The Unapologetic Mathematician

Mathematics for the interested outsider

(Pseudo-)Riemannian Metrics

Ironically, in order to tie what we’ve been doing back to more familiar material, we actually have to introduce more structure. It’s sort of astonishing in retrospect how much structure comes along with the most basic, intuitive cases, or how much we can do before even using that structure.

In particular, we need to introduce something called a “Riemannian metric”, which will move us into the realm of differential geometry instead of just topology. Everything up until this point has been concerned with manifolds as “shapes”, but we haven’t really had any sense of “size” or “angle” or anything else we could measure. Having these notions — and asking that they be preserved — is the difference between geometry and topology.

Anyway, a Riemannian metric on a manifold M is nothing more than a certain kind of tensor field g of type (0,2) on M. At each point p\in M, the field g gives us a tensor:

\displaystyle g_p\in\mathcal{T}_p^*M\otimes\mathcal{T}_p^*M\cong\left(\mathcal{T}_pM\otimes\mathcal{T}_pM\right)^*

We can interpret this as a bilinear function which takes in two vectors v_p,w_p\in\mathcal{T}_pM and spits out a number g_p(v_p,w_p). That is, g_p is a bilinear form on the space \mathcal{T}_pM of tangent vectors at p.

So, what makes g into a Riemannian metric? We now add the assumption that g_p is not just a bilinear form, but that it’s an inner product. That is, g_p is symmetric, nondegenerate, and positive-definite. We can let the last condition slip a bit, in which case we call g a “pseudo-Riemannian metric”. When equipped with a metric, we call M a “(pseudo-)Riemannian manifold”.

It’s common to also say “Riemannian” in the case of negative-definite metrics, since there’s little difference between the cases of signature (n,0) and (0,n). Another common special case is that of a “Lorentzian” metric, which is signature (n-1,1) or (1,n-1).

As we might expect, g is called a metric because it lets us measure things. Specifically, since g_p is an inner product it gives us notions of the length and angle for tangent vectors at p. We must be careful here; we do not yet have a way of measuring distances between points on the manifold M itself. The metric only tells us about the lengths of tangent vectors; it is not a metric in the sense of metric spaces. However, if two curves cross at a point p we can use their tangent vectors to define the angle between the curves, so that’s something.


September 20, 2011 Posted by | Differential Geometry, Differential Topology, Geometry, Topology | 20 Comments