Cited by 335Noncommutative and Quantum Gravity TheoriesRelativity and Gravitational TheoryBlack Holes and Theoretical Physics

Loop-Quantum-Gravity Vertex Amplitude

Open access
Engle, Jonathan, Pereira, Roberto, Rovelli, Carlo·2007·Physical Review Letters

Abstract

Spin foam models are hoped to provide the dynamics of loop-quantum gravity. However, the most popular of these, the Barrett-Crane model, does not have the good boundary state space and there are indications that it fails to yield good low-energy n-point functions. We present an alternative dynamics that can be derived as a quantization of a Regge discretization of Euclidean general relativity, where second class constraints are imposed weakly. Its state space matches the SO(3) loop gravity one and it yields an SO(4)-covariant vertex amplitude for Euclidean loop gravity.

Verified 2026-10-06 · 22 references not checked yet

AI snapshot

Written from the paper’s full text.

TL;DR

The paper proposes a new SO(4)-covariant spinfoam vertex amplitude for loop quantum gravity, obtained by imposing the off-diagonal simplicity constraints weakly rather than strongly as in the Barrett-Crane model. Its boundary state space matches SO(3) loop quantum gravity and it can be derived as a quantization of a Regge discretization of Euclidean general relativity.

Key findings

  • The model provides an SO(4)-covariant vertex amplitude for LQG whose boundary state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) Hamiltonian spin-network space, so “no degrees of freedom are lost.”
  • It modifies the Barrett-Crane model by replacing the single Barrett-Crane intertwiner \(i_{BC}\) with a space \(K_e\) of intertwiners, leaving intertwiner degrees of freedom free instead of constraining them entirely.
  • The vertex is built from an SO(4) \(15j\) symbol, which is the product of two conventional Wigner SU(2) \(15j\) symbols, together with a linear map from SO(3) intertwiners to SO(4) intertwiners.
  • The authors argue the model may yield the correct low-energy \(n\)-point functions because the degrees of freedom missing in Barrett-Crane are recovered, though this is presented as an expectation rather than a demonstrated result.

Methods

  • The theory is defined on a fixed 4d triangulation \(\Delta\), with a spinfoam partition function summing over integer SO(3) spins \(j_f\) on faces and intertwiners \(i_e\) on edges; the amplitude uses the SO(4) \(15j\) symbol and the map \(f\) from SO(3) to SO(4) intertwiners.
  • The derivation treats a Regge discretization of Euclidean GR: co-tetrad one-forms \(e^I(t)\) for tetrahedra and \(e^I(v)\) for simplices, SO(4) group elements \(V_{vt}\), face bivectors \(B_f(t)=\int_f \star(e(t)\wedge e(t))\), closure constraints, and simplicity constraints.
  • The canonical boundary variables are identified with those of SO(4) lattice gauge theory; imposing the simplicity constraints weakly reduces the state space to the SO(3) LQG spin-network space, and the single 4-simplex amplitude is evaluated by integrals and transformed back to the spin-network basis.

Limitations

  • The paper explicitly states: “We do not discuss here the issue of the recovery of triangulation independence,” and that it gives “only a rapid sketch of its derivation from Regge GR. Details will be given elsewhere.”
  • Evident from the text: the model is presented on a fixed triangulation, and no numerical low-energy \(n\)-point functions or quantitative tests are reported; the claim that the vertex “may yield the correct low-energy \(n\)-point functions” is not demonstrated here.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1103/physrevlett.99.161301

Cites

32 of 54 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 4 cited sources checked · 0 supported · 0 flagged

  • Findingnone given
    § The loop-quantum-gravity vertex-amplitude · p. 1

    The vertex can be seen as a vertex over SO(3) spin networks or SO(4) spin networks.

    “Third, the vertex can be seen as a vertex over SO (3) spin networks or SO (4) spin networks, and is both SO (3) and SO (4) covariant.”
    Quote in the paperSubstantial claim
  • Findingown data
    § The loop-quantum-gravity vertex-amplitude · p. 1

    In the presented model, the constraints are imposed weakly.

    “We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly .”
    • Also stated in: § The loop-quantum-gravity vertex-amplitude · p. 1 · plain
      “by properly imposing some of the constraints weakly ( 〈 φC n ψ 〉 = 0)”
    Quote in the paperSubstantial claim
  • Findingnone given
    § The loop-quantum-gravity vertex-amplitude · p. 3

    In the presented model, intertwiner degrees of freedom remain free.

