Some intuition for the formula above can be obtained as follows. Imagine that that we take the surface, S, to be a spherical surface so that the normal points radially outwards. At large distances from the source of the energy, r, the tensor h i j is expected to fall off as r − − 1 and the derivative with respect to r converts this into r − − 2 The area of the sphere at large radius also grows precisely as r 2 and therefore one obtains a finite value for the energy.