I understand that General Relativity is built on the Equivalence Principle. Apparently there is a approximation to Einstein's Field Equations called the GEM equations that are analogous to Maxwell's equations. They are valid for slowly moving particles far from gravitational sources. (http://en.wikipedia.org/wiki/Gravitomagnetism) Should the GEM equations also embody the Equivalence Principle somehow? Maybe one would also require some sort of cosmological boundary conditions as well as the GEM equations. My guess is that when one accelerates a mass, advanced gravitational waves from all the other masses in the Universe impinge on it providing the inertial reaction force. I believe that this effect can be described by the GEM equations. In order for this to provide enough reaction force one also requires that : G M / R = c^2 where M is the mass of the observable universe and R is its radius. This condition seems to be actually obeyed. It implies that the positive energy in each particle of mass m, +m c^ 2, is balanced by the negative gravitational potential energy between it and the rest of the Universe, -m c^2. Thus in total each particle costs the Universe zero energy!