nLab
geometry of physics -- integration

this page is going to contain one chapter of geometry of physics

previous chapters: differential forms, geometry of physics -- smooth homotopy types


Contents

Integration

By the discussion in Differential forms and Principal connections, differential forms and more generally connections may be regarded as infinitesimal measures of change, of displacement. The discussion in Differentiation showed how to extract from a finite but cohesive (e.g. smoothly continuous) displacement all its infinitesimal measures of displacements by differentiation.

Here we discuss the reverse operation: integration is a construction from a differential form of the corresponding finite cohesive displacement. More generally this applies to any connection and is then called the parallel transport of the connection, a term again referring to the idea that a finite displacement proceeds pointwise in parallel to a given infinitesimal displacement.

Under good conditions this construction can proceed literally by “adding up all the infinitesimal contributions” and therefore integration is traditionally thought of as a generalization of forming sums. Therefore one has the notation “ Σω\int_{\Sigma} \omega” for the integral of a differential form ω\omega over a space Σ\Sigma, as a variant of the notation “ Sf\sum_{S} f” for the sum of values of a function on a set SS. For the case of integrals of connections the corresponding parallel transport expression is often denoted by an exponentiated integral sign “𝒫exp( Σω)\mathcal{P} \exp(\int_\Sigma \omega)”, referring to the fact that the passage from infinitesimal to finite quantities involves also the passage from Lie algebra data to Lie group data (“exponentiated Lie algebra data”).

However, both from the point of view of gauge theory physics as well as from the general abstract perspective of cohesive homotopy type theory another characterization of integration is more fundamental: the integral Σω\int_\Sigma \omega of a differential form ω\omega (or more generally of a connection) is an invariant under those gauge transformations of ω\omega that are trivial on the boundary of Σ\Sigma, and it is the universal such invariant, hence is uniquely characterized by this property.

In traditional accounts this fact is referred to via the Stokes theorem and its generalizations (such as the nonabelian Stokes theorem), which says that the integral/parallel transport is indeed invariant under gauge transformations of differential forms/connections. That this invariance actually characterizes the integral and the parallel transport is rarely highlighted in traditional texts, but it is implicit for instance in the old “path method” of Lie integration (discussed below in Lie integration) as well as in the famous characterization of flat connections, discussed above in Flat 1-connections:

for XX a connected manifold and for GG a Lie group, the operation of sending a flat GG-principal connection \nabla to its parallel transport γhol γ()\gamma \mapsto hol_{\gamma}(\nabla) around loops γ:S 1X\gamma\colon S^1 \to X, hence to the integral of the connection around all possible loops (its holonomy), for any fixed basepoint

hol𝒫exp( ()()):H conn,flat 1(X,G)Hom Grp(π 1(X),G)/G hol \coloneqq \mathcal{P} \exp(\int_{(-)} (-)) \;\colon\; H^1_{conn, flat}(X,G) \stackrel{\simeq}{\to} Hom_{Grp}(\pi_1(X) , G)/G

exhibits a bijection between gauge equivalence classes of connections and group homomorphisms from the fundamental group π 1(X)\pi_1(X) of XX to the gauge group GG (modulo adjoint GG-action from gauge transformations at the base point, hence at the integration boundary). This is traditionally regarded as a property of the definition of the parallel transport 𝒫exp( ()())\mathcal{P} \exp(\int_{(-)}(-)) by integration. But being a bijection, we may read this fact the other way round: it says that forming equivalence classes of flat GG-connections is a way of computing their integral/parallel transport.

We saw a generalization of this fact to non-closed forms and non-flat connections already in the discussion at Differential 1-forms as smooth incremental path measures, where gauge equivalence classes of differential forms are shown to be equivalently assignments of parallel transport to smooth paths.

