Torsion tensor

In differential geometry, the torsion tensor is a tensor that is associated to any affine connection. The torsion tensor is a bilinear map of two input vectors, that produces an output vector representing the displacement within a tangent space when the tangent space is developed along an infinitesimal parallelogram whose sides are. It is skew symmetric in its inputs, because developing over the parallelogram in the opposite sense produces the opposite displacement, similarly to how a screw moves in opposite ways when it is twisted in two directions.
Torsion is particularly useful in the study of the geometry of geodesics. Given a system of parametrized geodesics, one can specify a class of affine connections having those geodesics, but differing by their torsions. There is a unique connection which absorbs the torsion, generalizing the Levi-Civita connection to other, possibly non-metric situations. The difference between a connection with torsion, and a corresponding connection without torsion is a tensor, called the contorsion tensor. Absorption of torsion also plays a fundamental role in the study of G-structures and Cartan's equivalence method. Torsion is also useful in the study of unparametrized families of geodesics, via the associated projective connection. In relativity theory, such ideas have been implemented in the form of Einstein–Cartan theory.

Definition

Let M be a manifold with an affine connection on the tangent bundle ∇. The torsion tensor of ∇ is the vector-valued 2-form defined on vector fields X and Y by
where is the Lie bracket of two vector fields. By the Leibniz rule, T = T = fT for any smooth function f. So T is tensorial, despite being defined in terms of the connection which is a first order differential operator: it gives a 2-form on tangent vectors, while the covariant derivative is only defined for vector fields.

Components of the torsion tensor

The components of the torsion tensor in terms of a local basis of sections of the tangent bundle can be derived by setting, and by introducing the commutator coefficients. The components of the torsion are then
Here are the connection coefficients defining the connection. If the basis is holonomic then the Lie brackets vanish,. So. In particular, while the geodesic equations determine the symmetric part of the connection, the torsion tensor determines the antisymmetric part.

The torsion form

The torsion form, an alternative characterization of torsion, applies to the frame bundle FM of the manifold M. This principal bundle is equipped with a connection form ω, a gl-valued one-form which maps vertical vectors to the generators of the right action in gl and equivariantly intertwines the right action of GL on the tangent bundle of FM with the adjoint representation on gl. The frame bundle also carries a canonical one-form θ, with values in Rⁿ, defined at a frame by
where is the projection mapping for the principal bundle and is its push-forward. The torsion form is then
Equivalently, Θ = Dθ, where D is the exterior covariant derivative determined by the connection.
The torsion form is a tensorial form with values in Rⁿ, meaning that under the right action of it transforms equivariantly:
where acts on the right-hand side by its canonical action on Rⁿ.

Torsion form in a frame

The torsion form may be expressed in terms of a connection form on the base manifold M, written in a particular frame of the tangent bundle. The connection form expresses the exterior covariant derivative of these basic sections:
The solder form for the tangent bundle is the dual basis of the e_i, so that . Then the torsion 2-form has components
In the rightmost expression,
are the frame-components of the torsion tensor, as given in the previous definition.
It can be easily shown that Θⁱ transforms tensorially in the sense that if a different frame
for some invertible matrix-valued function, then
In other terms, Θ is a tensor of type .
Alternatively, the solder form can be characterized in a frame-independent fashion as the TM-valued one-form θ on M corresponding to the identity endomorphism of the tangent bundle under the duality isomorphism. Then the torsion 2-form is a section
given by
where D is the exterior covariant derivative.

Irreducible decomposition

The torsion tensor can be decomposed into two irreducible parts: a trace-free part and another part which contains the trace terms. Using the index notation, the trace of T is given by
and the trace-free part is
where δⁱ_j is the Kronecker delta.
Intrinsically, one has
The trace of T, tr T, is an element of T^∗M defined as follows. For each vector fixed, T defines an element T of via
Then is defined as the trace of this endomorphism. That is,
The trace-free part of T is then
where ι denotes the interior product.

