Generalized Gauss' Law
Covers both gravity and EM.
Charge densities equal 1/2 divergences of E and e + time variation of g.
Implications for forces: Newton's field law implies an attractive force for mass, while Gauss' law indicates like electric charges repulse.
- Gauss' law results if no mass density and no change in the field e
Newton's [relativistic] gravitational field equation results if no electric charge density or field.