Also known as the strong interaction, this is the powerful but short-range force that holds protons and neutrons together in the nucleus of an atom despite the electromagnetic repulsion of same-chargeparticles, as well as holding together the constituent quarks which comprise neutrons and protons. It is one of the four fundamental forces of physics (along with the gravitational force, the electromagnetic force and weak nuclear force), and the most powerful, being 100 times the strength of the electromagnetic force, about 1013 times as great as that of the weak force and about 1038 times that of gravity. The force is mediated by elementary particles called gluons which shuttle back and forth between the particles being operated on and "glue" the particles together. Unlike the other forces, the strength of the strong force between quarks becomes stronger with distance, acting like an unbreakable elastic thread. However, it only operates over a very small distance (less than the size of the nucleus), outside of which it fades away abruptly.