Cartan Formalism (physics) - Constructions

Constructions

A (pseudo-)Riemannian metric is defined over M as the pullback of η by e. To put it in other words, if we have two sections of TM, X and Y,

g(X,Y) = η(e(X), e(Y)).

A connection over V is defined as the unique connection A satisfying these two conditions:

• dη(a,b) = η(d_Aa,b) + η(a,d_Ab) for all differentiable sections a and b of V (i.e. d_Aη = 0) where d_A is the covariant exterior derivative. This implies that A can be extended to a connection over the SO(p,q) principal bundle.
• d_Ae = 0. The quantity on the left hand side is called the torsion. This basically states that defined below is torsion-free. This condition is dropped in the Einstein-Cartan theory, but then we cannot define A uniquely anymore.

This is called the spin connection.

Now that we have specified A, we can use it to define a connection ∇ over TM via the isomorphism e:

e(∇X) = d_Ae(X) for all differentiable sections X of TM.

Since what we now have here is a SO(p,q) gauge theory, the curvature F defined as is pointwise gauge covariant. This is simply the Riemann curvature tensor in a different form.

An alternate notation writes the connection form A as ω, the curvature form F as Ω, the canonical vector-valued 1-form e as θ, and the exterior covariant derivative as D.