REVIEW
A General Definition of "Conserved Quantities" in General Relativity and Other Theories of Gravity
General Relativity and Quantum Cosmology · Submitted 24 Nov 1999 · 25 references
Pith's verdict: The paper that turned conserved quantities at null infinity into a Hamiltonian prescription: the modified Hamiltonian equation (eq. 26) and the flux formula (eq. 32) are the parts that lasted, and the Dray–Streubel derivation is the payoff.
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Referee report
The paper
The paper claims
Wald and Zoupas propose a replacement for the Hamiltonian equation that still defines mass, momentum and angular momentum at null infinity, where radiation destroys the ordinary Hamiltonian and the quantities stop being conserved.
The reading
What carries the argument
The load-bearing object is the boundary symplectic potential $Θ$, defined on the boundary $B$ by $ω̄(φ,δ_1φ,δ_2φ) = δ_1Θ(φ,δ_2φ) - δ_2Θ(φ,δ_1φ)$ for the pullback of the symplectic current. It is the extra term that repairs the failed consistency check $∫_{∂Σ} ξ·ω = 0$ for the existence of a Hamiltonian. Three conditions pin it down: it must be locally constructed from the dynamical fields and the fixed background structure, it must be independent of arbitrary choices in that background structure (in the null-infinity case, conformally invariant), and it must vanish on stationary solutions. In vacuum general relativity at null infinity those conditions select $Θ = -N_{ab}τ^{ab}$ times the boundary volume element, which is what makes the flux equal to the Bondi news contracted with the perturbation. The Noether charge two-form $Q_{ab}[ξ] = -(1/16π)ε_{abcd}∇^cξ^d$ supplies the other half of the definition, and the Geroch–Winicour gauge condition $∇_aξ^a = 0$ is what makes $∫_{∂Σ}Q$ well defined as a representative-independent charge.
Extended reading notes
Core claim
The central claim is that a 'conserved quantity' can be defined even where no Hamiltonian exists, by weakening the Hamiltonian equation rather than abandoning it. The proposed definition is $δH_ξ = ∫_{∂Σ}[δQ - ξ·θ] + ∫_{∂Σ} ξ·Θ$, where $Q$ is the Noether charge two-form, $θ$ the presymplectic potential, and $Θ$ a symplectic potential on the boundary whose variation reproduces the pullback of the symplectic current there. Requiring $Θ$ to vanish on stationary solutions makes the associated flux vanish when there is no radiation and when $ξ^a$ is an exact symmetry, and gives the simple flux formula $F_ξ = Θ(φ, L_ξφ)$. For vacuum general relativity at null infinity the authors argue that $Θ = -N_{ab}τ^{ab}$ (times the boundary volume element, with $N_{ab}$ the Bondi news) is the unique admissible choice, so that the flux is exactly the Ashtekar–Streubel news-tensor flux. The resulting charges are then shown, case by case, to agree with the Dray–Streubel prescription: for a generator tangent to the cross-section the answer reduces to the Geroch–Winicour linkage expression, and for a supertranslation it reduces to Geroch's formula built from the news and the Weyl tensor.
Load-bearing premise
The claim of a unique definition rests on the assertion that no other admissible boundary term $W$ exists whose variation vanishes on stationary solutions but not in general, and the paper supports that by a scaling-dimension counting argument introduced with the hedge 'presumably' rather than by a complete enumeration.
Editorial extensions
If this is right
- Every infinitesimal BMS generator and every cross-section of null infinity gets a definite 'conserved quantity', even though these quantities are not conserved when radiation is present.
- The flux of each such quantity is fixed by a single local formula, $F_ξ = Θ(g_{ab}, L_ξ g_{ab})$, which vanishes for stationary spacetimes and for exact symmetries regardless of radiation.
- In vacuum general relativity the resulting charges coincide with the Dray–Streubel quantities, which supplies a Hamiltonian-formulation derivation of a definition previously obtained from twistor-inspired considerations.
- The construction is stated for any diffeomorphism-covariant theory derived from a Lagrangian, so the same recipe is expected to extend to matter couplings, higher-derivative gravity, two-dimensional dilaton models, and asymptotically anti-de Sitter settings.
- The method gives a general diagnostic for when a genuine Hamiltonian does exist: precisely when the pullback of the symplectic current to the boundary vanishes, the 'case I' that covers spatial infinity and anti-de Sitter.
Reading between the lines
- The same recipe, applied to a horizon rather than an asymptotic boundary, would define a flux through the horizon and might constrain how entropy of a non-stationary black hole is defined; the authors raise this only as a possibility, and they doubt a unique $Θ$ exists in that setting.
- Because the whole scheme turns on whether an admissible $Θ$ exists and is unique, the method's reach into modified gravity depends on a case-by-case scaling-dimension count like the one done here for the news tensor; a theory where two independent boundary tensors share the same scaling weight would be the natural place to test the uniqueness argument.
