  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.
