Metric connection

In mathematics, a metric connection is a connection in a vector bundle E equipped with a bundle metric; that is, a metric for which the inner product of any two vectors will remain the same when those vectors are parallel transported along any curve.^[1] This is equivalent to:

A connection for which the covariant derivatives of the metric on E vanish.
A principal connection on the bundle of orthonormal frames of E.

A special case of a metric connection is a Riemannian connection; there exists a unique such connection which is torsion free, the Levi-Civita connection. In this case, the bundle E is the tangent bundle TM of a manifold, and the metric on E is induced by a Riemannian metric on M.

Another special case of a metric connection is a Yang–Mills connection, which satisfies the Yang–Mills equations of motion. Most of the machinery of defining a connection and its curvature can be worked through without requiring any compatibility with the bundle metric. However, once one does require compatibility, this metric connection defines an inner product, Hodge star (which additionally needs a choice of orientation), and Laplacian, which are required to formulate the Yang–Mills equations.

^ Jost, Jürgen (2011), Riemannian geometry and geometric analysis (PDF), Universitext (Sixth ed.), Springer, Heidelberg, doi:10.1007/978-3-642-21298-7, ISBN 978-3-642-21297-0, MR 2829653.(Third edition: see chapter 3; Sixth edition: see chapter 4.)

[jost-1] Jost, Jürgen (2011), Riemannian geometry and geometric analysis (PDF), Universitext (Sixth ed.), Springer, Heidelberg, doi:10.1007/978-3-642-21298-7, ISBN 978-3-642-21297-0, MR 2829653.(Third edition: see chapter 3; Sixth edition: see chapter 4.)

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