REVIEW 3 major objections 5 minor 44 references
Quasi-local energy and ADM mass in pure Lovelock gravity
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read In pure Lovelock gravity (m ≥ 2) the background-subtracted quasi-local energy vanishes at spatial infinity, so only its variation is well defined; the variation's large-surface limit equals the ADM mass variation.
desk verdict Useful perturbative ADM mass formula in pure Lovelock gravity, but the paper's claim that the absolute quasi-local energy vanishes at infinity is not proven and is likely wrong. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The workhorse is the generalized Gibbons-Hawking surface term (the boundary counterterm that makes the variational principle well-posed) for the codimension-two surface, together with the projected equation-of-motion tensor $\hat E^i{}_{j(m)}$ built from generalized Kronecker-delta contractions with surface Riemann tensors. The key identity, obtained by varying the surface term with the induced surface metric held fixed, is $2\bar P^{ab}{}_{cd(m)} n_a D_c \delta\gamma^d{}_b = -4m\,\hat E^i{}_{j(m-1)}\,\delta\hat K^j{}_i$; this converts the Hamiltonian boundary term into an integral over the variation of the extrinsic curvature $\delta\hat K^j{}_i$. What keeps the perturbation finite while the absolute background-subtracted energy collapses to zero is that $\hat E^i{}_{j(m-1)}$ in the limiting integral is not evaluated on the flat background. The asymptotic fall-off exponent $\beta = (D - 2m - 1)/m$ is engineered so that the decay of $\bar P^{ab}{}_{cd(m)}$ cancels the volume factor $r^{D-2}$ and the derivative $D_c\delta\gamma \sim r^{-\beta-1}$, making the boundary integral finite.
What would settle it
Take a non-spherically-symmetric asymptotically flat static solution of pure Lovelock gravity of order $m \ge 2$ in $D \ge 2m+3$ dimensions and compute the large-surface limit of the background-subtracted Brown-York energy (3.15); the paper's central claim predicts this limit is exactly zero for any solution satisfying the fall-off (2.19), so a nonzero result would falsify it. Independently, compute $\delta M^{(m)}_{\mathrm{ADM}}$ from the Hamiltonian formula (2.16) and from the surface formula (3.27) for a metric whose non-integrable term is nonzero; the two must agree for the central identity to hold.
Extended reading notes
Core claim
Working in dimensions $D > 2m+1$, the paper generalizes the Hamiltonian-surface derivation of the ADM mass to pure Lovelock gravity and finds that for asymptotically flat spacetimes the ADM mass is non-integrable: its variation $\delta M^{(m)}_{\mathrm{ADM}}$ is finite under the pure-Lovelock fall-off $\tilde\gamma_{ab} = O(r^{-\beta})$ with $\beta = (D - (2m+1))/m$, but it cannot be integrated to a function of the metric parameters because $\bar P^{ab}{}_{cd(m)}$ itself varies. The background-subtracted Brown-York energy is then shown to vanish in the large-surface limit for every $m \ge 2$, so the paper turns to the perturbative quantity $\delta M^{(m)}_B = \frac{m}{4\pi}\int_B \sqrt{\sigma}\,\hat E^i{}_{j(m-1)}\,\delta\hat K^j{}_i$ and proves its large-surface limit equals $\delta M^{(m)}_{\mathrm{ADM}}$. For spherical symmetry this integrates to $M^{(m)}_{\mathrm{ADM}} = \frac{1}{16\pi}\Omega_{D-2}\,a(m)\lim_{r\to\infty} r^{m\beta} F(r)^m$, which for pure Lovelock black holes gives $M \propto \alpha^m$, matching earlier integral-charge calculations. In asymptotically AdS spacetimes the obstruction disappears: the mass is integrable and proportional to the Einstein mass for all $m$, and the background-subtracted quasi-local energy has the correct ADM limit.
Load-bearing premise
The background-subtraction prescription assumes the boundary surface can be embedded in the reference flat (or AdS) spacetime with the same lapse and induced metric as in the physical spacetime, and the paper itself notes that this embedding is not possible in general.
Editorial extensions
If this is right
- For $m \ge 2$ in asymptotically flat spacetimes, the background-subtracted quasi-local energy has a zero large-surface limit, so only the perturbative variation $\delta M^{(m)}_B$ is well defined as a charge.
- The limit identity $\delta M^{(m)}_{\mathrm{ADM}} = \lim_{B\to\infty} \frac{m}{4\pi}\int_B \sqrt{\sigma}\,\hat E^i{}_{j(m-1)}\,\delta\hat K^j{}_i$ provides a practical boundary formula for the ADM mass, demonstrated on spherically symmetric static black holes and matching earlier integral-charge results.
- For static spherically symmetric metrics the perturbative mass integrates to $M^{(m)}_{\mathrm{ADM}} = \frac{1}{16\pi}\Omega_{D-2}\,a(m)\lim_{r\to\infty} r^{m\beta}F(r)^m$, and for the pure Lovelock black hole $F(r)=\alpha r^{-\beta}$ this yields $M^{(m)}_{\mathrm{ADM}}\propto \alpha^m$.
