Energy = mass times the speed of light squared does this mean energy and mass are completely interchangable? Where ever I see energy can I just divide it by c^2 to get that system's mass value? Take for example two systems; In the first system there are two bodies of mass moving towards each other at 1 m/s. there is a reference particle, initially at rest and located at large distance (r) from the area at which these two bodies of mass impact each other. From the time both bodies of mass are away from each other, to the point at which they impact, the displacement of the reference particle is measured and recorded and the the displacement is divided by the time interval between those two events to determine the average gravitational acceleration by the two bodies of mass. In the second system there are two bodies of mass moving towards each other (of equal mass as the first system) but they are moving towards each other at a much greater rate - .99c. Will the gravitational acceleration of the reference particle (located at distance r) be different than in the first system? In other words, does the kinetic energy of any system effect it's gravitational field?