REVIEW 2 major objections 5 minor 51 references
Given the covariant-phase-space dynamical entropy, the Wall entropy can be reconstructed locally by matching powers of the horizon affine parameter, making the two linearized definitions of black hole entropy equivalent up to standard ambig
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-01 02:24 UTC pith:QFD75TNT
load-bearing objection Useful technical comparison with a real new algorithm; the main gap is a terse 'always possible' step that needs a lemma, but the stress-test objection mostly does not land. the 2 major comments →
A comparison of two constructions for dynamical corrections to Wald entropy
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
The paper's claim is that the Wall entropy and the dynamical entropy are not independent definitions. Given a dynamical entropy S_dyn satisfying ∂_v S_dyn|_{v=0}=0, a local expression for the Wall entropy S_Wall can always be constructed for arbitrary linearized perturbations around any stationary geometry, without performing a v-integration. The S_Wall obtained this way differs from the original boost-symmetry construction only by terms of higher-than-linear order in the amplitude, which are negligible in the linearized framework. The paper also shows that the boundary condition defining S_dyn is a special case of the Noether-charge/presymplectic-potential relation underlying the Wall const
What carries the argument
The central object is the differential identity S_dyn = (1 - v d_v) S_Wall, linking the two entropies on a horizon cross-section. The algorithm expands S_dyn in powers of v, with v the affine parameter along the horizon's null generators, and matches coefficients of v^m to determine the terms of S_Wall recursively; each step requires rewriting a term with m+1 unpaired v-derivatives as a single total v-derivative. The recursion terminates because a 2N+2 derivative theory can only generate powers up to v^N. The boost-weight decomposition of the vv component of the equations of motion supplies the structural reason the matching works: both entropies are rearrangements of the same component of t
Load-bearing premise
That every term in the v-expansion of the dynamical entropy with k+1 unpaired v-derivatives can be rewritten as a single total v-derivative, so the recursion for the Wall entropy closes locally without any integration over v; this is asserted in general and verified only for the Riemann-squared theory.
What would settle it
Compute S_dyn for a six-derivative (Riemann-cubed) theory of gravity, expand it in powers of v, and check whether the coefficient of v^2 is the v-derivative of a local function. If it is not, the paper's algorithm cannot produce a local Wall entropy, and the claimed equivalence of the two constructions fails for that theory. A second check is to test the identity δΘ^r = (1+v∂_v)δB^r beyond the three four-derivative examples; a counterexample would break the recursion at the v^1 step.
If this is right
- Second-law statements proved for the dynamical entropy apply equally to the Wall entropy at linearized order, and vice versa, because the two differ only by the (1 - v d_v) map.
- Wall entropy can be obtained from covariant phase-space data alone, without invoking the boost-symmetry construction directly.
- For a 2N+2 derivative Lagrangian, the algorithm gives a finite recursion: N matching steps fix all local terms of S_Wall.
- The apparent tension between the physical-process first law and the zero-boost terms in the equations of motion is resolved at linear order, because those terms can be written as a double v-derivative.
- The condition ∂_v S_dyn|_{v=0}=0 is identified as a special case of the relation that guarantees the existence of S_Wall, tying the two formalisms together at the level of their foundational identities.
Where Pith is reading between the lines
- Inference: The recursion's critical step — that every v^{N+1} coefficient is a total v-derivative — is asserted rather than proved; if it fails for a six-derivative or mixed-derivative theory, a local Wall entropy may not exist and an integral expression would be unavoidable.
- Inference: A direct test is to compute S_dyn for a curvature-cubed Lagrangian and check whether the highest-v coefficient is a total v-derivative; the Riemann-squared example alone cannot distinguish a general mechanism from a coincidence.
- Inference: Since the relation holds only at linear order, a second-order calculation would expose exactly how the two prescriptions for fixing the Noether-charge ambiguities diverge; the toy model in the paper suggests the ambiguity is parametrized by constants that vanish on stationary backgrounds.