- The agreement with Dray–Streubel is derived by splitting a BMS generator into a cross-section-tangent part and a supertranslation; that split suggests the charge is naturally a sum of a 'Komar-like' piece and a 'news-and-Weyl' piece, which is a structural statement that could be checked independently of the derivation.
- If an admissible $W$ with $δW$ vanishing on stationary solutions but nonzero otherwise were found on null infinity, the definition would become a one-parameter family rather than a single prescription — a concrete target for anyone wanting to test the uniqueness claim.
Original abstract
In general relativity, the notion of mass and other conserved quantities at spatial infinity can be defined in a natural way via the Hamiltonian framework: Each conserved quantity is associated with an asymptotic symmetry and the value of the conserved quantity is defined to be the value of the Hamiltonian which generates the canonical transformation on phase space corresponding to this symmetry. However, such an approach cannot be employed to define `conserved quantities' in a situation where symplectic current can be radiated away (such as occurs at null infinity in general relativity) because there does not, in general, exist a Hamiltonian which generates the given asymptotic symmetry. (This fact is closely related to the fact that the desired `conserved quantities' are not, in general, conserved!) In this paper we give a prescription for defining `conserved quantities' by proposing a modification of the equation that must be satisfied by a Hamiltonian. Our prescription is a very general one, and is applicable to a very general class of asymptotic conditions in arbitrary diffeomorphism covariant theories of gravity derivable from a Lagrangian, although we have not investigated existence and uniqueness issues in the most general contexts. In the case of general relativity with the standard asymptotic conditions at null infinity, our prescription agrees with the one proposed by Dray and Streubel from entirely different considerations.
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
In general relativity, the notion of mass and other conserved quantities at spatial infinity can be defined in a natural way via the Hamiltonian framework: Each conserved quantity is associated with an asymptotic symmetry and the value of the conserved quantity is defined to be the value of the Hamiltonian which generates the canonical transformation on phase space corresponding to this symmetry. However, such an approach cannot be employed to define `conserved quantities' in a situation where symplectic current can be radiated away (such as occurs at null infinity in general relativity) because there does not, in general, exist a Hamiltonian which generates the given asymptotic symmetry. (This fact is closely related to the fact that the desired `conserved quantities' are not, in general, conserved!) In this paper we give a prescription for defining `conserved quantities' by proposing a modification of the equation that must be satisfied by a Hamiltonian. Our prescription is a very general one, and is applicable to a very general class of asymptotic conditions in arbitrary diffeomorphism covariant theories of gravity derivable from a Lagrangian, although we have not investigated existence and uniqueness issues in the most general contexts. In the case of general relativity with the standard asymptotic conditions at null infinity, our prescription agrees with the one proposed by Dray and Streubel from entirely different considerations.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | In general relativity, the notion of mass and other conserved quantities at spatial infinity can be defined in a natural way via the Hamiltonian framework: Each conserved quantity is associated with an asymptotic symmetry and the value of the conserved quantity is defined to be the value of the Hamiltonian which generates the canonical transformation on phase space corresponding to this symmetry. | background 1.00 | 0.09 | 0.05 | asserted 1.00 |
| s1 | However, such an approach cannot be employed to define `conserved quantities' in a situation where symplectic current can be radiated away (such as occurs at null infinity in general relativity) because there does not, in general, exist a Hamiltonian which generates the given asymptotic symmetry. | background 0.59 | 0.12 | 0.04 | asserted 0.99 |
| s2 | (This fact is closely related to the fact that the desired `conserved quantities' are not, in general, conserved!) In this paper we give a prescription for defining `conserved quantities' by proposing a modification of the equation that must be satisfied by a Hamiltonian. | contribution 0.94 | 0.78 | 0.51 | asserted 1.00 |
| s3 | Our prescription is a very general one, and is applicable to a very general class of asymptotic conditions in arbitrary diffeomorphism covariant theories of gravity derivable from a Lagrangian, although we have not investigated existence and uniqueness issues in the most general contexts. | limitation 0.51 | 0.73 | 0.17 | asserted 0.96 |
| s4 | In the case of general relativity with the standard asymptotic conditions at null infinity, our prescription agrees with the one proposed by Dray and Streubel from entirely different considerations. | result 0.96 | 0.67 | 0.12 | asserted 1.00 |
What was examined, and what was not
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References
How later papers used it
357 later papers cite it
12 later papers by other authors are listed below: 9 used its method or tool, 2 compared their own results with it and 1 cited it as support.
Uses its method or tool 9 of 113 papers with this label
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Calculated conserved charges and fluxes with the Wald-Zoupas formalism of this paper for BMS algebra.
