Black hole memory effect

We compute the memory effect produced at the black hole horizon by a transient gravitational shock wave. As shown by Hawking, Perry, and Strominger (HPS) such a gravitational wave produces a deformation of the black hole geometry which from future null infinity is seen as a Bondi-Metzner-Sachs supertranslation. This results in a diffeomorphic but physically distinct geometry which differs from the original black hole by their charges at infinity. Here we give the complementary description of this physical process in the near-horizon region as seen by an observer hovering just outside the event horizon. From this perspective, in addition to a supertranslation the shock wave also induces a horizon superrotation. We compute the associated superrotation charge and show that its form agrees with the one obtained by HPS at infinity. In addition, there is a supertranslation contribution to the horizon charge, which measures the entropy change in the process. We then turn to electrically and magnetically charged black holes and generalize the near-horizon asymptotic symmetry analysis to Einstein-Maxwell theory. This reveals an additional infinite-dimensional current algebra that acts nontrivially on the horizon superrotations. Finally, we generalize the black hole memory effect to Reissner-Nordström black holes.

Figure 1

Penrose diagram of a Schwarzschild black hole. The gravitational shock wave at $v=v_0$ describes a domain wall that divides the exterior geometry into two regions, each with different values of the asymptotic charges.