In differential geometry, the Lie derivative, named after Sophus Lie by Władysław Ślebodziński,[1] [2] evaluates the change of a tensor field (including scalar functions, vector fields and one-forms), along the flow defined by another vector field. This change is coordinate invariant and therefore the Lie derivative is defined on any differentiable manifold.
Functions, tensor fields and forms can be differentiated with respect to a vector field. If T is a tensor field and X is a vector field, then the Lie derivative of T with respect to X is denoted
l{L}XT
T\mapstol{L}XT
The Lie derivative commutes with contraction and the exterior derivative on differential forms.
Although there are many concepts of taking a derivative in differential geometry, they all agree when the expression being differentiated is a function or scalar field. Thus in this case the word "Lie" is dropped, and one simply speaks of the derivative of a function.
The Lie derivative of a vector field Y with respect to another vector field X is known as the "Lie bracket" of X and Y, and is often denoted [''X'',''Y''] instead of
l{L}XY
l{L}[X,Y]T=l{L}Xl{L}YT-l{L}Yl{L}XT,
valid for any vector fields X and Y and any tensor field T.
Considering vector fields as infinitesimal generators of flows (i.e. one-dimensional groups of diffeomorphisms) on M, the Lie derivative is the differential of the representation of the diffeomorphism group on tensor fields, analogous to Lie algebra representations as infinitesimal representations associated to group representation in Lie group theory.
Generalisations exist for spinor fields, fibre bundles with a connection and vector-valued differential forms.
A 'naïve' attempt to define the derivative of a tensor field with respect to a vector field would be to take the components of the tensor field and take the directional derivative of each component with respect to the vector field. However, this definition is undesirable because it is not invariant under changes of coordinate system, e.g. the naive derivative expressed in polar or spherical coordinates differs from the naive derivative of the components in Cartesian coordinates. On an abstract manifold such a definition is meaningless and ill defined. In differential geometry, there are three main coordinate independent notions of differentiation of tensor fields: Lie derivatives, derivatives with respect to connections, and the exterior derivative of totally antisymmetric covariant tensors, i.e. differential forms. The main difference between the Lie derivative and a derivative with respect to a connection is that the latter derivative of a tensor field with respect to a tangent vector is well-defined even if it is not specified how to extend that tangent vector to a vector field. However a connection requires the choice of an additional geometric structure (e.g. a Riemannian metric or just an abstract connection) on the manifold. In contrast, when taking a Lie derivative, no additional structure on the manifold is needed, but it is impossible to talk about the Lie derivative of a tensor field with respect to a single tangent vector, since the value of the Lie derivative of a tensor field with respect to a vector field X at a point p depends on the value of X in a neighborhood of p, not just at p itself. Finally, the exterior derivative of differential forms does not require any additional choices, but is only a well defined derivative of differential forms (including functions).
The Lie derivative may be defined in several equivalent ways. To keep things simple, we begin by defining the Lie derivative acting on scalar functions and vector fields, before moving on to the definition for general tensors.
Defining the derivative of a function
f\colonM\to{R}
style(f(x+h)-f(x))/h
x+h
The Lie derivative of a function
f\colonM\to{R}
X
p\inM
(l{L}Xf)(p)={d\overdt}r|t=0l(f\circ
t | |
\Phi | |
Xr)(p) |
=\limt\to
| ||||||||||
t |
t | |
\Phi | |
X(p) |
X
p
t.
t=0,
t | |
\Phi | |
X(p) |
d | |
dt |
r|t
t | |
\Phi | |
X(p) |
=
t | |
Xl(\Phi | |
X(p)r) |
0 | |
\Phi | |
X(p) |
=p.
Setting
l{L}Xf=\nablaXf
X(f):=l{L}Xf=\nablaXf
If X and Y are both vector fields, then the Lie derivative of Y with respect to X is also known as the Lie bracket of X and Y, and is sometimes denoted
[X,Y]
The Lie derivative is the speed with which the tensor field changes under the space deformation caused by the flow.
Formally, given a differentiable (time-independent) vector field
X
M,
t | |
\Phi | |
X |
:M\toM
t | |
\Phi | |
X |
t
* | |
\left(\Phi | |
p |
:
* | |||||||
T | |||||||
|
M\to
* | |
T | |
p |
M,
* | |
\left(\Phi | |
p |
\alpha(X)=\alphal(Tp
t | |
\Phi | |
X(X)r), |
\alpha\in
* | |||||||
T | |||||||
|
M,X\inTpM
-1 | |
\left(T | |
X\right) |
:
T | |||||||
|
M\toTpM
t | |
T | |
X |
t,
* | |
(\Phi | |
X) |
T
T
If
T
(r,0)
(0,s)
{\calL}XT
T
X
p\inM
{\calL}XT(p)=
d | |
dt |
l|t=0
* | |
\left(l(\Phi | |
Xr) |
T\right)p=
d | |
dt |
l|t=0
* | |
l(\Phi | |
p |
T | |||||||
|
=\limt
| ||||||||||||||||
t |
.