- Inference: The identity δΘ^r = (1+v∂_v)δB^r is verified only for three four-derivative theories; the algorithm likely extends no further than the class of theories in which that identity is a structural consequence of the Lagrangian.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper compares two recent constructions of black hole entropy for non-stationary perturbations in higher-derivative gravity: the Wall entropy S_Wall, based on a boost-weight rearrangement of the near-horizon equations of motion, and the dynamical entropy S_dyn, based on a covariant phase-space/Noether-charge analysis linearized about a stationary background. The paper's central claims are (i) that the two entropies satisfy the differential relation S_dyn = (1 - v∂_v) S_Wall up to O(epsilon^2), (ii) that the condition ∂_v S_dyn|_{v=0}=0 follows from the same structural relation used in the Wall construction, and (iii) that, given S_dyn, one can algorithmically reconstruct a local expression for S_Wall for arbitrary linearized perturbations around any stationary geometry. The algorithm is presented in Section 3.1 as a recursive solution of the power-in-v expansion of the differential relation, and the main consistency check is the Riemann-squared theory in Section 4, where the reconstructed S_Wall matches the known entropy current. The paper also includes appendices verifying a key identity for three four-derivative theories and a toy model illustrating the expected failure of the relation beyond linear order.
Significance. If the generic reconstruction claim were fully established, the paper would provide a useful bridge between two active approaches to dynamical black hole entropy: results proven for S_dyn could be transferred to S_Wall, and the relation between the two would clarify the role of JKM ambiguities and of the different approximation schemes. The paper has real strengths: the Riemann-squared example in Section 4 is explicit and consistent; Appendix B gives concrete, if limited, checks of the identity (3.22); and the central differential relation (1.5) is grounded in the external literature ([2,46]) rather than being an assumption introduced for this work. The main weakness is that the generic local-reconstruction claim rests on an unproved total-derivative assertion, and the identity needed for compatibility is only verified in low-derivative examples. These are load-bearing gaps for the advertised scope of the algorithm.
major comments (2)
- [§3.1, text after Eq. (3.10)] The assertion that the v^{N+1} coefficient can always be rewritten as a total ∂_v derivative is not justified. In the general decomposition (2.6), a term with N+1 unpaired ∂_v derivatives need not have all derivatives acting on a single factor; for example, a product A (∂_v^2 B)(∂_v C) with v-independent A has the correct total derivative count but is not generally a total derivative. This is precisely the situation that can occur in higher-derivative theories for N ≥ 2. The same problem recurs at each lower step of the recursion in (3.9), where a known combination must be a total derivative for S[n-1]T(n-1) to be obtained locally. The N=1 Riemann-squared check in §4.3, Eq. (4.24), uses a single factor ∂_v^2 h_ij and therefore does not test the generic case. The paper needs a lemma characterizing the structure of the coefficients tilde S[n] tilde T(n), or an explicit restriction of the a
- [§3.2, Eq. (3.22) and Appendix B] The identity δΘ^r(g, L_ξ g) = (1 + v∂_v) δB^r(g, ξ) is essential for the compatibility of the consistency condition (3.8) with the Wall construction, via Eq. (3.21). It is verified only for the three four-derivative theories in Appendix B: Riemann-squared, Ricci-tensor-squared, and Ricci-scalar-squared. No proof is supplied for a generic diffeomorphism-invariant higher-derivative Lagrangian, nor for theories with non-minimally coupled matter. If (3.22) fails in a higher-derivative theory, then the consistency condition derived from S_dyn need not match the corresponding consequence of the Wall relation, and the resulting S_Wall would not be the entropy of [1] even at linear order. This gap is load-bearing because it underpins the 'no further obstruction' claim at the end of §3.1.
minor comments (5)
- [Section 1, bullet list] Typo: 'for for any small-amplitude dynamical perturbation' should read 'for any small-amplitude dynamical perturbation'.
- [Section 3.1, Eqs. (3.5)–(3.10)] The notation S[n]T(n) with square brackets is defined in Appendix A, but the first use in the main text would benefit from a brief reminder that [n] denotes the minimum number of derivatives with respect to the product rv on the equilibrium background.
- [Section 4, Eq. (4.10)] The identity v∂_v \bar K^{ij}|_{r=0} = \bar K^{ij} + O(δg) is used in the last step. This is a property of the stationary background and should be flagged as such; otherwise the O(δg) term could be mistaken for a generic perturbative correction.
- [Appendix B, Eq. (B.14)] The Ricci-tensor-squared computation is terse; several intermediate simplifications leading from (B.12) to (B.14) are omitted, making the check harder to follow than the corresponding Riemann-squared case.
