I was thinking about a theoretical situation where you have two particles with some mass inside a vacuum and you were trying to model gravity between them. If they were both at rest, therefore 0 kinetic energy and then gravity acted upon them whether its through tensors in GR or gravitons in QFT they would have to increase the kinetic energy of the particles since once gravity acted upon them they would start to move a bit. But if there was no kinetic energy before, where did the energy come from that caused the particles to start moving? was it from their rest energy(mass)? and wouldn't that imply that the particles would be getting lighter over time? thanks in advance for any helpful replies.