The resulting tensor field
{\calL}XT
T
More generally, for every smooth 1-parameter family
\Phit
X
{d\overdt}r|t=0\Phit=X\circ\Phi0
We now give an algebraic definition. The algebraic definition for the Lie derivative of a tensor field follows from the following four axioms:
Axiom 1. The Lie derivative of a function is equal to the directional derivative of the function. This fact is often expressed by the formula
l{L}Yf=Y(f)
Axiom 2. The Lie derivative obeys the following version of Leibniz's rule: For any tensor fields S and T, we have
l{L}Y(S ⊗ T)=(l{L}YS) ⊗ T+S ⊗ (l{L}YT).
Axiom 3. The Lie derivative obeys the Leibniz rule with respect to contraction:
l{L}X(T(Y1,\ldots,Yn))=(l{L}XT)(Y1,\ldots,Yn)+T((l{L}XY1),\ldots,Yn)+ … +T(Y1,\ldots,(l{L}XYn))
Axiom 4. The Lie derivative commutes with exterior derivative on functions:
[l{L}X,d]=0
If these axioms hold, then applying the Lie derivative
l{L}X
df(Y)=Y(f)
l{L}XY(f)=X(Y(f))-Y(X(f)),
The Lie derivative acting on a differential form is the anticommutator of the interior product with the exterior derivative. So if α is a differential form,
l{L}Y\alpha=iYd\alpha+diY\alpha.
Explicitly, let T be a tensor field of type . Consider T to be a differentiable multilinear map of smooth sections α1, α2, ..., αp of the cotangent bundle T∗M and of sections X1, X2, ..., Xq of the tangent bundle TM, written T(α1, α2, ..., X1, X2, ...) into R. Define the Lie derivative of T along Y by the formula
(l{L}YT)(\alpha1,\alpha2,\ldots,X1,X2,\ldots)=Y(T(\alpha1,\alpha2,\ldots,X1,X2,\ldots))
-T(l{L}Y\alpha1,\alpha2,\ldots,X1,X2,\ldots) -T(\alpha1,l{L}Y\alpha2,\ldots,X1,X2,\ldots)-\ldots
-T(\alpha1,\alpha2,\ldots,l{L}YX1,X2,\ldots) -T(\alpha1,\alpha2,\ldots,X1,l{L}YX2,\ldots)-\ldots
The analytic and algebraic definitions can be proven to be equivalent using the properties of the pushforward and the Leibniz rule for differentiation. The Lie derivative commutes with the contraction.
See also: Interior product. A particularly important class of tensor fields is the class of differential forms. The restriction of the Lie derivative to the space of differential forms is closely related to the exterior derivative. Both the Lie derivative and the exterior derivative attempt to capture the idea of a derivative in different ways. These differences can be bridged by introducing the idea of an interior product, after which the relationships falls out as an identity known as Cartan's formula. Cartan's formula can also be used as a definition of the Lie derivative on the space of differential forms.
Let M be a manifold and X a vector field on M. Let
\omega\inΛk+1(M)
p\inM
\omega(p)
(TpM)k
iX\omega
(iX\omega)(X1,\ldots,Xk)=\omega(X,X1,\ldots,Xk)
The differential form
iX\omega
k+1 | |
i | |
X:Λ |
(M) → Λk(M)
is a \wedge \wedge
iX
iX(\omega\wedgeη)=(iX\omega)\wedgeη+(-1)k\omega\wedge(iXη)
for
\omega\inΛk(M)
f\inΛ0(M)
ifX\omega=fiX\omega
where
fX
l{L}Xf=iXdf
For a general differential form, the Lie derivative is likewise a contraction, taking into account the variation in X:
l{L}X\omega=iXd\omega+d(iX\omega).
This identity is known variously as Cartan formula, Cartan homotopy formula or Cartan's magic formula. See interior product for details. The Cartan formula can be used as a definition of the Lie derivative of a differential form. Cartan's formula shows in particular that
dl{L}X\omega=l{L}X(d\omega).
The Lie derivative also satisfies the relation
l{L}fX\omega=fl{L}X\omega+df\wedgeiX\omega.
