I’ve just read Schutz: “Gravitation from ground up”. He says that there are 4 (not all independent) sources of gravity: 1. density of active gravitational mass = (rho + 3 P) 2. active curvature mass (generating spacial curvature) = (rho - P/c^2) 3. ordinary momentum (gravito-magnetism) 4. Space-time curvature itself -> manifestation in non-linearity of EF I’m happy with 1. 3. and 4. But don’t know what he means with 2. Does anyone know what this means? If you answer, please consider that my (all self-taught) level is not above e.g Schutz: “A first course in GR”.