Frame-dragging
Frame dragging also has an effect that if one particle is next to another, they can both save energy if one is absorbing energy as the other one is emitting it. (This can also increase the mass of both particles, using Einstein's famous equation that energy is equal to a certain amount of mass). This would encourage particles to group together, which would explain what strong force (a force that holds protons and neutrons in an atom nucleus together) is.