The Black Hole Interior in Quantum Gravity

TL;DR

The paper addresses the tension between unitarity in black hole evaporation and the equivalence principle by positing a microscopic quantum-gravity description in which semiclassical spacetime arises after coarse-graining vacuum degrees of freedom. The authors introduce a vacuum index $k$ with entropy $S_0 = \frac{A}{4 l_{\rm P}^2}$ that counts microstates and show how a thermal reduced density matrix emerges for semiclassical operators. They propose two key features—extreme relativeness (reference-frame–dependent distribution of vacuum degrees) and spacetime–matter duality (vacuum degrees acting as both spacetime realization and apparent thermal radiation)—which together yield a unitary evolution of interior structure and Hawking radiation via a non-factorizable coupling to $k$. The framework preserves the equivalence principle across reference frames and clarifies how interior information can be encoded without invoking firewalls, with Hawking quanta purified by microstates and transferred through negative-energy excitations.

Abstract

We discuss the interior of a black hole in quantum gravity, in which black holes form and evaporate unitarily. The interior spacetime appears in the sense of complementarity because of special features revealed by the microscopic degrees of freedom when viewed from a semiclassical standpoint. The relation between quantum mechanics and the equivalence principle is subtle, but they are still consistent.