The above description is classical, but there is no impediment to its extension to the quantum domain [2]. By (9), the traditional sum over s-paths in 4D can if so desired be replaced by a sum over l-paths in 5D. However, metrics like (9) still require that the 5D path S be minimized around zero. The condition for this is given by the 5D geodesic equation, which for (9) can conveniently be presented in terms of the equation of motion in spacetime and the equation of motion in the extra dimension: