 One last item on the strong force answers the question I raised at the end of our segment on the atom. What holds the protons together in the nucleus? In 1934, a Japanese physicist, Ataki Yakawa, made the earliest attempt to explain the nature of the nuclear force. According to his theory, a particle was being shared between nucleons like molecules share electrons between atoms to bind them together. He even calculated the mass of this particle we now know as a pion. The shared particle is attached to both protons. The situation is similar to two people pulling on a ball. Each person exerts a force on the ball and the effect is as if each exerted a force on the other. Here's a two proton example of how we think it works. First, in one of the protons, an energetic gluon spontaneously creates a down quark-anti-down quark pair. Next the pion drifts into the other proton and the anti-down quark annihilates a down quark leaving the other down quark to take its place. The diameter of the proton is 1.662 femtometers. At a separation of less than a half a femtometer, the nuclear force is repulsive. This prevents nucleon collapse. It then becomes attractive over a short range, peaking at 1.3 femtometers, with a force much stronger than the electromagnetic repulsion. And it becomes negligible by around 3 femtometers where the electromagnetic repulsion takes over.