- In asymptotically AdS spacetimes the ADM mass is integrable and proportional to the Einstein value for every $m$, and the background-subtracted quasi-local energy has the correct ADM limit.
- The alternative quasi-local energy built from vacuum-subtracted extrinsic curvatures agrees with the ADM mass in AdS, but in flat space its large-surface limit equals $m$ times the ADM mass for spherically symmetric metrics and differs from the ADM variation for generic metrics.
Reading between the lines
- A testable consequence of the vanishing limit is that black hole thermodynamics in asymptotically flat pure Lovelock gravity must be formulated through variational quantities: the mass appearing in the first law is an integration parameter, not an absolute charge.
- The surface formula has the structure of a symplectic charge pairing a background equation-of-motion tensor with the variation of extrinsic curvature; the same pairing should yield angular momentum and other charges when the extrinsic-curvature variation is replaced by a rotational perturbation, and checking a first law would test the construction beyond the mass.
- Counterterm renormalization, already developed for AdS Lovelock, could be applied to the same spherically symmetric black holes in flat space; comparing the counterterm energy with the integrated mass formula would show whether the failure of absolute charges is specific to background subtraction or intrinsic to pure Lovelock.
- The obstruction to absolute charges is similar in spirit to the degeneracy seen in special Lovelock vacua, suggesting that only perturbations may be well defined quite generally in higher-curvature gravity; testing conformal-mass type definitions on these flat solutions would separate the two mechanisms.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper studies how the standard ADM mass and Brown-York quasi-local energy generalize to pure Lovelock gravity. It derives a non-integrable perturbative ADM mass formula (2.16), proposes a background-subtracted Brown-York type quasi-local energy (3.15), and argues that its large-surface limit vanishes for asymptotically flat pure Lovelock gravity with m ≥ 2. The main positive result is the perturbative formula (3.26)-(3.27), which expresses the variation of the quasi-local energy as an integral over the codimension-two surface and whose large-surface limit coincides with the variation of the ADM mass. The author applies this formula to static spherically symmetric solutions, recovering known black hole masses, and also treats asymptotically AdS spacetimes, where the mass becomes integrable and proportional to the Einstein value for all m.
Significance. If the central claims hold, the paper provides a practical new formula for computing ADM masses in pure Lovelock gravity and sharpens the statement that only perturbative energy variations are well defined for asymptotically flat m ≥ 2. The paper is explicit and cross-checks its formulas against the Einstein gravity limit m = 1, the known spherically symmetric black hole masses, and the Chakraborty-Dadhich quasi-local energy. These checks are genuine strengths. However, the paper's main conceptual conclusion about the vanishing of the non-perturbative quasi-local energy rests on an asymptotic expansion whose higher-order terms are not controlled; this weakens the stated significance until the gap is closed. The perturbative ADM formula (3.27) may survive independently of that conclusion, and the spherical application is an useful demonstration.
major comments (3)
- [Section 3.3, Eqs. (3.19)-(3.22)] The vanishing large-surface limit of the background-subtracted quasi-local energy is not established by the given argument. Equation (3.21) expands \hat B_(m) only to first order in the metric perturbation and concludes in (3.22) that the integral vanishes because \bar P_(m)|_(0) is zero on flat space. A first-order expansion is insufficient here: after multiplication by \sqrt{\sigma} ~ r^{D-2}, the next-order contribution can be finite or divergent under the pure Lovelock falloff (2.19). For the static spherically symmetric metric (4.1) with m = 2, the quadratic term in \hat B - \hat B_0 is of order F^2/r^3, so the integral scales as r^{D-5}F^2; with F ~ r^{-\beta}, \beta = (D-5)/2, this is a non-zero constant as r \to \infty, and for m > 2 the analogous contribution grows. Thus Eq. (3.22) does not follow from the stated expansion, and the paper's central claim that the non-perturbative quasi-local energy vanishes for m ≥ 2 is not proven. The perturbative formula (3.27) may still be correct, but the motivation for restricting to variations needs either a second-order computation showing the offending terms cancel, or a non-perturbative proof of the limit.
- [Section 3.2, Eq. (3.10)] The Hamiltonian decomposition of the total action with a timelike boundary is introduced as a proposal rather than derived. The quasi-local energy (3.15) and hence the perturbative formula (3.23) are built on this decomposition, so the status of the assumption is load-bearing. If (3.10) is meant as a definition of the physical Hamiltonian, the paper should state this explicitly and verify that the resulting charge is independent of the choices made; if it is meant to be derived from the action variation (3.3), the derivation should be supplied. As it stands, the central ADM-mass formula inherits the uncertainty of this proposed decomposition.