This is also implied by the above discussion: for H conn 1(X,G)\nabla \in H^1_{conn}(X,G) any non-flat connection and γ:S 1X\gamma \colon S^1 \to X a trajectory in XX, we may form the pullback of \nabla to S 1S^1. There it becomes a necessarily flat connection γ *H conn,flat 1(S 1,G)\gamma^* \nabla \in H^1_{conn, flat}(S^1,G), since the curvature differential 2-form necessarily vanishes on the 1-dimensional manifold S 1S^1. Accordingly, by the above bijection, forming the gauge equivalence class of γ *\gamma^* \nabla means to find a group homomorphism

π 1(S 1)G \mathbb{Z} \simeq \pi_1(S^1) \to G

modulo conjugation (modulo nothing if GG is abelian, such as G=U(1)G = U(1)) and since \mathbb{Z} is the free group on a single generator this is the same as finding an element

hol γ()=𝒫exp( γγ *)G. hol_\gamma(\nabla) = \mathcal{P} \exp(\int_\gamma \gamma^*\nabla) \in G \,.

This total operation of first pulling back the connection and then forming its integration (by taking gauge equivalence classes) is called the transgression of the original 1-form connection on XX to a 0-form connection on the loop space [S 1,X][S^1,X].

Below in the Model Layer we discuss the classical examples of integration/parallel transport and their various generalizations in detail. Then in the Semantic Layer we show how indeed all these constructions are obtained forming equivalence classes in the (∞,1)-topos of smooth homotopy types, hence by truncation (followed, to obtain the correct cohesive structure, by concretification, def. \ref{ConcreteObjectsAndConcretification}).

Model Layer

Integration

Integration over a coordinate patch

For nn \in \mathbb{N} let

C n{x n| i(0x i1)} n C^n \coloneqq \{ \vec x \in \mathbb{R}^n | \forall_i (0 \leq x_i \leq 1) \} \hookrightarrow \mathbb{R}^n

be the standard unit cube.

Let

ωΩ n( n) \omega \in \Omega^n(\mathbb{R}^n)

be a differential n-form.

ω=fdx 1dx 2dx n. \omega = f \mathbf{d} x^1 \wedge \mathbf{d} x^2 \wedge \cdots \wedge \mathbf{d} x^n \,.

Let Partitions(C k)Partitions(C^k) be the poset whose elements are partitions of the unit nncube C nC^n into N nN^n subcubes, for NN \in \mathbb{N}, ordered by inclusion.

Let

()ω:Partitions(C k) \sum_{(-)} \omega \colon Partitions(C^k) \to \mathbb{R}

be the function that sends

1N n x 1=0 N x 2=0 N k n=0 Nf(x 1,,x n). \frac{1}{N^n} \sum_{x^1 = 0}^N \sum_{x^2 = 0}^N \cdots \sum_{k^n = 0}^N f( x^1, \cdots, x^n ) \,.

Then

C kω:lim N Nω. \int_{C^k} \omega \colon \lim_{N} \sum_N \omega \,.
Integration of differential forms over a manifold

Let Σ\Sigma be a closed oriented smooth manifold of dimension kk

Definition

For nn \in \mathbb{N}, nkn \geq k, define the morphism of smooth spaces

Σ:[Σ,Ω n]Ω nk \int_{\Sigma} \colon [\Sigma, \Omega^n] \to \Omega^{n-k}

by declaring that over a coordinate chart UU \in CartSp it is the ordinary integration of differential forms over smooth manifolds

Σ,U:Ω n(Σ×U)Ω nk(U). \int_{\Sigma, U} : \Omega^n(\Sigma\times U) \to \Omega^{n-k}(U) \,.
Integration in ordinary differential cohomology

Holonomy

Parallel transport

given AΩ 1(Δ 1,𝔤)A \in \Omega^1(\Delta^1, \mathfrak{g})

we say fC (Δ 1,G)f \in C^\infty(\Delta^1, G) is the parallel transport of AA if

  1. f(0)=1f(0) = 1

  2. ff satisfies the differential equation

    df=Af \mathbf{d}f = A f

where on the right we have the differential of the left action of the group on itself.

In this case one writes

𝒫exp( Δ 1A)f(1) \mathcal{P} \exp\left(\int_{\Delta^1} A \right) \coloneqq f(1)

and calls it the path ordered integral? of AA. Here the enire left hand side is primitive notation.

