — can stay still. This simulator renders both surfaces in real 3D from the exact Kerr closed-form radii, then launches prograde and retrograde test particles at the exact Bardeen–Press–Teukolsky orbital angular velocity Ω = ±1/(r^1.5 ± a), so you can watch frame-dragging speed up co-rotating particles and slow down counter-rotating ones as you turn up the spin. Live readouts track the horizon, ergosphere, innermost stable circular orbit (ISCO) and photon-sphere radii for both orbital directions.