In local coordinate notation, for a type tensor field
T
X
\begin{align} (l{L}XT)
a1\ldotsar | |
{} | |
b1\ldotsbs |
={} &
c(\partial | |
X | |
c |
a1\ldotsar | |
T |
{} | |
b1\ldotsbs |
)\\ &{}-{}(\partialcX
a1 | |
)T
ca2\ldotsar | |
{} | |
b1\ldotsbs |
-\ldots-(\partialc
ar | |
X |
)T
a1\ldotsar-1c | |
{} | |
b1\ldotsbs |
\\ &+
(\partial | |
b1 |
Xc)T
a1\ldotsar | |
{} | |
cb2\ldotsbs |
+\ldots+
(\partial | |
bs |
Xc)T
a1\ldotsar | |
{} | |
b1\ldotsbs-1c |
\end{align}
here, the notation
\partiala=
\partial | |
\partialxa |
xa
\partiala
\partialaXb
\nablaaXb=
b | |
X | |
;a |
:=(\nabla
b | |
X) | |
a |
=\partialaXb+
b | |
\Gamma | |
ac |
Xc
a | |
\Gamma | |
bc |
=
a | |
\Gamma | |
cb |
The Lie derivative of a tensor is another tensor of the same type, i.e., even though the individual terms in the expression depend on the choice of coordinate system, the expression as a whole results in a tensor
(l{L}XT)
a1\ldotsar | |
{} | |
b1\ldotsbs |
\partial | |
a1 |
⊗ … ⊗ \partial | |
ar |
⊗
b1 | |
dx |
⊗ … ⊗
bs | |
dx |
T
The definition can be extended further to tensor densities. If T is a tensor density of some real number valued weight w (e.g. the volume density of weight 1), then its Lie derivative is a tensor density of the same type and weight.
\begin{align} (l{L}X
a1\ldotsar | |
T) |
{} | |
b1\ldotsbs |
={}
c(\partial | |
&X | |
c |
a1\ldotsar | |
T |
{} | |
b1\ldotsbs |
)-(\partialcX
a1 | |
)T
ca2\ldotsar | |
{} | |
b1\ldotsbs |
-\ldots-(\partialc
ar | |
X |
)T
a1\ldotsar-1c | |
{} | |
b1\ldotsbs |
+\\ &+
(\partial | |
b1 |
Xc)T
a1\ldotsar | |
{} | |
cb2\ldotsbs |
+\ldots+
(\partial | |
bs |
Xc)T
a1\ldotsar | |
{} | |
b1\ldotsbs-1c |
+w(\partialcXc)T
a1\ldotsar | |
{} | |
b1\ldotsbs |
\end{align}
Notice the new term at the end of the expression.
\Gamma=(
a | |
\Gamma | |
bc |
)
X
(l{L}X
a | |
\Gamma) | |
bc |
=
d\partial | |
X | |
d |
a | |
\Gamma | |
bc |
+\partialb\partialcXa-
d | |
\Gamma | |
bc |
\partialdXa+
a | |
\Gamma | |
dc |
\partialbXd+
a | |
\Gamma | |
bd |
\partialcXd
For clarity we now show the following examples in local coordinate notation.
\phi(xc)\inl{F}(M)
(l{L}X\phi)=X(\phi)=Xa\partiala\phi
Hence for the scalar field
\phi(x,y)=x2-\sin(y)
X=\sin(x)\partialy-
2\partial | |
y | |
x |
For an example of higher rank differential form, consider the 2-form
\omega=(x2+y2)dx\wedgedz
X
Some more abstract examples.
l{L}X(dxb)=diX(dxb)=dXb=\partialaXbdxa
Hence for a covector field, i.e., a differential form,
A=
b)dx | |
A | |
a(x |
a
l{L}XA=X(Aa)dxa+Abl{L}X(dxb)=(Xb\partialbAa+Ab\partiala(Xb))dxa
The coefficient of the last expression is the local coordinate expression of the Lie derivative.
For a covariant rank 2 tensor field
T=Tab(xc)dxa ⊗ dxb
If
T=g
(l{L}Xg)=(Xcgab;+gcb
c | |
X | |
;a |
+gac
c | |
X | |
;b |
)dxa ⊗ dxb=(Xb;a+Xa;b)dxa ⊗ dxb
The Lie derivative has a number of properties. Let
l{F}(M)
l{L}X:l{F}(M) → l{F}(M)
is a derivation on the algebra
l{F}(M)
l{L}X
l{L}X(fg)=(l{L}Xf)g+fl{L}Xg.
Similarly, it is a derivation on
l{F}(M) x l{X}(M)
l{X}(M)
l{L}X(fY)=(l{L}Xf)Y+fl{L}XY
which may also be written in the equivalent notation
l{L}X(f ⊗ Y)=(l{L}Xf) ⊗ Y+f ⊗ l{L}XY
where the tensor product symbol
⊗
Additional properties are consistent with that of the Lie bracket. Thus, for example, considered as a derivation on a vector field,
l{L}X[Y,Z]=[l{L}XY,Z]+[Y,l{L}XZ]
one finds the above to be just the Jacobi identity. Thus, one has the important result that the space of vector fields over M, equipped with the Lie bracket, forms a Lie algebra.
