I was thinking the other day, and came up with a scenario. Suppose you have two particles. The mass does not matter, but the distance between them is small enough for a gravitational field from one to have an effect on the other, but large enough that they do not immediately collide. Since they are separated, and in a gravitational field, they have a certain amount of potential energy given by, I believe, U=mgh. Suppose then that an anti-particle collides with one of the particles, completely annihilating both the particle and the antiparticle. Before the destruction of the particle, there was still a distance, still a mass, and still a gravitational field, so the other particle still has some potential energy. After the collision there is no gravitational field, so g=0 and the potential energy is 0. So where did this energy go? I asked my physics teacher, but he didn't know. Any thoughts?