I asked a less-well-posed version of this question here on the phyics stackexchange; however I believe the answers I recieved to be wrong. Please help enlighten me!

For those not aquainted, in General Relativity we consider spacetime as a 4-dimensional pseudo-Riemannian manifold with metric tensor components $g_{munu}$. There is an equivalent tetrad formulation of general relativity, wherein one utilizes (an often orthonormal) basis. In this case the metric tensor is written as:

$$g_{munu}=e_{mu}^{a}e_{nu}^{b}eta_{ab}$$

Where $eta_{ab}$ are components of the flat Minkowskian metric. In this case, the Greek and Latin indices refer to coordinate and orthonormal (generally non-coordinate) basis respectivly. One case use the tetrads to take quantities from coordinate to orthonormal indices. This formulation is extremely important, for example when using spinor fields in General relativity.

From a purely mathematical perspective, I perceive the tetrad formulation as a map from a spacetime manifold $M$ to Minkowskian spacetime $eta$.

$Mrightarroweta$

In which case it is rather natural that the spacetime metric tensor becomes the pullback of the metric from $eta$. Am I right in viewing it this way? The main arguments against this viewpoint on the physics stackexhange were:

“The tetrad formalism is not related to the pullback operation. The

similarity you point out has more to do with the statement that both

$g_{munu}=frac{partial X^{a}}{partial x^{mu}}frac{partial

> X^{b}}{partial x^{nu}}g_{ab}$ and

$g_{munu}=e_{mu}^{a}e_{nu}^{b}eta_{ab}$ transform covariantly

under diffeomorphisms (which is a pullback of the spacetime to itself

via some diffeomorphism, if you prefer that language).”

However; in reading about the pullback it would seem that this argument only applies to the pullback by a diffeomorphism (see section in link). The pullback is a much more robust operation than this.

So I’m asking the mathematicians now: Can I regard the tetrad formulation as a map from a spacetime manifold to Minkowskian space which implies the spacetime metric tensor is the pullback? I do understand there are issues with pulling back contravariant tensor fields, but we can just stick with differential form versions if preferred.

Also I’ll add that in a great book on General relativity “Formulations of General Relativity Gravitry, Spinors and differential forms” by Krasnov. Section 3.1.3 says:

“given a tetrad, the metric on M is defined to be the pullback of the

metric on E $$g_{munu}=e_{mu}^{a}e_{nu}^{b}eta_{ab}$$ “

Where Krasnov is referring to the base and fiber metric, by M and E respectively.