 The divergence of the non-gravitational stress–energy is zero.In other words, non-gravitational energy and momentum are conserved,
When gravity is negligible and using a Cartesian coordinate system for spacetime, this may be expressed in terms of partial derivatives as 

The integral form of this is

where "N" is any compact four-dimensional region of spacetime; formula_16 is its boundary, a three-dimensional hypersurface; and formula_17 is an element of the boundary regarded as the outward pointing normal.In flat spacetime and using Cartesian coordinates, if one combines this with the symmetry of the stress–energy tensor, one can show that angular momentum is also conserved