- [Section 3.3, after Eq. (3.14)] The background subtraction prescription assumes that the codimension-two surface B can be embedded in the flat reference spacetime with the same lapse and induced metric as in the physical spacetime. The paper itself notes in the Summary that this embedding is not possible in general. Since the quasi-local energy (3.15) is the central object of the paper, the precise class of surfaces for which the definition is valid should be stated. In particular, for surfaces that are not approximately round and large, the subtraction is not well defined by this prescription, and the error introduced by an approximate embedding should be controlled or at least discussed.
minor comments (5)
- [Section 2.2, text after Eq. (2.16)] The sentence 'Obtaining a non-perturbative expression for M_ADM_(m) in pure Lovelock gravity of order m ≤ 2 cannot be done in general' appears to contain a typo: the non-integrable case is m ≥ 2, not m ≤ 2, because \bar P_(1) is metric-independent. Please correct this.
- [Section 1.2, Eq. (1.4)] The definition of the projection ~\delta reads '~δa b = ~δa b − nanb', which is circular as typeset. It should presumably be ~\delta^a_b = \delta^a_b - n^a n_b. Please fix the notation.
- [Section 3.2, Eq. (3.10)] The term K_(m) appearing in the proposed Hamiltonian decomposition is not defined. Since it is said to contain terms proportional to the extrinsic curvature of the Cauchy slice, a brief definition or reference would help the reader.
- [Section 2.3, Eq. (2.19)] The exponent is written as β = D − (2m + 1)/m, which is ambiguous. The intended expression is β = (D − 2m − 1)/m, as used in Eq. (4.4). Please add parentheses.
- [Abstract and Introduction] The phrase that the perturbative quasi-local energy 'correctly approaches' the ADM mass is stronger than what is proven: by Eqs. (2.16) and (3.23), both sides are the same boundary integrand by construction, so the equality is a consistency relation rather than an independent check. The wording should be softened accordingly.
Circularity Check
No significant circularity: the perturbative quasi-local energy/ADM limit is a consistency relation, and the spherical-symmetric masses are verified against independent literature.
full rationale
The paper's main derivation chain is self-contained. Section 2 obtains the ADM mass variation (2.16) from the Hamiltonian boundary term of the pure Lovelock action; Section 3 obtains the Brown-York-like quasi-local energy (3.15) from the generalized Gibbons-Hawking term. The equality between the large-B limit of the perturbative quasi-local energy and the ADM mass variation (3.23 vs 2.16) uses the same boundary integrand by construction, so it is a consistency relation rather than an independently fitted prediction; it is not a circular dependency because neither quantity is defined in terms of the other, and the paper does not use this relation as an external benchmark. The genuinely new formula (3.27) is reached through the additional identity (3.24), which rewrites the boundary integrand in terms of the projected equation-of-motion tensor and the extrinsic-curvature variation; that step is nontrivial and is subsequently tested on static spherically symmetric metrics, where the resulting mass (4.5) and the black-hole value (4.7) agree with earlier independent calculations [13,17-20]. No self-citation is load-bearing; the surface-term and quasi-local-energy references [25,28] are prior independent work. The explicitly disclosed embedding assumption in the background-subtraction regularization is a limitation, not a circular step, and the possible higher-order terms in the large-surface expansion of \hat{B} are a correctness concern outside the circularity pass. Overall, no claim reduces to its own input by definition or by fit.
Assumptions & free parameters
assumptions (5)
- standard math The pure Lovelock action and its boundary variation (2.4)-(2.7) are the starting point, with c(m)/GN set to 1.
- ad hoc to paper The Hamiltonian decomposition of the total action with timelike boundary takes the proposed form (3.10) with boundary term B-hat(m)+K(m).
- domain assumption Spacetimes are static (shift vector zero) and asymptotically flat with fall-off gamma-tilde_{ab}=O(r^{-β}), β=(D-2m-1)/m, or asymptotically AdS.
- domain assumption For background subtraction, the surface B can be embedded in the reference spacetime with matching lapse and induced metric.
- standard math Generalized Kronecker delta identities in Appendix A are used for tensor contractions.
Cite this review
Pith. "Pith review of Quasi-local energy and ADM mass in pure Lovelock gravity." pith.science (2026). https://pith.science/paper/Q6F42PH2
@misc{pith2026190805522,
author = {Pith},
title = {Pith review of: Quasi-local energy and ADM mass in pure Lovelock gravity},
year = {2026},
howpublished = {\url{https://pith.science/paper/Q6F42PH2}},
note = {Machine review of arXiv:1908.05522}
}
read the original abstract
We study how the standard definitions of ADM mass and Brown-York quasi-local energy generalize to pure Lovelock gravity. The quasi-local energy is renormalized using the background subtraction prescription and we consider its limit for large surfaces. We find that the large surface limit vanishes for asymptotically flat fall-off conditions except in Einstein gravity. This problem is avoided by focusing on the variation of the quasi-local energy which correctly approaches the variation of the ADM mass for large surfaces. As a result, we obtain a new simple formula for the ADM mass in pure Lovelock gravity. We apply the formula to spherically symmetric geometries verifying previous calculations in the literature. We also revisit asymptotically AdS geometries.
Reference graph
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