© David Kaplan.
Quarks are bound together by the strong interaction. Bound quarks are constantly exchanging virtual gluons, which we can think of as forming a flux tube similar to a string stretching between the two particles. As the quarks are pulled apart, the potential energy between them grows, and is eventually equal to the amount of energy required to create two more quarks. At this point, the flux tube or string breaks, two more quarks are formed, and the quarks are once again bound in pairs. The string-like nature of the bond between quarks prompted theorists to think about what would happen if strings were fundamental quantum particles back in the 1960s. (Unit: 4)