Matter, Fields, and Reparametrization-Invariant Systems
  We study reparametrization-invariant systems, mainly the relativistic particle and its D-dimensional extended object generalization--d-brane. The corresponding matter Lagrangians naturally contain background interactions, like electromagnetism and gravity. For a d-brane that doesn't alter the background fields, we define non-relativistic equations assuming integral sub-manifold embedding of the d-brane. The mass-shell constraint and the Klein-Gordon equation are shown to be universal when gravity-like interaction is present. Our approach to the Dirac equation follows Rund's technique for the algebra of the $\gamma $-matrices that doesn't rely on the Klein-Gordon equation. 
Vesselin G. Gueorguiev