The Strong Force Carrier

With the discovery of the strong force, one could finally explain why the nucleus of an atom did not fly apart. Hideki Yukawa (The Nobel Prize in Physics 1949) proposed that the strong force between protons and neutrons in the nucleus occured through the exchange of pi (p) mesons (q q-bar particles). However, in the more modern view, there is a problem. The strong force is only supposed to affect particles with "color" charge, and protons and neutrons are "color" neutral.

Physicists realized that mesons and baryons, such as neutrons and protons in the nucleus can overlap. So while the "color" neutral protons and neutrons do not interact with each other, their quarks do. The strong force occurs through the exchange of gluons between quarks in different protons and neutrons. This process "glues" the nucleus together. It is similar to how neutral atoms bind together to make molecules.