String-scale black hole description in string theory

Determine the correct microscopic and Euclidean string-theoretic description of black holes when their horizon size approaches the string length, identifying the appropriate saddle (beyond Einstein gravity) that captures the physics at temperatures near the Hagedorn scale in asymptotically flat string theory.

Background

The paper studies the proposed correspondence between highly excited strings (string stars) and black holes, particularly as the black hole size shrinks toward the string scale due to Hawking radiation. In Euclidean string theory on asymptotically Rd × S1_β, the usual Einstein gravity description breaks down for β ∼ l_s, raising the broader question of the correct string-theoretic description of such small black holes.

The authors explore this question with angular momentum turned on, arguing for a rotating string star saddle that interpolates between free rotating strings and rotating black holes. The explicit open question highlights the general unresolved status of the black hole description at the string scale in string theory.

References

In string theory, the nature of black holes as their horizon size reaches the string scale remains an open question.

— A spin on Hagedorn temperatures and string stars  (2510.17951 - Seitz et al., 20 Oct 2025) in Introduction

Related to this, the fact that the horizon gas sits exactly at its Hagedorn point---which is what makes the entropy extensive in $E$ and hence linear in $A$---is reminiscent of the string/black-hole correspondence principle, where the transition occurs when the string temperature reaches $T_H$. Whether that resemblance can be made structural rather than suggestive is the natural next question.

— From arithmetic spectra to a quantum-corrected black hole geometry  (2608.23528 - Jusufi et al., 24 Aug 2026) in Section 6, Discussion

Stringy corrections on the black hole side also lift the special zero-Love property of four-dimensional Einstein gravity, but the presently available perturbative results do not yet reveal whether their multipolar response is organized in an analogous way.

— Love at the String Scale: Tidal Deformability Across the Black Hole-String Transition  (2608.23077 - Emparan et al., 24 Aug 2026) in Section 6, Conclusions and outlook

A further question we have not addressed is dissipation. The HP solution is horizonless and static, so it has no analogue of horizon absorption, unlike a black hole. Whether the effective description admits a meaningful dissipative response, and how it would behave across the transition, seems worth understanding: it would test the horizon interpretation from the opposite side, through the channel that the conservative Love numbers do not probe.

— Love at the String Scale: Tidal Deformability Across the Black Hole-String Transition  (2608.23077 - Emparan et al., 24 Aug 2026) in Section 6, Conclusions and outlook