Conserved charges in Chern-Simons modified theory and memory effects (2021) · 1.00 · paraphrased: the licence does not allow quoting
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Used the Wald-Zoupas prescription of this paper, along with the covariant phase space formalism, to compute boundary charges and their fluxes.
Symmetries, charges and conservation laws at causal diamonds in general relativity (2019) · 1.00 · paraphrased: the licence does not allow quoting
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Used the integral of the Noether charge 2-form given by Eq. (44) of this paper, the Komar formula, for the contribution to the charge.
Conserved charges of the extended Bondi-Metzner-Sachs algebra (2015) · 1.00 · paraphrased: the licence does not allow quoting
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Used the covariant phase space formalism of this paper to show that the charge is associated to divergent large gauge transformations.
Asymptotic Symmetries in $(d+2)$-Dimensional Gauge Theories (2019) · 1.00 · paraphrased: the licence does not allow quoting
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We list a set of criteria that these should satisfy to be physically reasonable and, using the prescription provided by Wald and Zoupas in [ref] , provide an explicit computation of the general expressions for these quantities.
The Wald-Zoupas prescription for asymptotic charges at null infinity in general relativity (2021) · Introduction · 0.99
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Used covariant phase space methods, including those of this paper, to analyze the Hamiltonian phase space near the background metric.
Schwarzschild/CFT from soft black hole hair? (2018) · 0.99 · paraphrased: the licence does not allow quoting
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Followed this paper to read off the (pre) symplectic potential current density and the symplectic form.
Lectures on the Infrared Structure of Gravity and Gauge Theory (2017) · 0.99 · paraphrased: the licence does not allow quoting
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Since the background radiative electromagnetic field vanishes, the electromagnetic flux has no first-order contribution, $n̂^an̂^bδT̂^EM_abℐ^+=0$, the contribution of Eq. [ref] on the $ℐ^+$ gives [equation] Following the scheme in [ref] , after canceling the boundary terms in the expression above, the modified canonical energy can be defined as [equation] Consequently, the change in $ℰ'[Σ(t)]$ between $Σ(t_1)$ and $Σ(t_2)$ is [equation] The first and third terms are the gravitational contributions associated with horizon shear and Bondi news, respectively.
The entropy of dynamical black holes deviating from electrovacuum in second order (2026) · Modified Canonical Energy · 0.83
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Identified the non-integrable piece as in this paper and treated it in the current algebra as in another work.
Comments on holographic current algebras and asymptotically flat four dimensional spacetimes at null infinity (2013) · 0.96 · paraphrased: the licence does not allow quoting
Compares with it 2 of 31 papers with this label
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Showed that the asymptotic limit of its own quasi-local BMS charges agrees with the BMS charges constructed by this paper at null infinity.
Quasi-local conserved charges in General Relativity (2019) · 0.99 · paraphrased: the licence does not allow quoting
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Checked that its own result matches the one given by this paper, here specialized to Bondi coordinates and for a shear associated with the u-foliation.
Center-less BMS charge algebra (2024) · 0.99 · paraphrased: the licence does not allow quoting
Cites it as support 1 of 5 papers with this label
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Justified its viewpoint that the boundary location has no physical meaning by the fact that a transversal vector field of compact support generates a field variation lying in the kernel of the pre-symplectic two-form, as in this paper.
Generating functional for gravitational null initial data (2019) · 1.00 · paraphrased: the licence does not allow quoting
How the literature uses it
333 arXiv papers cite this in 866 sentences
Papers citing it · 535 sentences without a confident label
Contests it 2 papers
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Otherwise, (94) of [ref] is missing an additional finite term coming from the $O(^2)$ terms of $ξ$.
Wald-Zoupas prescription with (soft) anomalies · Charges' archeology · contest 0.74
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The fact that one can assign a unique symplectic form to the pair $(L,ℓ)$ is an important point which was overlooked in the early references, such as [ref] , where this type of changes in Lagrangian and symplectic potential are often considered to be ambiguities of the covariant phase space.
Extended corner symmetry, charge bracket and Einstein's equations · Boundary Lagrangian shift · contest 0.72
Builds on it 6 papers
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Following with the discussion in Sec. [ref], [ref] and [ref], we may define [equation] The $ℰ_0$ is the divergent part from linear dilaton background, and $ℰ$ is the finite part that contributes to the first law of thermodynamics hence we may rewrite this as [equation] For more general case, we may generalise the Eq. ([ref]) and Eq. (20) in [ref] with [equation] Here the $ℰ_0$ and $𝒥_0$ describe the quantity defined by what we choose as background.
Weak cosmic censorship conjecture is not violated for a rotating linear dilaton black hole · Review of Wald's geometrical formulation · 0.83
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In developing the field theoretical description of the boundary spinors, we will also develop further the covariant Hamiltonian formalism [ref] for complex Ashtekar variables in the presence of inner null boundaries, thereby generalising the isolated horizon framework [ref] to generic lightlike boundaries.
