Einstein’s General Relativity predicts that pressure, in general stresses, play a similar role to energy density, ǫ = ρc (with ρ being corresponding mass density), in generating gravity. The source of gravitational field, the active gravitational mass density, sometimes referred to as Whittaker’s mass density, is ρgrav = ρ + 3p/c , where p is pressure in the case of an ideal fluid. Whittaker’s ...