Boundary unitarity and the black hole information paradox

TL;DR

The paper argues that boundary unitarity in quantum gravity does not conflict with the smoothness of black hole horizons, challenging the firewall argument by emphasizing the non-factorization of the state space.

Contribution

It demonstrates that boundary unitarity and horizon regularity are compatible, refuting the firewall paradox based on state space factorization assumptions.

Findings

Boundary observables evolve unitarily in quantum gravity.

The state space does not factorize into localized degrees of freedom.

Firewall argument relies on incorrect factorization assumptions.

Abstract

Both AdS/CFT duality and more general reasoning from quantum gravity point to a rich collection of boundary observables that always evolve unitarily. The physical quantum gravity states described by these observables must be solutions of the spatial diffeomorphism and Wheeler-deWitt constraints, which implies that the state space does not factorize into a tensor product of localized degrees of freedom. The "firewall" argument that unitarity of black hole S-matrix implies the presence of a highly excited quantum state near the horizon is based on such a factorization, hence is not applicable in quantum gravity. In fact, there appears to be no conflict between boundary unitarity and regularity of the event horizon.