(using geometrized units, where G = c = 1 ). For r > R > 0 (where R is the radius of some mass shell), mass acts as a delta function at the origin. For r < R , shells of mass may exist externally, but for the metric to be non-singular at the origin, M must be zero in the metric. This reduces the metric to flat Minkowski space ; thus external shells have no gravitational effect.