The Stretched Horizon and Black Hole Complementarity

TL;DR

This paper proposes a three-postulate framework to reconcile unitary quantum evolution, semi-classical gravity, and black-hole thermodynamics from a distant observer’s perspective, via a physically real yet coarse-grained stretched horizon. Using the 1+1D CGHS model, Hawking radiation is viewed as emerging from a microphysical, dissipative horizon with universal entropy proportional to area; information is conserved through long-time, non-thermal correlations rather than slow late-time emission or remnants. The work develops a semi-classical, solvable description (RST) of horizon dynamics, plus a causal, membrane-based account of information flow and entropy, culminating in a complementarity principle that unifies external and infalling observer viewpoints. A Brownian-motion analysis of horizon fluctuations reinforces the thermodynamic nature of the stretched horizon and its role as the backbone of information transfer in evaporation. The framework suggests deep connections between horizon microstructure, entanglement, and the ultimate fate of information in black hole evaporation.

Abstract

Three postulates asserting the validity of conventional quantum theory, semi-classical general relativity and the statistical basis for thermodynamics are introduced as a foundation for the study of black hole evolution. We explain how these postulates may be implemented in a ``stretched horizon'' or membrane description of the black hole, appropriate to a distant observer. The technical analysis is illustrated in the simplified context of 1+1 dimensional dilaton gravity. Our postulates imply that the dissipative properties of the stretched horizon arise from a course graining of microphysical degrees of freedom that the horizon must possess. A principle of black hole complementarity is advocated. The overall viewpoint is similar to that pioneered by 't~Hooft but the detailed implementation is different.