A black hole doesn't just spin: it's surrounded by bristles of an extra field (the Kalb–Ramond field). This paper explores how a massive field (waves with mass) behaves near such a black hole when it's almost spinning at its maximum rate.
When the black hole spins exactly at the limit, the problem simplifies to a known Whittaker form. At this limit, the field can form stationary clouds hovering just above the horizon: their frequencies exactly match the black hole's rotation frequency. If you step slightly away from the limit, the clouds turn into slowly decaying oscillations. The authors calculated how the black hole amplifies incoming waves (known as superradiance) and how strongly it absorbs them.
They also looked at light bending near the black hole and the shape of its shadow. It turns out the shadow depends on the direction of rotation: for prograde rotation, the logarithmic deflection coefficient differs significantly from that of a standard Schwarzschild black hole. Combining shadow calculations with Event Horizon Telescope data, the authors obtained tighter constraints on parameters that break Lorentz symmetry: these are the very numbers that make the black hole 'bristly.'
Clouds of a massive field near a black hole's horizon can exist indefinitely if the field's frequency exactly matches the black hole's rotation frequency, a perfect resonance.
In Michael Crichton's novel 'Sphere,' an alien spacecraft uses a rotating black hole for faster-than-light jumps — similar to superradiance, but on a starship scale.