In the case that G=U(1)G = U(1) this reproduces the ordinary integral

(G=)Righarrow𝒫exp( Δ 1A)=exp(i Δ 1A)U(1) \left(G = \mathbb{R}\right) \Righarrow \mathcal{P} \exp(\int_{\Delta^1} A) = \exp(i \int_{\Delta^1} A) \in U(1)

There is another way to express this parallel transport, related to Lie integration:

Define an equivalence relation on Ω 1(Δ 1,𝔤)\Omega^1(\Delta^1, \mathfrak{g}) as follows: two 1-forms A,AA,A' are taken to be equivalent if there is a flat 1-form A^Ω flat 1(D 2,𝔤)\hat A \in \Omega^1_{flat}(D^2, \mathfrak{g}) on the 2-disk such that its restriction to the upper semicircle is AA and the restriction to the lower semicircle is A˜\tilde A.

If GG is simply connected, then the equivalence classes of this relation form

Ω 1(Δ 1,𝔤) /G \Omega^1(\Delta^1,\mathfrak{g})_{/\sim} \simeq G

and the quotient map coincides with the parallel transport

𝒫exp( Δ 1()):Ω 1(Δ 1,𝔤)Ω 1(Δ 1,𝔤) /G \mathcal{P} \exp\left(\int_{\Delta^1} \left(-\right)\right) \colon \Omega^1(\Delta^1, \mathfrak{g}) \to \Omega^1(\Delta^1, \mathfrak{g})_{/\sim} \simeq G

Finally yet another perspective is this: consider the equivalence relation on Ω 1(Δ 1,𝔤)\Omega^1(\Delta^1, \mathfrak{g}) where two 1-forms are regarded as equivalent if there is a gauge transformation λC (Δ 1,G)\lambda \in C^\infty(\Delta^1, G) with λ(0)=e\lambda(0) = e and λ(1)=e\lambda(1) = e, then again

𝒫exp( Δ 1()):Ω 1(Δ 1,𝔤)Ω 1(Δ 1,𝔤) /G \mathcal{P} \exp\left(\int_{\Delta^1} \left(-\right)\right) \colon \Omega^1(\Delta^1, \mathfrak{g}) \to \Omega^1(\Delta^1, \mathfrak{g})_{/\sim} \simeq G

is the parallel transport

Holonomy of a flat principal connection

if XX is connected then forming the holonomy of flat GG-connections

hol:GBund ,flat(X)Hom Grp(π 1(X),G) hol \colon G Bund_{\nabla, flat}(X) \stackrel{\simeq}{\to} Hom_{Grp}(\pi_1(X), G)

is an equivalence, π 1(X)\pi_1(X) the fundamental group. If XX is not connected then

hol:GBund ,flat(X)Hom Grpd(Π 1(X),BG) hol \colon G Bund_{\nabla, flat}(X) \stackrel{\simeq}{\to} Hom_{Grpd}(\Pi_1(X), \mathbf{B}G)

is an equivalence.

Transgression

Given a differential form ω\omega of degree nn on some smooth space XX and given a closed smooth manifold Σ\Sigma of dimension knk \leq n, then there is canonically induced a differential form τ Σω\tau_\Sigma \omega of degree nkn-k on the mapping space [Σ,X][\Sigma,X]: its restriction to any smooth family Φ ()\Phi_{(-)} of smooth functions Φ u:ΣX\Phi_u \colon \Sigma \to X is the result of first forming the pullback of differential forms of ω\omega along Φ ()\Phi_{(-)} and then forming the integration of differential forms of the result over Σ\Sigma:

τ Σω| Φ ΣΦ () *ω. \tau_{\Sigma} \omega\vert_{\Phi} \coloneqq \int_\Sigma \Phi_{(-)}^\ast \omega \,.

This differential form τ Σω\tau_\Sigma \omega on the mapping space is called the transgression of ω\omega with respect to Σ\Sigma

This construction has a variety of immediate generalizations, for instance Σ\Sigma may have boundary and corners and the mapping space may be generalized to a space of sections of a given fiber bundle. Also it is immediate to generalize the construction from smooth sets to super formal smooth sets. Finally, the construction also generalizes to coefficients richer than differential forms, such as cocycles in differential cohomology, but this is no longer the topic of the present entry.

