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Black Hole Entropy is Noether Charge

General Relativity and Quantum Cosmology · Submitted 29 Jul 1993 · 19 references

Pith's verdict: The Noether-charge entropy formula is derived cleanly from diffeomorphism invariance alone, and the paper is honest about the stationary, bifurcate-horizon scope it does not cover.

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arXiv IDgr-qc/9307038 (version 1)
Pith ID56AKUVZP, Pith's permanent record of this paper and its review
CitationRobert M. Wald. "Black Hole Entropy is Noether Charge." arXiv:gr-qc/9307038 (1993). https://pith.science/paper/56AKUVZP
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The paper

The paper claims

Wald argues that in any gravity theory built from a diffeomorphism-invariant Lagrangian, black hole entropy is simply $2π$ times the Noether charge of the horizon Killing field, so the first law of black hole mechanics holds universally.

The reading

The paper asks whether the first law of black hole mechanics — the statement that a change in a black hole's mass and spin is balanced by a change in its entropy — is special to Einstein's equations or is instead a generic feature of any gravity theory that can be written as a Lagrangian without preferring any coordinate system. Earlier Hamiltonian derivations could show that some horizon surface term balances the changes in mass and angular momentum at infinity, but they could not show that this term is the change of a local, geometrical quantity on the horizon — which is what you need before you can call anything 'entropy'. Wald closes that gap. He shows that the horizon term is always $κ/2π$ times the change of $2π$ times the integral over the horizon cross-section of a Noether charge $(n-2)$-form, built from the horizon Killing field normalized to unit surface gravity. If this is right, every such theory has a black hole entropy defined by a local expression on the horizon, the first law follows from diffeomorphism invariance alone, and the Euclidean and Hamiltonian routes to entropy are the same calculation in different clothes.

What carries the argument

The Noether charge $(n-2)$-form $Q$. For any vector field $ξ^a$, diffeomorphism invariance makes $L_ξφ$ an infinitesimal local symmetry, so one can form the Noether current $(n-1)$-form $j = Θ(φ, L_ξφ) - ξ·L$, which is closed on solutions ($dj = -E L_ξφ$). Since $j$ is closed for every $ξ^a$, it can be written $j = dQ$ on solutions, with $Q$ locally constructed from the fields and $ξ^a$ and depending on at most $k-1$ derivatives of $ξ^a$ when the Lagrangian contains at most $k$ derivatives of the dynamical fields. The second piece of machinery is the reduction of $Q$ on the bifurcation surface: every higher derivative $∇_{a_1}…∇_{a_n}ξ_b$ of a Killing field can be rewritten in terms of $ξ^a$ and $∇_aξ_b$ with coefficients built from the curvature and its derivatives, so on $Σ$ one may set $ξ^a = 0$ and replace $∇_aξ_b$ by the binormal $ε_{ab}$, producing $Q̃$ — the Noether charge of the horizon Killing field at unit surface gravity. The third piece is the identification of the asymptotic surface terms with $δE$ and $δJ$ via $δE = ∫_∞(δQ[t] - t·Θ)$ and $δJ = -∫_∞ δQ[φ]$, which is what turns the integrated identity into a first law.

Extended reading notes

Core claim

The central claim is eq. (26): the entropy of a stationary black hole in an $n$-dimensional diffeomorphism-invariant Lagrangian theory of gravity is $S = 2π ∫_Σ Q̃$, where $Q̃$ is the Noether charge $(n-2)$-form associated with the horizon Killing field $ξ^a$ normalized so that its surface gravity is unity, integrated over the bifurcation surface $Σ$. With this $S$, the first law takes the standard form $(κ/2π) δS = δE - Ω^{(μ)}_H δJ_{(μ)}$ for perturbations to nearby stationary black hole solutions (eq. 25). The derivation starts from the identity $δj = Ω(φ, δφ, L_ξφ) + d(ξ·Θ)$, integrates it over an asymptotically flat hypersurface bounded inside by $Σ$, and identifies the horizon surface term as $δ∫_Σ Q$; the step to eq. (25) uses the fact that on $Σ$ the Killing field vanishes and its first derivative is $κ$ times the binormal, so that $δQ = κ δQ̃$. Because $Q̃$ is fixed by an algorithm whose only input is a diffeomorphism-invariant Lagrangian, it is a local geometrical quantity on the horizon — which is what licenses calling it entropy, and what lets the author propose the same integral over any cross-section of a dynamical horizon as a candidate entropy there.

Load-bearing premise

The derivation assumes the black hole has a bifurcate Killing horizon with constant surface gravity, and the paper itself notes that the constancy of surface gravity is proved in general relativity using the specific form of Einstein's equations and does not look likely to carry over to other theories of gravity.