Curvature and the Bianchi identities

The curvature tensor of ∇ is a mapping defined on vector fields X, Y, and Z by
For vectors at a point, this definition is independent of how the vectors are extended to vector fields away from the point.
The Bianchi identities relate the curvature and torsion as follows. Let denote the cyclic sum over X, Y, and Z. For instance,
Then the following identities hold

Bianchi's first identity:
:
Bianchi's second identity:
:
The curvature form and Bianchi identities

The curvature form is the gl-valued 2-form
where, again, D denotes the exterior covariant derivative. In terms of the curvature form and torsion form, the corresponding Bianchi identities are

Moreover, one can recover the curvature and torsion tensors from the curvature and torsion forms as follows. At a point u of F_xM, one has
where again is the function specifying the frame in the fibre, and the choice of lift of the vectors via π⁻¹ is irrelevant since the curvature and torsion forms are horizontal.

Characterizations and interpretations

The torsion is a manner of characterizing the amount of slipping or twisting that a plane does when rolling along a surface or higher dimensional affine manifold.
For example, consider rolling a plane along a small circle drawn on a sphere. If the plane does not slip or twist, then when the plane is rolled all the way along the circle, it will also trace a circle in the plane. It turns out that the plane will have rotated, an effect due to the curvature of the sphere. But the curve traced out will still be a circle, and so in particular a closed curve that begins and ends at the same point. On the other hand, if the plane were rolled along the sphere, but it was allowed it to slip or twist in the process, then the path the circle traces on the plane could be a much more general curve that need not even be closed. The torsion is a way to quantify this additional slipping and twisting while rolling a plane along a curve.
Thus the torsion tensor can be intuitively understood by taking a small parallelogram circuit with sides given by vectors v and w, in a space and rolling the tangent space along each of the four sides of the parallelogram, marking the point of contact as it goes. When the circuit is completed, the marked curve will have been displaced out of the plane of the parallelogram by a vector, denoted. Thus the torsion tensor is a tensor: a function of two input vectors v and w that produces an output vector. It is skew symmetric in the arguments v and w, a reflection of the fact that traversing the circuit in the opposite sense undoes the original displacement, in much the same way that twisting a screw in opposite directions displaces the screw in opposite ways. The torsion tensor thus is related to, although distinct from, the torsion of a curve, as it appears in the Frenet–Serret formulas: the torsion of a connection measures a dislocation of a developed curve out of its plane, while the torsion of a curve is also a dislocation out of its osculating plane. In the geometry of surfaces, the geodesic torsion describes how a surface twists about a curve on the surface. The companion notion of curvature measures how moving frames roll along a curve without slipping or twisting.

Example

Consider the Euclidean space. On it, we put a connection that is flat, but with non-zero torsion, defined on the standard Euclidean frame by the cross product:
Consider now the parallel transport of the vector along the axis, starting at the origin. The parallel vector field thus satisfies, and the differential equation
Thus, and the solution is.
Now the tip of the vector, as it is transported along the axis traces out the helix
Thus we see that, in the presence of torsion, parallel transport tends to twist a frame around the direction of motion, analogously to the role played by torsion in the classical differential geometry of curves.

Development

One interpretation of the torsion involves the development of a curve. Suppose that a piecewise smooth closed loop is given, based at the point, where. We assume that is homotopic to zero. The curve can be developed into the tangent space at in the following manner. Let be a parallel coframe along, and let be the coordinates on induced by. A development of is a curve in whose coordinates sastify the differential equation
If the torsion is zero, then the developed curve is also a closed loop. On the other hand, if the torsion is non-zero, then the developed curve may not be closed, so that. Thus the development of a loop in the presence of torsion can become dislocated, analogously to a screw dislocation.
The foregoing considerations can be made more quantitative by considering a small parallelogram, originating at the point, with sides. Then the tangent bivector to the parallelogram is. The development of this parallelogram, using the connection, is no longer closed in general, and the displacement in going around the loop is translation by the vector, where is the torsion tensor, up to higher order terms in. This displacement is directly analogous to the Burgers vector of crystallography.
More generally, one can also transport a moving frame along the curve. The linear transformation that the frame undergoes between is then determined by the curvature of the connection. Together, the linear transformation of the frame and the translation of the starting point from to comprise the holonomy of the connection.