Important examples of transgression of differential forms appear in Lagrangian field theory (in the sense of physics) defined by a Lagrangian form on the jet bundle of a field bundle. Here the transgression of the Lagrangian itself is the corresponding action functional, the transgression of its Euler-Lagrange variational derivative is the 1-form whose vanishing is the equations of motion and the transgression of the induced pre-symplectic current is the pre-symplectic form on the covariant phase space of the field theory.

Transgression of differential forms
Defintion

There are two definitions of transgression of differential forms: A traditional formulation is def. 4 below, which transgresses by pullback of differential forms along the evaluation map, followed by integration of differential forms.

Another definition is useful, which makes more use of the existence of smooth classifying spaces for differential forms in smooth sets, this we consider as def. 2 below.

That these two definitions are indeed equivalent is the content of prop. 1 below

Definition

(parameterized integration of differential forms)

Let

  1. XX be a smooth set;

  2. nkn \geq k \in \mathbb{N};

  3. Σ k\Sigma_k be a compact smooth manifold of dimension kk.

Then we write

Σ:[Σ k,Ω n]Ω nk \int_{\Sigma} \;\colon\; [\Sigma_k, \mathbf{\Omega}^n] \longrightarrow \mathbf{\Omega}^{n-k}

for the smooth function which takes a plot ω ():U[Σ,Ω k]\omega_{(-)} \colon U \to [\Sigma, \mathbf{\Omega}^k], hence equivalently a differential nn-form ω ()()\omega_{(-)}(-) on U×ΣU \times \Sigma to the result of integration of differential forms over Σ\Sigma:

Σω ()() Σω (). \int_{\Sigma} \omega_{(-)}(-) \coloneqq \int_\Sigma \omega_{(-)} \,.
Definition

(transgression of differential forms to mapping spaces)

Let

  1. XX be a smooth set;

  2. nkn \geq k \in \mathbb{N};

  3. Σ k\Sigma_k be a compact smooth manifold of dimension kk.

Then the operation of transgression of differential nn-forms on XX with respect to Σ\Sigma is the function

τ Σ Σ[Σ,]:Ω n(X)Ω nk([Σ,X]) \tau_\Sigma \coloneqq \int_\Sigma [\Sigma,-] \;\colon\; \Omega^n(X) \to \Omega^{n-k}([\Sigma,X])

from differential nn-forms on XX to differential nkn-k-forms on the mapping space [Σ,X][\Sigma,X] which takes the differential form corresponding to the smooth function

(XωΩ n)Ω n(X) (X \stackrel{\omega}{\to} \Omega^n) \in \Omega^n(X)

to the differential form corresponding to the following composite smooth funtion:

τ Σω Σ[Σ,ω]:[Σ,X][Σ,ω][Σ,Ω n] ΣΩ nk, \tau_\Sigma \omega \coloneqq \int_{\Sigma} [\Sigma,\omega] \;\colon\; [\Sigma, X] \stackrel{[\Sigma, \omega]}{\to} [\Sigma, \Omega^n] \stackrel{\int_{\Sigma}}{\to} \Omega^{n-k} \,,

where [Σ,ω][\Sigma,\omega] is the mapping space functor on morphisms and Σ\int_{\Sigma} is the parameterized integration of differential forms from def. 2.

More explicitly in terms of plots this means equivalently the following

A plot of the mapping space

ϕ ():U[Σ,X] \phi_{(-)} \;\colon\; U \to [\Sigma, X]

is equivalently a smooth function of the form

ϕ ()():U×ΣX. \phi_{(-)}(-) \;\colon\; U \times \Sigma \to X \,.

The smooth function [Σ,ω][\Sigma,\omega] takes this smooth function to the plot

U×ΣXϕ ()()XωΩ n U \times \Sigma \to X \overset{\phi_{(-)}(-)}{\longrightarrow} X \overset{\omega}{\longrightarrow} \mathbf{\Omega}^{n}

which is equivalently a differential form

(ϕ ()()) *ωΩ n(U×Σ). (\phi_{(-)}(-))^\ast \omega \in \Omega^n(U \times \Sigma) \,.

