The strong force is one of the four fundamental forces of nature, along with gravity, electromagnetism, and the weak force. It is the strongest of these four and also the one with the shortest range. The strong force is the interaction responsible for binding quarks and gluons to make hadrons.
Residual strong interactions provide the nuclear binding energy that holds protons and neutrons together in the atomic nucleus; in nuclear physics, the term strong interaction is also used for this residual effect. (As a parallel, the force between electrically charged particles is an electromagnetic interaction, the force between neutral atoms that leads to the formation of molecules is a residual electromagnetic effect.)
Scientists began hypothesizing the presence of a strong force in the mid-1930s, when they learned that atoms are made up of electrons orbiting a nucleus of protons and neutrons. Theorizing that electromagnetic repulsion between the protons should blow the nucleus apart, they posited the existence of another force much stronger than electromagnetism to glue the nucleus together. In the 1960s, with the discovery of quarks (the subatomic particles that make up protons and neutrons), the argument shifted to how quarks stick together. In 1973, David Gross (1941–), Frank Wilczek (1951–), and David Politzer (1949–) published two papers solving a key part of the mystery. They realized that, unlike other forces, the strong force appears to become stronger with distance; the force between two quarks in close proximity is so weak that they behave as if they were free particles, but as they move apart the force between them increases as if they are connected by an elastic band. The team won the 2004 Nobel Prize in Physics for this discovery.
The strong force may act only within a minuscule space but it is now understood to be the most powerful of all forces. In fact, at these distances, scientists calculate that it is 100 times stronger than the next strongest force, electromagnetism.