# What is the historical significance of Riemann’s discovery of non-flat metrics?

From a perspective of math history, what discoveries resulted directly from Riemann’s discovery and codification of non-flat metrics on Riemannian manifolds? After Albert Einstein modeled the Universe on a pseudo-Riemannian manifold, Riemann’s work was rediscovered. But Einstein was a physicist. Math concepts like the Cartan connection and bundle and fiber model for the tangent space seem related to curved metrics, but did not emerge from it directly.

One motivation for this question is the fact that a curved metric does not change the result for total arc length when integrated compared to the Euclidean one from which it is derived (for simple real curves). That made me think there must be some other important significance to the idea that the distance between nearby points can be something other than straight. A non-flat metric seems to be such a cornerstone of Riemannian geometry.

As an example of arc length invariance, see https://math.stackexchange.com/questions/4441646/do-the-curvature-properties-of-exotic-spheres-result-in-necessary-new-techniques . The circumference of a great circle on a sphere is the same as that of a circle produced by a planar intersection at the equator (this also applies to curves on higher dimensional manifolds).

The question arises: what calculations did change directly as a result of the fact that curved spaces have non-flat metrics?

The following things did result from Riemann’s overall body of work on Riemannian manifolds:

1. Established that many geometries exist
2. That Riemann curvature and metric are intrinsic to a surface, the same way Gaussian curvature is.
3. It led to new branches of mathematics such as algebraic geometry, the Cartan and other connections, and the bundle and fiber concepts of the tangent space.

None of these proceeds directly from the fact that a metric itself can be conceived of as curved, geometrically. It seems to me that there must have been something additional that resulted. For example, did Riemann’s work enable arc lengths on manifolds to be calculated simply with a single integral of his metric? Using a Euclidean metric on portions of a curve on a curved space would be tedious, as each planar intersection segment (especially for a higher dimensional manifold) would have to be added to others to arrive at total arc length. Geometers other than Riemann in 1860 didn’t spend time on curves on higher dimensional manifolds, rendering my suggestion sort of moot. But if they had...

Or did the aforementioned discovery have a direct result on the calculation of complex curves - curves in complex spaces that use the imaginary number i? I mean would arc lengths not be the same for the curve on the manifold compared to that on a planar (or several) intersection(s)? This is an area I know nothing about, and requires expert mathematical advice to answer. I would obviously have to define which axes are imaginary and which are real first.

Is it better to view the concept of a curved metric simply as part of the greater picture, and not separated – put in context along with curved spaces, the curvature tensor, Christoffel symbols, and higher dimensional shapes? Then answer the question of how these changed the course of mathematics development.

• Several people knew about hyperbolic geometry, and spherical geometry, before Riemann: Gauss, Lobachevsky, Beltrami, ... May 15, 2022 at 2:34
• I know. But dimension 3 was as high as they went. Did they perform tedious arc lengtb calculations?.Specifically im asking if the result of any calculation, using differential geometry such as for volume, area or arc length, had a diffetent answer than before riemann. Or within the 19th century. Or if any calculation was greatly simpliied. May 15, 2022 at 3:00
• Much of your question appear to be due to some misconceptions about Riemannian geometry. My suggestion is to read a RG textbook first, say, do Carmo's "Riemannian Geometry." May 15, 2022 at 8:30
• One recent achievement is the proof of the Poincare conjecture due to Perelman. It is a pure topological statement, but Riemannian metric was used in the proof. May 15, 2022 at 14:01
• Thanks for your comments. I dont have the time (or skill) to read a whole math textbook. Would you say my two suggestions as to what followed are wrong? Ive heard of perelmans proof. Was his theorem about loops reducing to points related to gravitational energy and curvature of the universe? May 15, 2022 at 16:22