Soft Hair
Hawking, Perry, and Strominger's 2016 proposal that information is stored in 'soft hair', infinitely many very-low-energy quantum excitations at the horizon. Hawking's last published response to the paradox before his death in 2018.
Placeholder for a 3D visualisation of Black Hole Information Paradox. The interactive scene will land in Phase 3. Hawking showed in 1974 that black holes radiate. If the radiation is purely thermal (random) and the black hole eventually evaporates, the information about what fell in is destroyed. That breaks one of the foundational rules of quantum mechanics, that information is preserved by any allowed physical process. Hawking himself argued for decades that information really is lost and quantum mechanics has to be modified at black hole horizons. Most of the field disagreed and treated the apparent contradiction as a paradox to be solved within standard quantum theory. Hawking conceded the bet in 2004. Today nearly everyone agrees information must be preserved; the contested question is by what mechanism. Black Hole Complementarity, the Firewall paradox, ER=EPR, Soft Hair, and Fuzzballs are five proposals for that mechanism. The 2019 Page curve calculations using replica wormholes (Penington; Almheiri, Engelhardt, Marolf, Maxfield) showed that unitarity can be recovered within semiclassical gravity, which is the strongest concrete progress the field has made in two decades.
In one sentence
The no-hair theorem says a classical black hole is characterized by only mass, charge, and angular momentum. Hawking, Perry, and Strominger argued in 2016 that this is technically incomplete: black holes also carry infinitely many conserved 'soft' charges, very-low-energy quantum excitations sitting at the horizon. Information about what fell in is stored in this soft hair.
The claim
The classical no-hair theorem of the 1970s established that a stationary black hole in general relativity is fully characterized by three numbers: mass, electric charge, and angular momentum. Two black holes with the same three numbers are indistinguishable to any outside observer. If this is the complete classical story and quantum corrections don't change it, then most of the information about what fell in really is lost when the black hole evaporates. The 2016 Hawking-Perry-Strominger paper challenges the assumption that the no-hair theorem is complete. They show that the asymptotic symmetry group of flat spacetime is richer than previously appreciated, including infinitely many supertranslations and superrotations, each with an associated conserved charge.
The hair the proposal adds is 'soft' in a precise technical sense: these are zero-energy excitations of the gravitational field, related to the asymptotic symmetries of spacetime at infinity. Each piece of matter that falls into a black hole leaves an imprint on the horizon by sourcing one of these soft modes. The information about what fell in is encoded across an infinite tower of these soft charges, similar to how a hologram stores three-dimensional information on a two-dimensional surface, except here the encoding lives in the soft-mode spectrum rather than directly on the geometric horizon. The Bondi-van der Burg-Metzner-Sachs (BMS) supertranslation symmetries are the formal apparatus.
Hawking died in 2018, two years after this paper. Soft hair was his last published response to the paradox he had created in 1974. The proposal remains active but is treated by most of the field as a partial sketch rather than a complete mechanism. The technical questions: do the soft modes carry enough information to encode the full state of what fell in? Is the encoding accessible to an outside observer collecting Hawking radiation? How does soft hair fit into the post-2019 replica-wormhole picture of unitarity recovery? None of these have been fully settled. The proposal is more developed in the related context of asymptotic flat-space holography and celestial CFT, which is its own active research program in 2026.
The family stance
Most physicists now accept that information is preserved when matter falls into a black hole. Hawking conceded this point publicly in 2004, paying off a 1997 bet with John Preskill. The contested question is the mechanism. Black hole complementarity, ER=EPR, soft hair, and fuzzballs each propose different machinery for how information escapes; the firewall paradox is the argument that exposed why a naive resolution cannot work. The 2019 Page curve calculations using replica wormholes have shown that unitarity can be recovered within semiclassical gravity, but the question of what an infalling observer experiences at the horizon locally remains open.
Predictions
- Black holes carry infinitely many conserved soft charges associated with BMS supertranslations and superrotations; these can in principle be measured by detectors at asymptotic infinity
- Two black holes with the same mass, charge, and angular momentum but different histories of what fell in differ in their soft-hair spectra; the difference is recoverable from sufficiently sensitive asymptotic measurements
- Hawking radiation carries correlations matching the soft-charge spectrum of the emitting black hole; in principle measurable, in practice astronomically far from current detector capabilities
- Memory effects in gravitational waves (permanent strain in distant detectors after a passing wave) are imprints of the same BMS structure that underpins soft hair, and have been observationally targeted by LIGO
Evidence
- BMS supertranslations are mathematically real symmetries of asymptotically flat spacetime; their existence is not contested, only their physical role
- Strominger and collaborators have developed an extensive program (celestial CFT, asymptotic symmetries, gravitational memory) showing the soft-mode sector is structurally significant and connects to scattering amplitudes via soft theorems
- LIGO has searched for gravitational-wave memory effects, which are observational signatures of BMS supertranslations; bounds exist and are improving
- The soft-hair proposal grew out of decades of prior work by Strominger and collaborators on infrared structure and asymptotic symmetries in QED and gravity, giving it strong theoretical context
Counterpoints
- The original 2016 paper sketched an information-encoding mechanism but did not demonstrate that the soft modes actually carry enough information to encode arbitrary matter falling into the black hole; the counting argument was incomplete
- Several follow-up papers (Bousso-Porrati 2017 among others) argued that the soft-hair charges are pure gauge in a precise sense and therefore cannot carry the information they were proposed to carry; this critique is contested but unresolved
- Soft hair does not connect cleanly to the replica-wormhole / entanglement-wedge picture that has dominated the post-2019 literature; the two approaches are not known to be reconcilable or to be incompatible
- Most of the technical development of soft modes since 2016 has happened in the celestial-CFT and asymptotic-symmetry program, somewhat separately from the information-paradox debate proper
Variants in this family
▸Go deeperTechnical detail with proper terminology
BMS supertranslations: Bondi, van der Burg, Metzner, and Sachs identified in the 1960s that the asymptotic symmetry group of flat spacetime is not the Poincare group but a much larger algebra including angle-dependent translations of asymptotic null infinity. These supertranslations have associated conserved charges that label distinct vacuum states.
Soft theorems: Weinberg's soft theorems (1965) state that scattering amplitudes with an additional zero-energy graviton are related to amplitudes without it by a universal factor. Strominger and collaborators showed in 2013-2016 that these soft theorems are Ward identities for BMS supertranslations; soft hair extends this to black hole horizons.
Memory effect: passage of a gravitational wave through a region of asymptotic infinity leaves a permanent change in the relative positions of test particles there. This is a direct observational consequence of BMS supertranslations and is targeted by LIGO and proposed space-based detectors like LISA.
Celestial CFT: a 2020s program proposing that 4D scattering amplitudes have an alternative formulation as 2D conformal field theory correlators on the celestial sphere at asymptotic infinity. The BMS supertranslation structure shows up there as a Virasoro symmetry; soft hair is the black hole signature of the same algebra.
References
Last reviewed May 18, 2026
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