nLab conserved current



Variational calculus


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Via the variational bicomplex

The following discusses the formulation of conserved currents in terms of variational calculus and the variational bicomplex.

The context

Let XX be a spacetime of dimension nn, EXE \to X a bundle, j EXj_\infty E \to X its jet bundle and

Ω ,(j E),(D=δ+d) \Omega^{\bullet,\bullet}(j_\infty E), (D = \delta + d)

the corresponding variational bicomplex with δ\delta being the vertical and d=d dRd = d_{dR} the horizontal differential.


For LΩ n,0(j E)L \in \Omega^{n,0}(j_\infty E) a Lagrangian we have that

δL=E(L)+dΘ \delta L = E(L) + d \Theta

for EE the Euler-Lagrange operator.

The covariant phase space of the Lagrangian is the locus

{ϕΓ(E)|E(L)(j ϕ)=0}. \{\phi \in \Gamma(E) | E(L)(j_\infty \phi) = 0\} \,.

For ΣX\Sigma \subset X any (n1)(n-1)-dimensional submanifold,

δθ:=δ ΣΘ \delta \theta := \delta \int_\Sigma \Theta

is the presymplectic structure on covariant phase space



A conserved current is an element

jΩ n1,0(j E) j \in \Omega^{n-1, 0}(j_\infty E)

which is horizontally closed on covariant phase space

dj| E(L)=0=0. d j|_{E(L) = 0} = 0 \,.

For ΣX\Sigma \hookrightarrow X a submanifold of dimension n1n-1, the charge of the conserved current jj with respect to Σ\Sigma is the integral

Q Σ:= Σj. Q_\Sigma := \int_\Sigma j \,.



If Σ,ΣX\Sigma, \Sigma' \subset X are homologous, the associated charge is the same

Q Σ=Q Σ. Q_{\Sigma} = Q_{\Sigma'} \,.

By Stokes' theorem.


Every symmetry of the Lagrangian induces a conserved current.

This is Noether's theorem. See there for more details.

In higher prequantum geometry

The following discusses conserved currents in the context of higher prequantum geometry, closely related to Azcarraga-Izquierdo 95, section 8.1. This follows (classicalinhigher, section 3.3., going back to Schreiber 13). Similar observations have been made by Igor Khavkine.

this section needs much polishing. For the moment better see classicalinhigher, section 3.3


Let H\mathbf{H} be the ambient (∞,1)-topos. For FieldsH\mathbf{Fields} \in \mathbf{H} a moduli ∞-stack of fields a local Lagrangian for an nn-dimensional prequantum field theory is equivalently a prequantum n-bundle given by a map

L:FieldsB nU(1) conn \mathbf{L} \;\colon\; \mathbf{Fields} \longrightarrow \mathbf{B}^n U(1)_{conn}

to the moduli ∞-stack of smooth circle n-bundles with connection. The local connection differential n-form is the local Lagrangian itself as in traditional literature, the rest of the data in \mathcal{L} is the higher gauge symmetry equivariant structure.

The following is effectively the direct higher geometric analog of the Hamiltonian version of Noether’s theorem.


A transformation of the fields is an equivalence

FieldsϕFields. \mathbf{Fields} \underoverset{\simeq}{\phi}{\longrightarrow} \mathbf{Fields} \,.

That the local Lagrangian \mathcal{L} be preserved by this, up to (gauge) equivalence, means that there is a diagram in H\mathbf{H} of the form

Fields ϕ Fields L α L B nU(1) conn. \array{ \mathbf{Fields} &&\underoverset{\simeq}{\phi}{\longrightarrow}&& \mathbf{Fields} \\ & {}_{\mathllap{\mathbf{L}}}\searrow &\swArrow^\simeq_\alpha& \swarrow_{\mathrlap{\mathbf{L}}} \\ && \mathbf{B}^n U(1)_{conn} } \,.