Finally the smooth function Σ\int_\Sigma takes this to the result of integration of differential forms over Σ\Sigma:

τ Σω| ϕ ()= Σ(ϕ ()()) *ωΩ nk(U). \tau_{\Sigma}\omega\vert_{\phi_{(-)}} \;=\; \int_\Sigma (\phi_{(-)}(-))^\ast \omega \;\in\; \Omega^{n-k}(U) \,.
Definition

(transgression of differential forms to mapping space via evaluation map)

Let

  1. XX be a smooth set;

  2. nkn \geq k \in \mathbb{N};

  3. Σ k\Sigma_k be a compact smooth manifold of dimension kk.

Then the operation of transgression of differential nn-forms on XX with respect to Σ\Sigma is the function

τ Σ Σev *:Ω n(X)ev *Ω n(Σ×[Σ,X]) ΣΩ nk([Σ,X]) \tau_\Sigma \coloneqq \int_\Sigma ev^\ast \;\colon\; \Omega^n(X) \overset{ev^\ast}{\longrightarrow} \Omega^n(\Sigma \times [\Sigma, X]) \overset{\int_\Sigma}{\longrightarrow} \Omega^{n-k}([\Sigma,X])

from differential nn-forms on XX to differential nkn-k-forms on the mapping space [Σ,X][\Sigma,X] which is the composite of forming the pullback of differential forms along the evaluation map ev:[Σ,X]×ΣXev \colon [\Sigma, X] \times \Sigma \to X with integration of differential forms over Σ\Sigma.

Proposition

The two definitions of transgression of differential forms to mapping spaces from def. 3 and def. 4 are equivalent.

Proof

We need to check that for all plots γ:U[Σ,X]\gamma \colon U \to [\Sigma, X] the pullbacks of the two forms to UU coincide.

For def. 4 we get

γ * Σev *A= Σ(γ,id Σ) *ev *AΩ n(U) \gamma^\ast \int_\Sigma \mathrm{ev}^\ast A = \int_\Sigma (\gamma,\mathrm{id}_\Sigma)^\ast \mathrm{ev}^\ast A \; \in \Omega^n(U)

Here we recognize in the integrand the pullback along the (()×Σ[Σ,])( (-)\times \Sigma \dashv [\Sigma,-])-adjunct γ˜:U×ΣΣ\tilde \gamma : U \times \Sigma \to \Sigma of γ\gamma, which is given by applying the left adjoint ()×Σ(-)\times \Sigma and then postcomposing with the adjunction counit ev\mathrm{ev}:

U×Σ (γ,id Σ) [Σ,X]×Σ ev X. \array{ U \times \Sigma & \overset{(\gamma, \mathrm{id}_\Sigma)}{\longrightarrow} & [\Sigma,X] \times \Sigma & \overset{\mathrm{ev}}{\longrightarrow} & X } \,.

Hence the integral is now

= Σγ˜ *A. \cdots = \int_{\Sigma} \tilde \gamma^\ast A \,.

This is the operation of the top horizontal composite in the following naturality square for adjuncts, and so the claim follows by its commutativity:

γ˜ H(U×Σ,X) H(U×Σ,A) H(U×Σ,Ω n+k) Σ(U) Ω n(U) γ H(U,[Σ,X]) H(U,[Σ,A]) H(U,[Σ,Ω n+k]) H(U, Σ) H(U,Ω n) \array{ \tilde \gamma \in & \mathbf{H}(U \times\Sigma, X) & \overset{\mathbf{H}(U \times \Sigma,A)}{\longrightarrow} & \mathbf{H}(U \times \Sigma, \mathbf{\Omega}^{n+k}) & \overset{\int_\Sigma(U)}{\longrightarrow} & \Omega^n(U) \\ & {}^{\mathllap{\simeq}}\downarrow && {}^{\mathllap{\simeq}}\downarrow && {}^{\mathllap{\simeq}}\downarrow \\ \gamma \in & \mathbf{H}(U,[\Sigma,X]) & \overset{\mathbf{H}(U,[\Sigma,A])}{\longrightarrow} & \mathbf{H}(U,[\Sigma,\mathbf{\Omega}^{n+k}]) & \overset{\mathbf{H}(U,\int_\Sigma)}{\longrightarrow} & \mathbf{H}(U,\mathbf{\Omega}^n) }

(here we write H(,)\mathbf{H}(-,-) for the hom functor of smooth sets).

