Geometric meaning of symmetric connection

If $(M, g)$ is Riemannian manifold, there is unique connection $\nabla$, called Levi-Civita connection, satisfying the following conditions:

1) Compatibility with Riemannian metric, i.e. $\nabla(g)$=0

2) Symmetricity, i.e. $\nabla_X(Y)-\nabla_Y(X)=[X, Y]$

While the former seems quite natural (equally, parallel transform of a vector along a curve does not change its length), what geometric intuition is hidden behind the later?

The only idea I have, except for the fact it makes a compatible connection unique, is that it is in some way similar to the equality of partial derivates along commuting fields.

Solution 1:

This is an excellent question. As indicated by the MathOverflow link in the comments, there are many ways to think about torsion and torsion-freeness. At the risk of being repetitive, allow me to summarize some of these, adding my own thoughts.

Throughout, we let $M$ be a smooth manifold, $\nabla$ a connection on $TM$, and $$T^\nabla(X,Y) = \nabla_XY - \nabla_YX - [X,Y]$$ its torsion tensor field. We let $X$, $Y$ denote vector fields.

Initial Observations

(1) Parallel coordinates

Torsion (at a point) can be seen as the obstruction to the existence of parallel coordinates (at that point):

Fact: Let $p \in M$. Then $T^\nabla|_p = 0$ if and only if there exists a coordinate system $(x^i)$ centered at $p$ such that $\nabla \partial_i |_p = 0$.

The point here is that if $T^\nabla = 0$, then any parallel frame is commuting (i.e.: $\nabla E_i = 0$ $\forall i$ $\implies$ $[E_i, E_j] = 0$ $\forall i,j$), hence is a coordinate frame (by the "Flowbox Coordinate Theorem").

(2) Commuting of second partials

The following two facts indicate that torsion can be thought of as the obstruction to (certain types of) second partial derivatives commuting.

For a smooth function $f \colon M \to \mathbb{R}$, recall that its covariant Hessian (or second covariant derivative) is the covariant $2$-tensor field defined by $$\text{Hess}(f) := \nabla \nabla f = \nabla df.$$ Explicitly, $\text{Hess}(f)(X,Y) = (\nabla_X df)(Y) = X(Yf) - (\nabla_XY)(f)$.

Fact [Lee]: The following are equivalent:

(i) $T^\nabla = 0$

(ii) The Christoffel symbols of $\nabla$ with respect to any coordinate system are symmetric: $$\Gamma^k_{ij} = \Gamma^k_{ji}$$

(iii) The covariant Hessian of any smooth function $f$ is symmetric: $$\text{Hess}(f)(X,Y) = \text{Hess}(f)(Y,X)$$

Torsion-freeness also implies another kind of symmetry of second partials:

Symmetry Lemma [Lee]: If $T^\nabla = 0$, then for every smooth family of curves $\Gamma \colon (-\epsilon, \epsilon) \times [a,b] \to M$, we have $$\frac{D}{ds} \frac{d}{dt} \Gamma(s,t) = \frac{D}{dt} \frac{d}{ds} \Gamma(s,t).$$

I don't know for certain whether the converse to the Symmetry Lemma is true, but I imagine it is.

Some Heuristic Interpretations

(i) "Twisting" of parallel vector fields along geodesics

Suppose we have a connection $\nabla$ on $\mathbb{R}^n$ whose geodesics are lines, but that has torsion. One could then imagine that parallel translating a vector along a line results in the vector "spinning" along the line, as if one were holding each end of a string and rolling it between our fingers.

An explicit example of such a connection is in the MathOverflow answer linked in the comments.

The justification for why this interpretation should be believed in general will be discussed below in (B).

On the MO thread, Igor Belegradek points out two related facts:

Fact [Spivak]:

(1) Two connections $\nabla^1$, $\nabla^2$ on $TM$ are equal if and only if they have the same geodesics and torsion tensors.

(2) For every connection on $TM$, there is a unique torsion-free connection with the same geodesics.