(With L\mathbf{L} equivalently regarded as prequantum n-bundle this is equivalently a higher quantomorphism. These are the transformations studied in (Fiorenza-Rogers-Schreiber 13))

For ϕ\phi an infinitesimal operation an LL locally the Lagrangian nn-form, this means that the Lie derivative δϕ\mathcal{L}_{\delta \phi} of LL has a potential,

δϕL=dα \mathcal{L}_{\delta \phi} L = \mathbf{d} \alpha

hence that the Lagrangian changes under the Lie derivative by an exact term, hence by a divergence on the worldvolume (since the degree of the Lagrangian form is the dimension of the worldvolume). This defines an infinitesimal symmetry of the Lagrangian. See also (Azcarraga-Izquierdo 95 (8.1.13)).

This is the situation of the Noether theorem for the general case of “weak” symmetries (see at Noether theorem – schematic idea – weak symmetries).

By Cartan's magic formula the above means

d(αι δϕL)=ι δϕω. \mathbf{d}\left( \alpha - \iota_{\delta\phi} \mathbf{L} \right) = \iota_{\delta \phi} \omega \,.

and hence the combination jαι δϕLj \coloneqq \alpha - \iota_{\delta\phi} \mathbf{L} (a Hamiltonian form for δϕ\delta \phi with respect to ω\omega) is conserved on trajectories in the kernel of the n-plectic form ω\omega (which are indeed the classical trajectories of L\mathbf{L}, see (Azcarraga-Izquierdo 95 (8.1.14))).

This is the first stage in the Poisson bracket Lie n-algebra, the current algebra (see there at As a homotopy Lie algebra).


Energy-momentum tensor

Dirac current

Of Green-Schwarz super pp-brane sigma models

The WZW term of the Green-Schwarz super p-brane sigma models is invariant under supersymmetry only up to a divergence, hence here the general Noether theorem for “weak” symmetries applies and yields a current algebra which is an polyvector extension of the supersymmetry algebra. See at Green-Schwarz action functional – Conserved currents for more.


Dickey-Lie bracket on currents

The Dickey Lie bracket on conserved currents is due to

  • Leonid Dickey, Soliton equations and Hamiltonian systems, Advanced Series in Mathematical Physics, Vol. 12 (World Scientific 1991).

and is reviewed in

The statement that the Dickey bracket Lie algebra of currents is a central Lie algebra extension of the algebra of symmetries by de Rham cohomology of the jet bundle appears as theorem 11.2 in (Part II of)

  • Alexandre Vinogradov, The 𝒞\mathcal{C}-spectral sequence, Lagrangian formalism, and conservation laws. I. the linear theory, Journal of Mathematical Analysis and Applications 100, 1-40 (1984) (doi90071-4))

  • Alexandre Vinogradov, The 𝒞\mathcal{C}-spectral sequence, Lagrangian formalism, and conservation laws. II. The nonlinear theory, Journal of Mathematical Analysis and Applications 100, Issue 1, 30 April 1984, Pages 41-129 (publisher)

and is stated as exercise 2.28 on p. 203 of

  • Alexandre Vinogradov, Joseph Krasil'shchik (eds.), Symmetries and conservation laws for differential equations of mathematical physics, vol. 182 of Translations of Mathematical Monographs, AMS (1999)

A lift of the Dickey Lie bracket on cohomologically trivial spaces to an equivalent L-infinity equivalent L-infinity bracket is constructed, under some assumptions, in

The cohomologically non-trivial lift is discussed in

In variational calculus

A general discussion as above is around definition 9 of

  • G. J. Zuckerman, Action Principles and Global Geometry , in Mathematical Aspects of String Theory, S. T. Yau (Ed.), World Scientific, Singapore, 1987, pp. 259–284. (pdf)

The relation of conserved currents to moment maps in symplectic geometry is highlighted for instance in

  • Huijun Fan, Lecture 8, Moment map and symplectic reduction (pdf)

Higher conserved currents

Higher conserved currents are discussed for instance in

In higher prequantum theory

Conserved currents for Lagrangians written as WZW terms are discussed in

Building on that, in the context of higher prequantum geometry conserved currents of the WZW model and in ∞-Wess-Zumino-Witten theory are briefly indicated on the last page of

The same structure is considered in

as higher quantomorphisms and Poisson bracket Lie n-algebras of local currents.

category: physics

Last revised on May 1, 2020 at 17:16:36. See the history of this page for a list of all contributions to it.