Light from afar bends around a black hole like a cyclist taking a sharp turn. Get closer, and it starts looping around an invisible sphere of photons. Every black hole has such a sphere, but usually it's wobbly: the slightest nudge sends light flying away or falling in.
The authors considered black holes with 'hair'—the name for an extra scalar field characterized by the curvature parameter Q. Such objects arise in theories that extend general relativity. The scientists derived exact formulas for the radius of the photon sphere and its stability. It turns out that almost always it remains unstable. But there's an exception: when Q is exactly equal to minus twice the black hole's mass, a stable ring for light emerges right at the event horizon.
Such a ring acts like a bottomless well for particles: light and ultra-fast particles can circle indefinitely right on the horizon, redshifted to zero energy. It's like frozen ripples on water: there's motion, but the wave doesn't travel anywhere.
Next, the physicists checked what happens in head-on collisions of particles falling into such a hole. They found that the energy at the collision center grows without limit as you approach the horizon — the so-called Banados–Silk–West effect. The formula is simple: the collision energy is inversely proportional to the square root of the distance to the horizon. In the extremal state, when the horizon and photon sphere coincide, this distance can be made arbitrarily small.
Thus, a 'hairy' black hole in its extremal mode becomes a perfect trap — for both light and matter. Light that starts circling near the horizon can neither escape nor fall in, marking the boundary where space and time swap roles.
The 'hair' parameter Q can be negative, and precisely at Q = –2M does the black hole behave completely differently from an ordinary one.
In Michael Crichton's novel 'Sphere', a giant sphere at the ocean floor warps space and time — why not a stable horizon for light?