Editorial extensions

If this is right

  • The first law of black hole mechanics holds, with entropy given by $S = 2π∫_Σ Q̃$, in any theory of gravity derived from a diffeomorphism-invariant Lagrangian — including higher-derivative theories — provided the theory admits stationary black holes with bifurcate Killing horizons and well-defined canonical mass and angular momentum at infinity.
  • Black hole entropy is a local geometrical quantity on the horizon rather than an artifact of a particular asymptotic or Hamiltonian construction, which supplies a natural candidate definition of entropy for a dynamical black hole: integrate $Q̃$ over an arbitrary cross-section of the horizon.
  • For a black hole that starts stationary, evolves, and settles to a stationary final state, the net change in entropy equals the total flux of Noether current through the horizon, so the second law of black hole mechanics becomes the statement that this Noether flux is positive.
  • The Euclidean path-integral route to entropy and the Noether-charge route agree: writing the Euclidean action as $I = -(2π/κ)[∫_C ξ·L + ∫_∞ t·B]$ reproduces $S = 2π∫_Σ Q$ exactly.
  • In general relativity and in $(1+1)$-dimensional theories, where $Q$ depends only on $ξ^a$ and its first antisymmetrized derivative, the first law in the form of eq. (25) continues to hold even for non-stationary perturbations.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Because the entropy formula depends only on the Lagrangian and the horizon geometry, the same bookkeeping should reproduce known entropy expressions in specific higher-curvature theories once the Noether charge is evaluated for their Lagrangians — a check that would test the claim without needing new physics.
  • If the second law really reduces to positivity of a Noether flux, then a theory that violates a positive-energy property should be a candidate for violating the second law; that pairing is a concrete, searchable link between energy conditions and horizon thermodynamics.
  • The proposal to define entropy on an arbitrary horizon cross-section by the same algorithm is testable in principle against time-dependent numerical black hole evolutions, where the candidate entropy should be monotonic in any theory whose Noether flux is positive.
  • The derivation leans on the horizon being bifurcate with constant surface gravity, so a theory whose stationary black holes have non-bifurcate Killing horizons is the natural place to look for a counterexample to the universality of the formula.
Original abstract

We consider a general, classical theory of gravity in $n$ dimensions, arising from a diffeomorphism invariant Lagrangian. In any such theory, to each vector field, $\xi^a$, on spacetime one can associate a local symmetry and, hence, a Noether current $(n-1)$-form, ${\bf j}$, and (for solutions to the field equations) a Noether charge $(n-2)$-form, ${\bf Q}$. Assuming only that the theory admits stationary black hole solutions with a bifurcate Killing horizon, and that the canonical mass and angular momentum of solutions are well defined at infinity, we show that the first law of black hole mechanics always holds for perturbations to nearby stationary black hole solutions. The quantity playing the role of black hole entropy in this formula is simply $2 \pi$ times the integral over $\Sigma$ of the Noether charge $(n-2)$-form associated with the horizon Killing field, normalized so as to have unit surface gravity. Furthermore, we show that this black hole entropy always is given by a local geometrical expression on the horizon of the black hole. We thereby obtain a natural candidate for the entropy of a dynamical black hole in a general theory of gravity. Our results show that the validity of the ``second law" of black hole mechanics in dynamical evolution from an initially stationary black hole to a final stationary state is equivalent to the positivity of a total Noether flux, and thus may be intimately related to the positive energy properties of the theory. The relationship between the derivation of our formula for black hole entropy and the derivation via ``Euclidean methods" also is explained.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 6,565 input tokens, $0.0003 reported.

What the abstract itself states, as probabilities: claims first 0.05 · states limitation 0.67 · human participants 0.01 · controlled comparison 0.02 · randomized assignment 0.01 · states quantitative result 0.64 · compares with prior results 0.92 · mentions code or data availability 0.01.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

We consider a general, classical theory of gravity in $n$ dimensions, arising from a diffeomorphism invariant Lagrangian. In any such theory, to each vector field, $\xi^a$, on spacetime one can associate a local symmetry and, hence, a Noether current $(n-1)$-form, ${\bf j}$, and (for solutions to the field equations) a Noether charge $(n-2)$-form, ${\bf Q}$. Assuming only that the theory admits stationary black hole solutions with a bifurcate Killing horizon, and that the canonical mass and angular momentum of solutions are well defined at infinity, we show that the first law of black hole mechanics always holds for perturbations to nearby stationary black hole solutions. The quantity playing the role of black hole entropy in this formula is simply $2 \pi$ times the integral over $\Sigma$ of the Noether charge $(n-2)$-form associated with the horizon Killing field, normalized so as to have unit surface gravity. Furthermore, we show that this black hole entropy always is given by a local geometrical expression on the horizon of the black hole. We thereby obtain a natural candidate for the entropy of a dynamical black hole in a general theory of gravity. Our results show that the validity of the ``second law" of black hole mechanics in dynamical evolution from an initially stationary black hole to a final stationary state is equivalent to the positivity of a total Noether flux, and thus may be intimately related to the positive energy properties of the theory. The relationship between the derivation of our formula for black hole entropy and the derivation via ``Euclidean methods" also is explained.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 We consider a general, classical theory of gravity in $n$ dimensions, arising from a diffeomorphism invariant Lagrangian. background 0.85 0.27 0.02 asserted 0.66
s1 In any such theory, to each vector field, $\xi^a$, on spacetime one can associate a local symmetry and, hence, a Noether current $(n-1)$-form, ${\bf j}$, and (for solutions to the field equations) a Noether charge $(n-2)$-form, ${\bf Q}$. background 0.98 0.23 0.04 asserted 0.98
s2 Assuming only that the theory admits stationary black hole solutions with a bifurcate Killing horizon, and that the canonical mass and angular momentum of solutions are well defined at infinity, we show that the first law of black hole mechanics always holds for perturbations to nearby stationary black hole solutions. result 0.91 0.93 0.16 asserted 1.00
s3 The quantity playing the role of black hole entropy in this formula is simply $2 \pi$ times the integral over $\Sigma$ of the Noether charge $(n-2)$-form associated with the horizon Killing field, normalized so as to have unit surface gravity. result 0.99 0.92 0.79 asserted 1.00
s4 Furthermore, we show that this black hole entropy always is given by a local geometrical expression on the horizon of the black hole. result 1.00 0.94 0.18 asserted 1.00
s5 We thereby obtain a natural candidate for the entropy of a dynamical black hole in a general theory of gravity. interpretation 0.36 0.90 0.13 hedged possibility 0.50
s6 Our results show that the validity of the ``second law" of black hole mechanics in dynamical evolution from an initially stationary black hole to a final stationary state is equivalent to the positivity of a total Noether flux, and thus may be intimately related to the positive energy properties of the theory. interpretation 0.91 0.92 0.12 hedged possibility 0.55
s7 The relationship between the derivation of our formula for black hole entropy and the derivation via ``Euclidean methods" also is explained. interpretation 0.55 0.66 0.04 asserted 0.62

What kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 1.00.

What was examined, and what was not

sources read
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not examined
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  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References
How later papers used it

1,003 later papers cite it

Evidence: substantial. 1,002 of the citing papers share no author with it, and 422 of those carry a confident label from Pith's citation classifier.

Pith read 37 of the 1,790 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

Uses its method or tool 13 of 369 papers with this label
How the literature uses it

888 arXiv papers cite this in 1,607 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 946 sentences without a confident label

Contests it 5 papers
  • Despite its elegance and very wide success, it has been observed that Wald's entropy formula [ref] and/or the first law does not work for a family of solutions to certain Horndeski theories, e.g. see [ref] .

    On Black Hole Temperature in Horndeski Gravity · Introduction · contest 0.83

  • In [ref] Wald proposes an algorithm for $X_dyn^cd$ that gives simply [equation] Although this prescription satisfies the conditions (i) and (ii) (because, as we argued above, for a stationary black hole with bifurcate surface $B$, $(X^cd)_a_3⋯a_n=-E_R^abcdϵ_aba_3⋯a_n$ on $B$, and the entropy is independent of the cross-section), it fails to satisfy (iii), in fact the addition of $dμ$ to $L$ with a suitable dependence on the Riemann tensor can change $E_R^abcd$ in such a way that the change in $S_dyn$ is nonvanishing for non-stationary black holes.

    The First Law of Black Hole Mechanics · Entropy of a dynamical black hole and the second law of black hole mechanics for diffeomorphism invariant theories · contest 0.84

  • We observe that the standard Wald formulation [ref] fails to address the issue of entropy for extremal black holes.

    Entropy of Black Holes in N=2 Supergravity · Discussions · contest 0.88

Builds on it 14 papers
Uses its method or tool 321 papers
  • In Section [ref] we use the covariant phase space formalism [ref] to construct the conserved charges for the solution.

    Thermodynamics of accelerating AdS$_4$ black holes from the covariant phase space · Introduction · 1.00

  • The coefficient of shear viscosity can be very easily computed using the prescription given in [ref] and the result is same as in [ref] η=r^3_02κ^2(1-24 c_1 q^2r^6_0) and the entropy density in terms of $q/r^3_0$, can be calculated using the Wald's entropy formula [ref] s=4πr^3_02κ^2[1+8 c_1-q^2r^6_0(28 c_1+24 c_2)]

    Weak Gravity Conjecture, Central Charges and $\eta/s$ · shear viscosity and $η/s$ · 0.91

  • For a general variation of the fields \(Ψ={g_μν,A_μ,ϕ}\), the Lagrangian four-form satisfies [ref] δL = E_ΨδΨ+ dΘ(Ψ,δΨ) , where $E_ψ=0$ gives the equations of motion.

    First Law of Black Hole Interior Dynamics · Iyer-Wald derivation of the interior first law · 1.00

Uses its data or values 1 paper
  • The conserved charge $Ẽ$ for a time-like killing vector, the area $Ã_H$, and the Hawking temperature $T̃_H$ of the black string are calculated as $Ẽ=2√(M/ℓ)$, $Ã_H=∫dx̂=2π/√(ℓM),$ and $T̃_H=1/(2πℓ)$ [ref] , and the Wald entropy is given as $S̃_W=4π√(ℓM)$ [ref] .

    Validity of black hole complementarity in the BTZ black hole · No-cloning theorem in the black string and the BTZ black hole · 0.89

Compares with it 31 papers
Cites it as support 6 papers
Cites it as background 160 papers

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