Properties
Example

(relative transgression over manifolds with boundary)

  1. XX be a smooth set;

  2. nkn \geq k \in \mathbb{N};

  3. Σ k\Sigma_k be a compact smooth manifold of dimension kk with boundary Σ\partial \Sigma

  4. ωΩ X n\omega \in \Omega^n_{X} a closed differential form.

Write

(| Σ)[ΣΣ,X]:[Σ,X][Σ,X] (-\vert_{\partial \Sigma}) \coloneqq [\partial \Sigma \hookrightarrow \Sigma, X] \;\colon\; [\Sigma, X] \longrightarrow [\partial \Sigma, X]

for the smooth function that restricts smooth functions on Σ\Sigma to smooth functions on the boundary Σ\partial \Sigma.

Then the operations of transgression of differential forms (def. 3) to Σ\Sigma and to Σ\partial \Sigma, respectively, are related by

d(τ Σ(ω))=(1) k+1(| Σ) *τ Σ(ω)AAAAAAAA[Σ,X] τ Σ(ω) Ω nk (| Σ) (1) k+1d [Σ,X] τ Σ(ω) Ω nk+1. d \left( \tau_{\Sigma}(\omega) \right) = (-1)^{k+1} (-\vert_{\partial \Sigma})^\ast \tau_{\partial \Sigma}(\omega) \phantom{AAAAAAAA} \array{ [\Sigma, X] &\overset{ \tau_{\Sigma}(\omega) }{\longrightarrow}& \mathbf{\Omega}^{n-k} \\ {}^{\mathllap{(-\vert_{\partial \Sigma}) }}\downarrow && \downarrow^{\mathrlap{ (-1)^{k+1} d}} \\ [\partial \Sigma, X] &\underset{ \tau_{\partial\Sigma}(\omega) }{\longrightarrow}& \mathbf{\Omega}^{n-k+1} } \,.

In particular this means that if the compact manifold Σ\Sigma happens to have no boundary (is a closed manifold) then transgression over Σ\Sigma takes closed differential forms to closed differential forms.

Proof

Let ϕ ()():U×ΣX\phi_{(-)}(-) \colon U \times \Sigma \to X be a plot of the mapping space [Σ,X][\Sigma, X]. Notice that the de Rham differential on the Cartesian product U×ΣU \times \Sigma decomposes as

d=d U+d Σ. d = d_U + d_\Sigma \,.

Now we compute as follows:

dτ Σω| ϕ () =d U Σ(ϕ ()()) *ω =(1) k Σd U(ϕ ()()) *ω =(1) k Σ(ddΣ)(ϕ ()()) *ω =(1) k Σd(ϕ ()()) *ω(1) k Σd Σ(ϕ ()()) *ω =(1) k Σ(ϕ ()()) *dω=0(1) k Σd Σ(ϕ ()()) *ω =(1) k Σd Σ(ϕ ()()) *ω =(1) k Σ(ϕ ()()) *ω =(1) kτ Σω| ϕ () \begin{aligned} d \tau_{\Sigma}\omega\vert_{\phi_(-)} & = d_U \int_\Sigma (\phi_{(-)}(-))^\ast \omega \\ & = (-1)^k \int_\Sigma d_U (\phi_{(-)}(-))^\ast \omega \\ & = (-1)^k \int_\Sigma (d - d \Sigma) (\phi_{(-)}(-))^\ast \omega \\ & = (-1)^k \int_\Sigma d (\phi_{(-)}(-))^\ast \omega - (-1)^k \int_\Sigma d_\Sigma (\phi_{(-)}(-))^\ast \omega \\ & = (-1)^k \int_\Sigma (\phi_{(-)}(-))^\ast \underset{= 0}{\underbrace{d \omega}} - (-1)^k \int_\Sigma d_\Sigma (\phi_{(-)}(-))^\ast \omega \\ & = - (-1)^k \int_\Sigma d_\Sigma (\phi_{(-)}(-))^\ast \omega \\ & = -(-1)^k \int_{\partial \Sigma} (\phi_{(-)}(-))^\ast \omega \\ & = -(-1)^k \tau_{\partial \Sigma} \omega \vert_{\phi_{(-)}} \end{aligned}

where in the second but last step we used Stokes' theorem.