    “In the model (1), instead, intertwiner degrees of freedom remain free.”
    Quote in the paperSubstantial claim
  • Findingreasoning
    § The loop-quantum-gravity vertex-amplitude · p. 1

    The state space of the presented model matches the SO(3) hamiltonian spin network space.

    “Its state space is a natural subspace of the SO (4) spin-network space and matches the SO (3) hamiltonian spin network space.”
    • Also stated in: § The loop-quantum-gravity vertex-amplitude · p. 1 · boosted
      “First, its boundary quantum state space matches exactly the one of SO (3) loop quantum gravity: no degrees of freedom are lost.”
    Quote in the paperSubstantial claim
  • Findingnone given
    § The loop-quantum-gravity vertex-amplitude · p. 3

    The states (4) span a subspace K_e of H_e.

    “More precisely, the states (4) span a subspace K e of H e .”
    Quote in the paperNot a substantial claim
  • Classified as probably not a substantial claim. 1
  • May combine two claims. 5
  • Probably not a substantial claim: may not pass S4 (based).

    Findingnone given
    § The loop-quantum-gravity vertex-amplitude · p. 2

    The boundary states of the theory are spanned by trivalent graphs colored with SO(3) spins and intertwiners.

    “First, the boundary states of the theory are spanned by trivalent graphs colored with SO (3) spins and intertwiners.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingnone given
    § The loop-quantum-gravity vertex-amplitude · p. 1

    The vertex is both SO(3) and SO(4) covariant.

    “Third, the vertex can be seen as a vertex over SO (3) spin networks or SO (4) spin networks, and is both SO (3) and SO (4) covariant.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § The loop-quantum-gravity vertex-amplitude · p. 4

    The closure constraint gives gauge-invariance at each tetrahedron and reduces the space of states to the space of SO(4) spin networks on the graph dual to the boundary triangulation.

    “The closure constraint (15) gives gaugeinvariance at each tetrahedron, and reduces the space of states to the space of the SO (4) spin networks on the graph dual to the boundary triangulation.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Findingreasoning
    § The loop-quantum-gravity vertex-amplitude · p. 4

    The simplicity constraints reduce each SO(4) link representation to a simple one, and the intertwiner spaces to K_e.

    “The simplicity constraints (11), as seen above, reduce each SO (4) link representation to a simple one, and the intertwiners spaces to K e .”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Premisecitation
    § The loop-quantum-gravity vertex-amplitude · p. 3

    In the Barrett-Crane theory, the off-diagonal simplicity constraints C_ff'=0 are imposed strongly on H_e, and the only solution is i_BC.

    “In BC theory, the constraints C ff ′ = 0 are imposed strongly on H e , and the only solution of these constraint equations is i BC [17].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [17] MP Reisenberger, 'On relativistic spin network vertices', J Math Phys 40 (1999) 2046 [arXiv:gr-qc/9809067] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationreasoning
    § The loop-quantum-gravity vertex-amplitude · p. 4

    The resulting space of states can be physically identified with the corresponding one of SO(3) loop quantum gravity, because there is an explicit identification of quantum operators on the two spaces with the same classical analogues, such as the area of the faces.

    “The resulting space of states is not only mathematical isomorphic to the corresponding one of SO (3) loop quantum gravity, but it can also be physically identified with it, because we have an explicit identification of the quantum operators on the two spaces with the same classical analogues, such as the area of the faces.”
    Quote in the paperSubstantial claim
6 of 6
  • The paper relies on this without support.

    The Barrett-Crane model's boundary state space does not exactly match that of loop quantum gravity.

    “Its boundary state space is similar, but does not exactly match, that of loop quantum gravity; in particular the volume operator is ill-defined.”
  • The paper relies on this without support.

    The volume operator is ill-defined in the Barrett-Crane model.

    “Its boundary state space is similar, but does not exactly match, that of loop quantum gravity; in particular the volume operator is ill-defined.”
  • The paper relies on this without support.

    The spinfoam picture has never been fully implemented in 4d.

    “Compelling and popular as it is, however, this picture has never been fully implemented in 4d.”
  • The paper relies on this without support.

    The classical limit of the simplicity constraints yields precisely the constraints that change BF theory into general relativity.

    “by means of constraints -called simplicity constraints- whose classical limit yields precisely the constraints that change BF theory into general relativity (GR).”
  • The paper relies on this without support.