The Lie derivative also has important properties when acting on differential forms. Let α and β be two differential forms on M, and let X and Y be two vector fields. Then
l{L}X(\alpha\wedge\beta)=(l{L}X\alpha)\wedge\beta+\alpha\wedge(l{L}X\beta)
[l{L}X,l{L}Y]\alpha:=l{L}Xl{L}Y\alpha-l{L}Yl{L}X\alpha=l{L}[X,Y]\alpha
[l{L}X,iY]\alpha=[iX,l{L}Y]\alpha=i[X,Y]\alpha,
Various generalizations of the Lie derivative play an important role in differential geometry.
A definition for Lie derivatives of spinors along generic spacetime vector fields, not necessarily Killing ones, on a general (pseudo) Riemannian manifold was already proposed in 1971 by Yvette Kosmann.[4] Later, it was provided a geometric framework which justifies her ad hoc prescription within the general framework of Lie derivatives on fiber bundles[5] in the explicit context of gauge natural bundles which turn out to be the most appropriate arena for (gauge-covariant) field theories.[6]
In a given spin manifold, that is in a Riemannian manifold
(M,g)
\psi
l{L}X\psi:=Xa\nablaa\psi-
14\nabla | |
a |
Xb\gammaa\gammab\psi,
where
\nablaaXb=\nabla[aXb]
X=Xa\partiala
\gammaa
It is then possible to extend Lichnerowicz's definition to all vector fields (generic infinitesimal transformations) by retaining Lichnerowicz's local expression for a generic vector field
X
\nablaaXb
l{L}X\psi:=Xa\nablaa\psi-
18\nabla | |
[a |
Xb][\gammaa,\gammab]\psi=\nablaX\psi-
14 | |
(d |
X\flat) ⋅ \psi,
where
[\gammaa,\gammab]=\gammaa\gammab-\gammab\gammaa
d
X\flat=g(X,-)
X
⋅
It is worth noting that the spinor Lie derivative is independent of the metric, and hence also of the connection. This is not obvious from the right-hand side of Kosmann's local expression, as the right-hand side seems to depend on the metric through the spin connection (covariant derivative), the dualisation of vector fields (lowering of the indices) and the Clifford multiplication on the spinor bundle. Such is not the case: the quantities on the right-hand side of Kosmann's local expression combine so as to make all metric and connection dependent terms cancel.
To gain a better understanding of the long-debated concept of Lie derivative of spinor fields one may refer to the original article,[8] [9] where the definition of a Lie derivative of spinor fields is placed in the more general framework of the theory of Lie derivatives of sections of fiber bundles and the direct approach by Y. Kosmann to the spinor case is generalized to gauge natural bundles in the form of a new geometric concept called the Kosmann lift.
If we have a principal bundle over the manifold M with G as the structure group, and we pick X to be a covariant vector field as section of the tangent space of the principal bundle (i.e. it has horizontal and vertical components), then the covariant Lie derivative is just the Lie derivative with respect to X over the principal bundle.
Now, if we're given a vector field Y over M (but not the principal bundle) but we also have a connection over the principal bundle, we can define a vector field X over the principal bundle such that its horizontal component matches Y and its vertical component agrees with the connection. This is the covariant Lie derivative.
See connection form for more details.
Another generalization, due to Albert Nijenhuis, allows one to define the Lie derivative of a differential form along any section of the bundle Ωk(M, TM) of differential forms with values in the tangent bundle. If K ∈ Ωk(M, TM) and α is a differential p-form, then it is possible to define the interior product iKα of K and α. The Nijenhuis–Lie derivative is then the anticommutator of the interior product and the exterior derivative:
l{L}K\alpha=[d,iK]\alpha=
k-1 | |
di | |
K\alpha-(-1) |
iKd\alpha.
In 1931, Władysław Ślebodziński introduced a new differential operator, later called by David van Dantzig that of Lie derivation, which can be applied to scalars, vectors, tensors and affine connections and which proved to be a powerful instrument in the study of groups of automorphisms.
The Lie derivatives of general geometric objects (i.e., sections of natural fiber bundles) were studied by A. Nijenhuis, Y. Tashiro and K. Yano.
For a quite long time, physicists had been using Lie derivatives, without reference to the work of mathematicians. In 1940, Léon Rosenfeld[10] —and before him (in 1921[11]) Wolfgang Pauli[12] —introduced what he called a ‘local variation’
\delta\astA
A
X
\delta\astA
-l{L}X(A)