We next discuss some examples and applications:

Gauge coupling action functional of charged particle

Let XHX \in \mathbf{H} and consider a circle group-principal connection :XBU(1) conn\nabla \colon X \to \mathbf{B}U(1)_{conn} over XX. By the discussion in Dirac charge quantization and the electromagnetic field above this encodes an electromagnetic field on XX. Assume for simplicity here that the underlying circle principal bundle is trivialized, so that then the connection is equivalently given by a differential 1-form

=A:XΩ 1, \nabla = A \colon X \to \Omega^1 \,,

the electromagnetic potential.

Let then Σ=S 1\Sigma = S^1 be the circle. The transgression of the electromagnetic potential to the loop space of XX

S 1[S 1,A]:[S 1,X][S 1,A][S 1,Ω 1] S 1Ω 0 \int_{S^1} [S^1, A] \;\colon\; [S^1, X] \stackrel{[S^1, A]}{\to} [S^1 , \Omega^1] \stackrel{\int_{S^1}}{\to} \Omega^0 \simeq \mathbb{R}

is the action functional for an electron or other electrically charged particle in the background gauge field AA is S em= S 1[S 1,A]S_{em} = \int_{S^1} [S^1, A].

The variation of this contribution in addition to that of the kinetic action of the electron gives the Lorentz force law describing the force exerted by the background gauge field on the electron.

Transgression of Killing form to symplectic form of Chern-Simons theory

Let 𝔤\mathfrak{g} be a Lie algebra with binary invariant polynomial ,:𝔤𝔤\langle -,-\rangle \colon \mathfrak{g} \otimes \mathfrak{g} \to \mathbb{R}.

For instance 𝔤\mathfrak{g} could be a semisimple Lie algebra and ,\langle -,-\rangle its Killing form. In particular if 𝔤=𝔰𝔲(n)\mathfrak{g} = \mathfrak{su}(n) is a matrix Lie algebra such as the special unitary Lie algebra, then the Killing form is given by the trace of the product of two matrices.

This pairing ,\langle -,-\rangle defines a differential 4-form on the smooth space of Lie algebra valued 1-forms

F ()F ():Ω 1(,𝔤)F ()Ω 2(,𝔤)()()Ω 4(,𝔤𝔤),Ω 4 \langle F_{(-)} \wedge F_{(-)} \rangle \colon \Omega^1(-,\mathfrak{g}) \stackrel{F_{(-)}}{\to} \Omega^2(-, \mathfrak{g}) \stackrel{(-)\wedge (-)}{\to} \Omega^4(-, \mathfrak{g}\otimes \mathfrak{g}) \stackrel{\langle-,-\rangle}{\to} \Omega^4

Over a coordinate patch UU \in CartSp this sends a differential 1-form AΩ 1(U)A \in \Omega^1(U) to the differential 4-form

F AF AΩ 4(U). \langle F_A \wedge F_A \rangle \in \Omega^4(U) \,.

The fact that ,\langle -, - \rangle is indeed an invariant polynomial means that this indeed extends to a 4-form on the smooth groupoid of Lie algebra valued forms

F ()F ():BG connΩ 4. \langle F_{(-)} \wedge F_{(-)}\rangle \colon \mathbf{B}G_{conn} \to \Omega^4 \,.

Now let Σ\Sigma be an oriented closed smooth manifold. The transgression of the above 4-form to the mapping space out of Σ\Sigma yields the 2-form

ω ΣF ()F ():Ω 1(Σ,𝔤)[Σ,BG conn][Σ,F ()F ()][Σ,Ω 4] ΣΩ 2 \omega \coloneqq \int_{\Sigma} \langle F_{(-)}\wedge F_{(-)}\rangle \colon \mathbf{\Omega}^1(\Sigma,\mathfrak{g}) \hookrightarrow [\Sigma, \mathbf{B}G_{conn}] \stackrel{[\Sigma, \langle F_{(-)}\wedge F_{(-)}\rangle]}{\to} [\Sigma, \Omega^4] \stackrel{\int_{\Sigma}}{\to} \Omega^2

to the moduli stack of Lie algebra valued 1-forms on Σ\Sigma.

