Cartan structural equations

Elie Cartan was a French mathematician who made fundamental contributions in differential geometry and group theory.

Today I want to continue the discussion regarding curvature and talk about Cartan's modern approach to differential geometry. Cartan died in 1951, three years before Yang and Mills wrote their seminal paper on gauge theory and he did not see how his ideas are used by Nature in all the four fundamental interactions: gravity, electromagnetism, weak force, and strong force.

Last time I presented the Christoffel symbols and I noted that they depend on the coordinate system used. Riemann uncovered an intrinsic geometric object called Riemann curvature tensor which expressed in terms of the Cristoffel symbols is:

\( R^{\delta}_{\alpha \beta \gamma} = \partial_\alpha \Gamma^{\delta}_{\beta \gamma} - \partial_\beta \Gamma^{\delta}_{\alpha \gamma} + \Gamma^{\delta}_{\alpha \mu} \Gamma^{\mu}_{\beta \gamma} - \Gamma^{\delta}_{\beta \mu} \Gamma^{\mu}_{\alpha \gamma} \)

Now recall the exterior product: \( a \wedge b = a\otimes b -b \otimes a\) which is skew-symmetric: \( a\wedge b = - b \wedge a\) 

If we introduce Cartan's local connection forms:

\( \omega^\delta_\gamma = \Gamma^{\delta}_{\beta \gamma} dx^\beta \)

and compute \( d \omega^\delta_\gamma \) and \( \omega^\delta_\mu \wedge \omega^\mu_\gamma \) and compare the sum with the curvature equation in terms of \( \Gamma \) we get Cartan's local structural equation:

\( \Omega^\delta_\gamma = d \omega^\delta_\gamma + \omega^\delta_\mu \wedge \omega^\mu_\gamma \)

where \( \Omega^\delta_\gamma = \frac{1}{2} R^{\delta}_{\alpha \beta \gamma} d x^\alpha \wedge d x^\beta \)

In matrix form: \( A = \omega^\delta_\gamma \) is the local connection, and \( F = \Omega^\delta_\gamma \) is the local curvature.  

Cartan's local structural equation is:

\( F = d A + A \wedge A\)

which differentiated yields Bianchi's local identity:

\( d F = F \wedge A - A \wedge F \)

Now those two equations are a thing of maximal physical beauty. Sure, they look alien and vaguely interesting in this abstract mathematical form which Cartan derived, but inside them hides Maxwell's equations, Yang-Mills gauge theory, and Einstein's general relativity. In the next posts it will be our job to extract the physics from them.  

For example if \( A \) is the electromagnetic 4-potential because the electromagnetic gauge theory is abelian, \( A \wedge A = 0 \) and the two equations from above are nothing but Maxwell's equations as we will show later. Please stay tuned.