Edge states in gravity and black hole physics

We show in the context of canonical gravity that the removal of a spatial region leads to the appearance of an infinite set of observables and their associated edge states localized at its boundary. Such a boundary occurs in certain approaches to the physics of black holes like the one based on the membrane paradigm. The edge states can contribute to the black hole entropy in these models. A ''complementarity principle'' is also shown to emerge whereby certain ''edge'' observables are accessible only to certain observers. The physical significance of edge observables and their states are discussed using their similarities to the corresponding quantities in the quantum Hall effect, The coupling of the edge to the bulk states of the gravitational field is demonstrated in the context of (2+1)-dimensional gravity.