Resolving the Structure of Black Holes: Philosophizing with a Hammer

TL;DR

Resolving the Structure of Black Holes argues that unitarity in black-hole evaporation requires horizon-scale structure organized as microstate geometries or fuzzballs, rather than a classical horizon. The authors synthesize string theory, supergravity, holography, and quantum information to identify two crucial scales, the transition length ${\lambda_T}$ and the energy gap ${E_{gap}}$, which govern the existence and fluctuations of horizonless microstate configurations. They classify three incarnations of the underlying mechanism—geometric transitions with cohomological fluxes, brane polarization, and non-Abelian effects—and discuss near-BPS and non-extremal extensions that could describe Schwarzschild- and Kerr-like microstates. The work argues that microstate geometries encode black-hole entropy and yield Hawking-like radiation carrying microstate information, with holography providing a consistent IR-phase interpretation in the dual CFT framework.

Abstract

We give a broad conceptual review of what we have learned about black holes and their microstate structure from the study of microstate geometries and their string theory limits. We draw upon general relativity, supergravity, string theory and holographic field theory to extract universal ideas and structural features that we expect to be important in resolving the information problem and understanding the microstate structure of Schwarzschild and Kerr black holes. In particular, we emphasize two conceptually and physically distinct ideas, with different underlying energy scales: a) the transition that supports the microstate structure and prevents the formation of a horizon and b) the representation of the detailed microstate structure itself in terms of fluctuations around the transitioned state. We also show that the supergravity mechanism that supports microstate geometries becomes, in the string theory limit, either brane polarization or the excitation of non-Abelian degrees of freedom. We thus argue that if any mechanism for supporting structure at the horizon scale is to be given substance within string theory then it must be some manifestation of microstate geometries.