- [Appendix E] The toy example illustrates the non-linear mismatch, but it also shows explicitly that S_dyn can contain products of ∂_v derivatives acting on different factors (see Eq. (E.11), e.g. v(∂_r h)(∂_v^2 h)). This is a helpful illustration of the representability issue in §3.1, and the paper could use it to sharpen the discussion of when the total-derivative step is guaranteed.
Circularity Check
No significant circularity: the central differential relation is an external input from [2], independently checked in [46]; the paper solves it rather than assuming its own conclusion.
full rationale
The central claim is that, given S_dyn, one can reconstruct S_Wall using relation (1.5). This relation is not manufactured inside the paper: the introduction states, 'In fact it has also been shown in [2] that S_dyn is not exactly equal to S_Wall but rather they satisfy a differential relation S_dyn = (1-v d_v) S_Wall.' The algorithm in Section 3.1 is a coefficient-wise solution of that external relation: (3.6)-(3.10) equate powers of v in (1-v d_v)S_Wall = S_dyn and recursively solve for the S_[m]T_(m) pieces. This is a reconstruction, not a fit passed off as prediction, and the same relation was independently verified in [46] and re-derived in the Riemann-squared check of Section 4. The uses of the authors' prior work, especially identity (2.11) and relation (3.11) from [1], are uses of the already-existing Wall-entropy construction being compared, not self-referential definitions of the conclusion. Self-citation is present but load-bearing only in the sense that [1] is the object of comparison; it does not by itself force the result. The actual weaknesses are mathematical gaps, not circularity: after eq. (3.10) the paper asserts 'But this is always possible since according to our notation, ~T_(N+1) has (N+1) unpaired d_v derivatives and ~S_[N+1] is v-independent', a representability lemma that is not proved and is not obvious for products of d_v-derivative factors; and eq. (3.22) is verified only for three four-derivative theories in Appendix B. These are omitted-proof/scope issues that affect the rigor of the generic claim, but they do not reduce the derivation to its inputs by construction. No specific circular step can be exhibited from the paper's own equations, so the appropriate finding is no significant circularity.
Axiom & Free-Parameter Ledger
free parameters (2)
- Integration constant C in eq. (3.1) =
0 (fixed by matching to Wald entropy in the stationary limit)
- JKM ambiguity prescription =
prescription-dependent; undetermined by stationary data
axioms (6)
- standard math Existence of Gaussian null coordinates (2.1) around the horizon with v as affine parameter
- domain assumption Exact boost-weight decomposition of E^v_v, eqs. (2.5)-(2.6), and the structure result (2.11) are taken from [1]
- standard math Identity (2.9)/(2.13): the diffeomorphism-invariance Noether identity
- domain assumption Linearized split g = gbar + epsilon delta g, with all O(epsilon^2) terms dropped
- domain assumption Regularity at the bifurcation surface: delta E^v_v|_{v=0} finite, from which (1.6) follows
- domain assumption Identity (3.22): delta Theta^r(g, L_xi g) = (1 + v d_v) delta B^r(g, xi)
read the original abstract
In this work, we analyze the differences and similarities between two recent constructions, which are distinct in their methodologies for extending the Wald entropy of stationary black holes to non-stationary situations in general higher-derivative gravity. One of them, denoted by $S_\text{Wall}$, is constructed by exploiting the boost symmetry of the near-horizon geometry, whereas the other, denoted by $S_\text{dyn}$, is obtained from a covariant phase-space analysis based on the Wald-Iyer Noether charge formalism. While $S_\text{dyn}$ is, by construction, defined only for linearized fluctuations around a stationary black hole solution, $S_\text{Wall}$ does not require such a linearization for its construction. Although the linearization is necessary to interpret $S_\text{Wall}$ as a well-defined notion of entropy, the construction itself naturally contains terms that are higher order in the dynamical fluctuations. By comparing the technical structures underlying the two constructions, we clarify the fundamental differences between the methods on which they are based. We demonstrate that while the construction of $S_\text{dyn}$ given the $S_\text{Wall}$ is straightforward, the converse is more subtle. We develop an algorithm to obtain a local expression for $S_\text{Wall}$ from a known expression for $S_\text{dyn}$ in a generic diffeomorphism-invariant theory of gravity, provided certain technical conditions are satisfied. We justify our analytical findings with explicit demonstrations in a particular case: the Riemann-squared example of the higher-derivative theory of gravity.
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discussion (0)
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