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# 3: Differential Geometry

Differential geometry uses the techniques of differential calculus, integral calculus, linear algebra and multilinear algebra to study problems in geometry.

• 3.0: Introduction to Differential Geometry
General relativity is described mathematically in the language of differential geometry. Let’s take those two terms in reverse order.  The geometry of spacetime is non-Euclidean.
• 3.1: Tangent Vectors
The freshman physics notion of a vector carries all kinds of baggage, including ideas like rotation of vectors and a magnitude that is positive for nonzero vectors. We also used to assume the ability to represent vectors as arrows, i.e., geometrical figures of finite size that could be transported to other places — but in a curved geometry, it is not in general possible to transport a figure to another location without distorting its shape, so there is no notion of congruence.
• 3.2: Affine Notions and Parallel Transport
We want to be able to measure things in curved spacetime. There turn out to be two complementary systems of measurement we can apply: affine measure and metric measure.
• 3.3: Models
A typical first reaction to the phrase “curved spacetime” — or even “curved space,” for that matter — is that it sounds like nonsense. How can featureless, empty space itself be curved or distorted?
• 3.4: Intrinsic Quantities
Models can be dangerous, because they can tempt us to impute physical reality to features that are purely extrinsic, i.e., that are only present in that particular model. This is as opposed to intrinsic features, which are present in all models, and which are therefore logically implied by the axioms of the system itself. The existence of lunes is clearly an intrinsic feature of non-Euclidean geometries, because intersection of lines was defined before any model has even been proposed.
• 3.5: The Metric (Part 1)
The purely affine notion of vectors and their duals is not enough to define the length of a vector in general; it is only sufficient to define a length relative to other lengths along the same geodesic. When vectors lie along different geodesics, we need to be able to specify the additional conversion factor that allows us to compare one to the other. The piece of machinery that allows us to do this is called a metric.
• 3.5: The Metric (Part 2)
The set of all transformations that can be built out of successive translations, rotations, and reflections is called the group of isometries. It can also be defined as the group that preserves dot products, or the group that preserves congruence of triangles.
• 3.6: The Metric in General Relativity
When masses are present, finding the metric is analogous to finding the electric field made by charges, but the interpretation is more difficult. In the electromagnetic case, the field is found on a preexisting background of space and time. In general relativity, there is no preexisting geometry of spacetime. The metric tells us how to find distances in terms of our coordinates, but the coordinates themselves are completely arbitrary.
• 3.7: Interpretation of Coordinate Independence
This section discusses some of the issues that arise in the interpretation of coordinate independence. It can be skipped on a first reading.
• 3.E: Differential Geometry (Exercises)

Thumbnail: A triangle immersed in a saddle-shape plane (a hyperbolic paraboloid), as well as two diverging ultraparallel lines. Image used with permission (Public Domain; LucasVB).