Near-horizon instability of black holes

I had the pleasure of working with Sam Gralla and Peter Zimmerman (no relation) on a paper this summer that was recently published. In this work, we returned to a topic I’ve explored in the past, and asked what happens when a rapidly spinning black hole is perturbed. Black holes have a maximum rate of spin above which they shed their horizons and cease to make much physical sense. Just before this point, a black hole takes on all sorts of interesting properties, with a unique spectrum of resonant modes and a long-lived emission towards faraway regions (like say, a gravitational wave detector) when matter falls into it. There’s also a lot of interest in these black holes in the particle physics community thanks to a famous “duality” that connects black holes that spin at maximum and other theories (the Kerr/conformal field theory duality).

In our recent work, we asked what the situation is near the horizon of the spinning black holes when we send energy in from far away. The result was surprising: normally when a black hole is disturbed in this way, energy is trapped in the strong gravity region but always decays away as it falls into the black hole or escapes to infinity. But when the hole is spinning at nearly maximum, we found that electromagnetic fields and gravitational radiation actually build, potentially to large amplitudes over a long time, before finally dissipating away as expected. We’re still working out the possible observational signatures of this effect, which is in some ways a generalization to the idea that high-energy particle collisions can occur near rapidly spinning black holes.