(ii) Closing of geodesic parallelograms (to second order)

Let $v, w \in T_pM$ be tangent vectors. Let $\gamma_v$ and $\gamma_w$ be the geodesics whose initial tangent vectors are $v$, $w$, respectively. Consider parallel translating the vector $w$ along $\gamma_v$, and also the vector $v$ along $\gamma_w$. Then the tips of the resulting two vectors agree to second order if and only if $T^\nabla|_p = 0$.

Heuristic reasons for this (and a picture!) are given in this excellent answer by Sepideh Bakhoda.

A precise proof of this fact is outlined by Robert Bryant at the end of this MO answer of his.

More Reasons We Like $T^\nabla = 0$

(A) Submanifolds of $\mathbb{R}^N$ come with torsion-free connections

Suppose $(M,g)$ is isometrically immersed into $\mathbb{R}^N$.

As hinted in the comments, the euclidean connection $\overline{\nabla}$ on $\mathbb{R}^N$ is torsion-free. It is a fact that the tangential component of $\overline{\nabla} = \nabla^\top + \nabla^\perp$ defines an induced connection on $M \subset \mathbb{R}^N$. This induced connection on $M$ will then also be torsion-free (and compatible with the induced metric).

Point: If $(M,g) \subset \mathbb{R}^N$ is an isometrically immersed submanifold, then its induced connection is torsion-free.

This example is more general than it seems: by the Nash Embedding Theorem, every Riemannian manifold $(M,g)$ can be isometrically embedded in some $\mathbb{R}^N$.

(B) $T = d^\nabla(\text{Id})$

[I'll add this another time.]

(C) Simplification of identities

Finally, I should mention that $T^\nabla = 0$ greatly simplifies many identities.

First, we have the Ricci Formula $$\nabla^2_{X,Y}Z - \nabla^2_{Y,X}Z = R(X,Y)Z - \nabla_{T^\nabla(X,Y)}Z.$$ Thus, in the case where $T^\nabla = 0$, we can interpret the curvature $R(X,Y)$ as the obstruction to commuting second covariant derivatives of vector fields.

In the presence of torsion, the First and Second Bianchi Idenities read, respectively, $$\mathfrak{S}(R(X,Y)Z) = \mathfrak{S}[ T(T(X,Y),Z) + (\nabla_XT)(Y,Z)],$$ $$\mathfrak{S}[(\nabla_XR)(Y,Z) + R(T(X,Y),Z)] = 0,$$ where $\mathfrak{S}$ denotes the cyclic sum over $X,Y,Z$.

References

[Lee] "Riemannian Manifolds: An Introduction to Curvature"

[Spivak] "A Comprehensive Introduction to Differential Geometry: Volume II"

Solution 2:

The presence of torsion in a connection allows you to extend parallel transport in a manifold from being a linear mapping to a more general affine mapping. The torsion then generates the translation part of the affine mapping. But parallel transport is defined between tangent spaces which are of course linear spaces and these do not admit affine mappings in a natural way. So instead we think of each tangent space to be the associated vector space of an underlying affine space and the affine parallel transport is between these tangent affine spaces.

These tangent affine spaces capture perhaps in some sense the classical notion of infinitesimal close points. The points of a tangent affine space are all infinitesimally close to each other as well as being infinitesimally close to the manifold point which the affine space is the fiber of.

The points of each affine space are on equal footing, there is no distinguished point among them. We may ask if it is possible to choose an origin in each tangent affine space which could be identified with the base manifold point, thus "attaching" the tangent affine spaces to the manifold. This can be carried out by choosing some initial point in one tangent affine space and then trying to parallel transport this point throughout the whole manifold. If there is no torsion this succeeds, the manifold is generated as a submanifold in the tangent affine space bundle and the bundle becomes isomorphic to the usual tangent bundle. But if there is torsion this is not possible, the tangent affine spaces "slip", they are not "attachable".

This can be expressed by

$D^2P=T(X,Y)$

where $D$ is the exterior covariant derivative, $P$ is a point function mapping a manifold point to a point in its tangent affine space and $T(X,Y)$ is the torsion two-form.

Thus a non-vanishing torsion two-form means that $DP$ is not closed and there is not a well defined point function $P$ allowing us to consistently choose origins in the tangent affine spaces.