Over a coordinate chart U= nU = \mathbb{R}^n \in CartSp an element AΩ 1(Σ,𝔤)( n)A \in \mathbf{\Omega}^1(\Sigma,\mathfrak{g})(\mathbb{R}^n) is a 𝔤\mathfrak{g}-valued 1-form AA on Σ×U\Sigma \times U with no leg along UU. Its curvature 2-form therefore decomposes as

F A=F A Σ+δA, F_A = F_A^{\Sigma} + \delta A \,,

where F A ΣF_A^{\Sigma} is the curvature component with all legs along Σ\Sigma and where

δA i=1 nx iAdx i \delta A \coloneqq - \sum_{i = 1}^n \frac{\partial}{\partial x^i} A \wedge \mathbf{d}x^i

is the variational derivative of AA.

This means that in the 4-form

F AF A=F A ΣF A Σ+2F A ΣδA+δAδAΩ 4(Σ×U) \langle F_A \wedge F_A\rangle = \langle F_A^\Sigma \wedge F_A^\Sigma \rangle + 2 \langle F_A^\Sigma \wedge \delta A\rangle + \langle \delta A \wedge \delta A\rangle \in \Omega^4(\Sigma \times U)

only the last term gives a 2-form contribution on UU. Hence we find that the transgressed 2-form is

ω= ΣδAδA:Ω 1(Σ,𝔤)Ω 2. \omega = \int_\Sigma \langle \delta A \wedge \delta A\rangle \colon \mathbf{\Omega}^1(\Sigma, \mathfrak{g}) \to \Omega^2 \,.

When restricted further to flat forms

Ω 1 flat(Σ,𝔤)Ω 1(Σ,𝔤) \mathbf{\Omega^1}_{flat}(\Sigma,\mathfrak{g}) \hookrightarrow \mathbf{\Omega^1}(\Sigma,\mathfrak{g})

which is the phase space of 𝔤\mathfrak{g}-Chern-Simons theory, then this is the corresponding symplectic form (by the discussion at Chern-Simons theory – covariant phase space).

Transgression of circle nn-bundles with connection
Action functionals from transgression

(…)

Lie integration

Semantic Layer

Integration and higher holonomy

integration/higher holonomy is

exp(2πi Σ()):[Σ n,B nU(1) conn]Concτ 0[Σ,B nU(1)]U(1) \exp(2 \pi i \int_{\Sigma}(-)) \colon [\Sigma_n, \mathbf{B}^n U(1)_{conn}] \to Conc \tau_0 [\Sigma, \mathbf{B}^n U(1)] \simeq U(1)

Transgression

transgression is

[Σ k,B nU(1) conn]exp(2πi Σ k)()B nkU(1) conn [\Sigma_k, \mathbf{B}^n U(1)_{conn}] \stackrel{\exp(2 \pi i \int_{\Sigma_k}) (-) }{\to} \mathbf{B}^{n-k} U(1)_{conn}

Action functionals from Lagrangeans

and higher Chern-Simons action functionals induced from

L:BG connB nU(1) conn \mathbf{L} \colon \mathbf{B}G_{conn} \to \mathbf{B}^n U(1)_{conn}

are

exp(iS())exp(2π Σ k[Σ k,L]):[Σ k,BG][Σ k,L][Σ k,B nU(1)]exp(2πi())B nkU(1) conn \exp\left(i S\left(-\right)\right) \coloneqq \exp(2 \pi \in \int_{\Sigma_k} [\Sigma_k, \mathbf{L}] ) \colon [\Sigma_k, \mathbf{B}G] \stackrel{[\Sigma_k, \mathbf{L}]}{\to} [\Sigma_k, \mathbf{B}^n U(1)] \stackrel{\exp(2 \pi i\left(-\right))}{\to} \mathbf{B}^{n-k} U(1)_{conn}

here L\mathbf{L} is the Lagrangean.

Syntactic Layer

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Revised on September 19, 2017 17:25:13 by Urs Schreiber (77.56.177.247)