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arXiv · 2604.28050

Black-Hole Microstate Hair Requires a Horizon Measurement

Sudhanva Joshi·Sunil Kumar Mishra
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Abstract

Can a smooth horizon hide its microstates? At fixed classical and superselected data, the exterior distinguishability $D_{\max}$ of same-sector infalling states and the departure $\varepsilon$ from isometric infall obey $D_{\max}^{2}+(1-2\varepsilon)^{2}\leq 1$, and every point is realized. Below maximal disruption, the boundary is reached only by which-state measurements: the distinguished pair crosses undisturbed while the exterior records which one fell in. The least a horizon can do to have microstate hair is measure what falls in. Classically hairy black holes are labeled backgrounds, not microstate hair.

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Sudhanva Joshi, Sunil Kumar Mishra. 2026-04-30. Black-Hole Microstate Hair Requires a Horizon Measurement. https://arxiv.org/abs/2604.28050

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