I want to understand why pressure creates gravity under GR (not how, why). What kind of pressure are we talking about here? Is this just the potential energy of pressure acting as mass? And the second part: I've been trying to model gravity as refraction of masses moving with constant velocity through a spacetime that has a varying density. Spacetime is more dense around masses, and other masses travelling through spacetime have their paths refracted due to the density changes, and so appear to fall "in" towards the mass causing the increase in density. Does this model of gravity concur with GR?