New boundary variables for classical and quantum gravity on a null surface · Introduction: Why spinors? · 0.96
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We are proposing a new perspective on these questions which builds on the Wald–Zoupas approach [ref] and on the recent work of Chandrasekaran and Speranza [ref] .
Extended corner symmetry, charge bracket and Einstein's equations · Introduction · 0.81
Uses its method or tool 102 papers
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The prescription of Wald and Zoupas [ref] provides a method of determining charges and fluxes at null infinity, and can be applied to any local and covariant theory.
The Wald-Zoupas prescription for asymptotic charges at null infinity in general relativity · Asymptotic charges and fluxes: The Wald-Zoupas prescription · 0.90
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Its value on the desired solution will be our definition of the mass of the metric [We are specifying here to a particular symmetry generator since we are interested in the mass, but the same procedure con be applied to the most general asymptotic symmetry generator [ref] .] $ℋ_ξ$ is defined via its variation with respect to a generic fluctuation $δϕ$, obeying the linearised equations of motion in a given background obeying the full equations of motion [ref] : [equation] where $Σ$ is a 9 dimensional submanifold of the spacetime without boundary, a “slice” corresponding to the vector field $ξ$.
Half BPS states in AdS_5 x Y^{p,q} · Definition of charges in asymptotically $AdS_5 ×X^5$ · 0.99
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In this paper, we apply the prescription set out in Ref. [ref] , which uses the covariant phase space formalism [ref] to propose a systematic method for determining, in principle, all possible gravitational charges, to give a Hamiltonian derivation of a recently discovered tower of dual BMS charges [ref] .
Hamiltonian derivation of dual gravitational charges · Introduction · 0.98
Compares with it 28 papers
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These covariant expressions are equal to those given by [ref] , and are therefore independent of foliation; however, picking a choice of foliation allows us to write the charge in terms of quantities defined entirely at null infinity.
The Wald-Zoupas prescription for asymptotic charges at null infinity in general relativity · Introduction · 0.92
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For vacuum GR, the charge expression <ref> coincides with the charges defined by Wald and Zoupas [ref] .
Angular momentum at null infinity in Einstein-Maxwell theory · Computation of the Wald-Zoupas charges and fluxes at null infinity for Einstein-Maxwell theory · 0.95
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Here we show that the asymptotic limit of the quasi-local BMS charges [ref] agrees with the BMS charges constructed by Wald and Zoupas [ref] at null infinity.
Quasi-local conserved charges in General Relativity · Asymptotic behaviour · 0.99
Cites it as support 3 papers
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In general relativity it can be shown that the \(3\)-form \(θ(δĝ)\) is finite at \(\) [ref] and thus \((Ω^2 W) ·θ(δĝ) 0\).
The Wald-Zoupas prescription for asymptotic charges at null infinity in general relativity · Asymptotic charges and fluxes: The Wald-Zoupas prescription · 0.97
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This viewpoint is motivated by background invariance, which implies that the location of the boundary with respect to a given coordinate system has no physical meaning, and is justified by the fact that a transversal vector field $ξ^a∈Tℳ:ξ^a|_𝒩∉T𝒩$ of compact support on the null boundary generates a field-variation $δ_ξ[·]=ℒ_ξ(·)$ on the covariant phase space that lies in the kernel of the pre-symplectic two-form [ref] .
Generating functional for gravitational null initial data · Field strength, constraints, evolution equations · 1.00
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This interpretation is supported by explicit examples such as the Kerr-Newman and Vaidya metric, the Landau-Lifschitz approach to defining balance laws [ref] and put on a firm ground by an analysis of the covariant phase space of asymptotically flat spacetimes [ref] .
BMS-like symmetries in cosmology · Conformal Bondi-Sachs coordinates in the unphysical spacetime · 0.98
Cites it as background 104 papers
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The seminal Wald-Zoupas (WZ) paper [ref] provides a prescription for the gravitational charges in the two cases of conservative boundary conditions and of leaky boundary conditions, with the latter that make the system non-conservative and thus the infinitesimal Hamiltonian generators non-integrable.
Wald-Zoupas prescription with (soft) anomalies · Introduction · 0.97
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It is well known that $Θ(𝒩)$ is subject to the so-called $W$ and $Y$ freedoms (or ambiguities) [ref] .
Dynamical Entropy Is a Noether Charge · Introduction · 0.80
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They were first derived in [ref] using the Barnich–Brandt formalism [ref] and later in [ref] from a split following the prescription laid out by Wald–Zoupas [ref] .
The Weyl BMS group and Einstein's equations · BMSW group and Einstein's equations · 0.85
Discussion
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