REVIEW
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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Referee report
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
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
- 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.
The abstract, sentence by sentence
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
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| 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 was examined, and what was not
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References
How later papers used it
1,003 later papers cite it
13 later papers by other authors are listed below: 13 used its method or tool.
Uses its method or tool 13 of 369 papers with this label
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Followed the procedure proposed in this paper and another work in its derivation.
Gravity as a constrained BF theory: Noether charges and Immirzi parameter (2011) · 1.00 · paraphrased: the licence does not allow quoting
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Used effective field theoretical methods to calculate quantum gravitational corrections to the entropy of a Schwarzschild black hole using this paper's Wald entropy formula.
Quantum Gravitational Corrections to the Entropy of a Schwarzschild Black Hole (2021) · 1.00 · paraphrased: the licence does not allow quoting
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Instead, we need to use the formula obtained by Wald's prescription [ref] , namely [equation] where $h$ is determinant of the induced metric on the horizon, $ϵ_ab$ is the binormal to the horizon with $ϵ_abϵ^ab=-2$ and $P_abcd$ is defined in ([ref]).
Taub-NUT Black Hole in Higher Derivative Gravity and Its Thermodynamics (2024) · Thermodynamics · 1.00
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The entropy of our model is calculated using the Wald formula [ref] as follows, [equation] where, [equation] Furthermore, [equation] [equation] and [equation] Calculations give the following relation for the Ricci scalar, [equation] where, at the black hole horizon takes the form [equation] because, $f(r_h)=0$.
Exponential Modification of AdS Black Hole and Thermodynamic Behavior (2024) · Exponential Modification of the AdS Black Hole · 1.00
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Constructed gravitational Noether charges following this paper for several supergravity theories.
Charges from Attractors (2007) · 1.00 · paraphrased: the licence does not allow quoting
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Used a dynamical definition of entropy as proposed in this paper.
The horizon-entropy increase law for causal and quasi-local horizons and conformal field redefinitions (2011) · 1.00 · paraphrased: the licence does not allow quoting
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Found the entropy from this paper's Wald formula.
Entropy Function for Non-extremal D1D5 and D2D6NS5-branes (2007) · 1.00 · paraphrased: the licence does not allow quoting
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Throughout this section we will employ Wald's Noether charge formalism [ref] .
Gravitational Thermodynamics of Causal Diamonds in (A)dS (2018) · Smarr formula for causal diamonds · 1.00
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Proceeded with this paper's proposal to calculate the entropy.
Information Geometry on the Space of Equilibrium States of Black Holes in Higher Derivative Theories (2018) · 1.00 · paraphrased: the licence does not allow quoting
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Calculated the entropy density using this paper's Wald entropy formula.
Weak Gravity Conjecture, Central Charges and $\eta/s$ (2010) · 0.91 · paraphrased: the licence does not allow quoting
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Computed the black hole entropy using this paper's Wald formula.
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Followed this paper by starting with the definition of the pre-symplectic (d-1)-form.
The first law and Wald entropy formula of heterotic stringy black holes at first order in alpha prime (2020) · 0.99 · paraphrased: the licence does not allow quoting
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At first order in $β$ we find [equation] Therefore, the Hawking temperature is given by [equation] According to the Wald formula [ref] , the entropy is [equation] where $dΣ=r^2sinθdθdϕ$, $ϵ_μν$ is the Levi-Civita tensor in two dimensions and $ℒ$ is the Lagrangian density of the theory.
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How the literature uses it
888 arXiv papers cite this in 1,607 sentences
Papers citing it · 946 sentences without a confident label
Contests it 5 papers
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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
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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
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These charges as shown in [ref] are integrable.
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The covariant phase space [ref] has yet to be applied to accelerating AlAdS black holes, although analysing their charges and associated thermodynamics using different techniques have been the study of a slew of recent work [ref] which we will follow closely (see also [ref] for related works).
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Discussion
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