    The reduction of the intertwiner space to the sole i_BC vector is commonly motivated by the imposition of the off-diagonal simplicity constraints.

    “The reduction of the intertwiner space to the sole i BC vector is commonly motivated by the imposition of the off-diagonal simplicity constraints.”
  • The paper relies on this without support.

    A simple SO(4) representation (j,j) transforms under the SO(3) subgroup in the representation j⊗j = 0⊕...⊕2j.

    “A simple SO (4) representation ( j, j ) transforms under the SO (3) subgroup in the representation j ⊗ j = 0 ⊕ ... ⊕ 2 j .”
  • The paper relies on this without support.

    All SO(3) subgroups in SO(4) are conjugate to one another.

    “because all SO (3) subgroups in SO (4) are conjugate to one another.”
7 of 7

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • Imposing the constraints strongly, as in the Barrett-Crane model, implies additional conditions that are not present in the classical theory.

    “Imposing the constraints strongly, as in the Barrett-Crane model, implies the imposition of additional conditions that are not present in the classical theory.”

    From The Spin-Foam Approach to Quantum Gravity (2013) · cites this paper as [168]

  • Relaxing the constraints naturally leads to a simple model that has a clearer relationship with the canonical picture provided by LQG.

    “There is a natural way to relax the constraints and this leads to a simple model that has a more clear relationship with the canonical picture provided by LQG.”

    From The Spin-Foam Approach to Quantum Gravity (2013) · cites this paper as [168]

  • The EPRL model is a modification of the Barrett-Crane model that was recently introduced and extended to arbitrary Immirzi parameter.

    “A modification of the Barrett-Crane model was recently introduced in [168, 169] and extended for arbitrary Immirzi parameter in [164].”

    From The Spin-Foam Approach to Quantum Gravity (2013) · cites this paper as [168]

  • The basic idea of the EPRL model was to relax the imposition of the Plebanski constraints that reduce BF theory to general relativity in the quantum theory.

    “The basic idea was to relax the imposition of the Plebanski constraints that reduce BF theory to general relativity in the quantum theory.”

    From The Spin-Foam Approach to Quantum Gravity (2013) · cites this paper as [168]

  • The model of [168] corresponds to the sector γ = ∞, which has been shown to be topological.

    “They also emphasize how the model [168] corresponds to the sector γ = ∞, which has been shown to be topological [246].”

    From The Spin-Foam Approach to Quantum Gravity (2013) · cites this paper as [168]

Cites

Flagged first, then by citations

Research works printed but not linked: 3

  1. C. Rovelli, 1995, Nucl. Phys., B456, 734
  2. M. Reisenberger, 2003, Florence 2001, A Relativistic Spacetime Odyssey
  3. G. Ponzano, 1968, Spectroscopy and Group Theoretical Methods in Physics

Cited by

Sorted by citations

Authors
Engle, J., Pereira, R., & Rovelli, C.
Affiliations
Centre de Physique Théorique (FR)
Venue
Physical Review Letters
Published
2007-10-16
Volume
99
Issue
16
Pages
161301-161301
ISSN
  • 0031-9007 (ISSN-L)
  • 1079-7114
OpenAlex
W2006259415
Cited by
335
References
53
Citations by year
2026: 1 · 2025: 4 · 2024: 11 · 2023: 9 · 2022: 7 · 2021: 36 · 2020: 7 · 2019: 6 · 2018: 5 · 2017: 10 · 2016: 11 · 2015: 10 · 2014: 17 · 2013: 29 · 2012: 31
FWCI
26.04
Type
journal article
Language
en
Open access
Locations
  • Physical Review Letters (publishedVersion)
  • PubMed (publishedVersion)
  • arXiv (Cornell University) (submittedVersion)
  • HAL (submittedVersion, other-oa)
  • arXiv (Cornell University)
Funders
Agence Nationale de la Recherche (ANR-06-BLAN-0050)
Topics
  • Physical Sciences › Physics and Astronomy › Statistical and Nonlinear Physics › Noncommutative and Quantum Gravity Theories
  • Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Relativity and Gravitational Theory
  • Physical Sciences › Physics and Astronomy › Nuclear and High Energy Physics › Black Holes and Theoretical Physics
Keywords
loop quantum gravity, spin foam models, vertex amplitude, Barrett-Crane model, Regge calculus, second-class constraints, soft constraints

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