Resolving the Structure of Black Holes: Philosophizing with a Hammer

TL;DR

This paper reviews how microstate geometries from string theory and related frameworks shed light on black hole structure, horizon formation, and the information paradox, emphasizing two key conceptual ideas and their physical mechanisms.

Contribution

It introduces a unified perspective on black hole microstates, highlighting the roles of transition mechanisms and fluctuations, and connects supergravity support to string theory phenomena like brane polarization.

Findings

Microstate geometries prevent horizon formation.

Supergravity mechanisms relate to brane polarization.

Microstate fluctuations encode detailed black hole microstructure.

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.