Dynamics of extremal black holes

Particle scattering and radiation by a magnetically charged, dilatonic black hole is investigated near the external limit at which the mass is a constant times the charge. Near this limit a neighborhood of the horizon of the black hole is closely approximated by a trivial product of a two-dimensional black hole with a sphere. This is shown to imply that the scattering of long-wavelength particles can be described by a (previously analyzed) two-dimensional effective field theory, and is related to the formation and/or evaporation of two-dimensional black holes. The scattering proceeds via particle capture followed by Hawking reemission, and naively appears to violate unitarity. However this conclusion can be altered when the effects of back reaction are included. Particle-hole scattering is discussed in the light of a recent analysis of the two-dimensional back-reaction problem. It is argued that the quantum-mechanical possibility of scattering off of extermal black holes implies the potential existence of additional quantum numbers, referred to as "quantum whiskers," characterizing the black hole.