nLab Riemannian metric

Definition

In terms of a 2-tensor

A Riemannian metric on a smooth manifold $M$ is defined as a covariant symmetric 2-tensor $(., .)_p, p \in M$ – a section of the symmetrized second tensor power of the tangent bundle – such that $(v,v)_p \gt 0$ for all $v \in T_p(M)$. For convenience, we will write $(v,w)$ for $(v,w)_p$. In other words, a Riemannian metric is a collection of (positive) inner products on each of the tangent spaces $T_p(M)$ such that if $X,Y$ are (smooth) vector fields, the function $(X,Y): M \to \mathbb{R}$ defined by taking the inner product at each point, is smooth. A manifold together with a Riemannian metric is called a Riemannian manifold.

In terms of a Vielbein

for the moment see Poincare Lie algebra and first-order formulation of gravity

Examples

There are several ways to get Riemannian metrics:

1. On $\mathbb{R}^n$, there is a standard Riemannian metric coming from the usual inner product. More generally, if $g_{i j}: \mathbb{R}^n \to \mathbb{R}$ are smooth functions such that the matrix $(g_{i j}(x))$ is symmetric and positive definite for all $x \in \mathbb{R}^n$, we get a Riemannian metric $\sum_{i,j} g_{i j} d x^i \otimes d x^j$ on $\mathbb{R}^n$, where the sum is to be interpreted as a covariant tensor.

2. Given an immersion $N \to M$, a Riemannian metric on $M$ induces one on $N$ in the natural way, simply by pulling back. For instance, any surface in $\mathbb{R}^3$ has a Riemannian structure based upon the standard Riemannian structure on $\mathbb{R}^3$—based simply on the usual inner product—and induced on the surface.

3. Given an open covering $U_i$ on $M$, Riemannian metrics $(\cdot, \cdot)_i$ on $U_i$, and a partition of unity $\phi_i$ subordinate to the covering $U_i$, we get a Riemannian metric on $M$ by

   $$(v,w)_p := \sum_i \phi_i(p) (v,w)_{i,p}.$$

   Thus, using 1) above, any smooth manifold—which necessarily admits partitions of unity—can be given a Riemannian metric.

Lengths of Curves

A Riemannian metric allows us to take the length of a curve in a manner resembling the standard case. Given $v \in T_p(M)$, use the notation $\left \Vert{v} \right \Vert := (v,v) = (v,v)_p$. If $c: I \to M$ is a smooth curve for $I$ an interval in $\mathbb{R}$, we define

$$l(c) := \int_I \left \Vert{c'(t)}\right \Vert d t;$$

this is easily checked to be independent of parametrization, just as in the usual case. Using this, we can make a Riemannian manifold $M$ into a metric space: for $p,q \in M$, let

$$d(p,q) := \inf_{c \mid c(a)=p,c(b)=q} l(c).$$

The metric on $M$ induces the standard topology on $M$. To see this, first note that it is a local question, so we can reduce to the case of $M$ an open ball in euclidean space $\mathbb{R}^n$. Each tangent vector $v \in T_p(M)$ can be viewed as an element of $\mathbb{R}^n$ in a natural way. Now let $\left \Vert{\cdot}\right \Vert_{\mathbb{R}^n}$ be the standard norm on $\mathbb{R}^n$. By continuity, we can find $\delta \gt 0$ by shrinking $M$ if necessary such that for all $v \in T_p(M), p \in K$,

$$\delta \left \Vert{v}\right \Vert_{\mathbb{R}^n} \leq \left \Vert{v}\right \Vert_p \leq \delta^{-1} \left \Vert{v}\right \Vert_{\mathbb{R}^n} ;$$

in particular, the lengths of curves in $M$ are necessarily comparable to the usual lengths in $\mathbb{R}^n$. The result now follows.

Related concepts

• signature of a metric

• invariant Riemannian metric

• orthogonal structure

• Riemannian manifold, pseudo-Riemannian manifold

• Levi-Civita connection

• moduli space of Riemannian metrics

References

An introduction in terms of synthetic differential geometry is in

• Gonzalo Reyes, Metrics, connections and curvature (pdf)