Of the Exterior Calculus and Relativistic Quantum Mechanics

In 1960-62, E. K\"ahler developed what looks as a generalization of the exterior calculus, which he based on Clifford rather than exterior algebra. The role of\ the exterior derivative, $du$, was taken by the more comprehensive derivative $\partial u$ ($\equiv dx^{\mu }\vee d_{\mu }u$), where `$\vee $' stands for Clifford product. The $d_{\mu }u$ represents a set of quantities to which he referred as covariant derivative, and for which he gave a long, ad hoc expression. We provide the geometric foundation for this derivative, based on Cartan's treatment of the structure of a Riemannian differentiable manifold without resort to the concept of the so called affine connections. Buried at advanced points in his presentations is the implied statement that $\partial u=du+\ast ^{-1}d$ $u\ast $, the sign at the front of the coderivative term is a matter of whether we include the unit imaginary or not in the definition of Hodge dual, $\ast $. We extract and put together the pieces of theory that go into his derivation of that statement, which seems to have gone unnoticed in spite of its relevance for a quick understanding of what his `K\"ahler calculus'. K\"ahler produced a most transparent, compelling and clear formulation of relativistic quantum mechanics (RQM) as a virtual concomitant of his calculus. We shall enumerate several of its notable features, which he failed to emphasize. The exterior calculus in K\"ahler format thus reveals itself as the computational tool for RQM, making the Dirac calculus unnecessary and its difficulties spurious.