"non-stationary" means in an expanding universe ? http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ I didn't get this part. For the system in my example of two massive particles, can we not say that the density of gravitational energy decreases as the particles move away from each other ? maybe I didn't get what "density" of gravitational energy means. I didn't get what "matter field" actually means, is it like saying (with respect to my example) kinetic energy is positive and gravitational potential energy is negative so they cancel each other out and so energy is conserved ?