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Principles of Physics

Strong force

by Marianne Moss Madsen, MS

Fields of Study

Quantum Electrodynamics; Quantum Field Theory; Quantum Mechanics; Relativity; String theory; Superstring theory

Summary

The strong force is one of the four fundamental forces (electromagnetic, gravitational, strong, and weak) that work throughout the universe. It is effective only over a very short range and only at the level of subatomic particles, but it is the strongest of these forces. It holds quarks and/or antiquarks, and thus protons and neutrons, close to each other, forming the basis of atoms and, by extension, everything in the universe.

Principal Terms

  • color force: the force of the strong interaction that operates on the quark level.

  • hadron: a subatomic particle that is made of either three quarks or one quark and one antiquark and held together by the strong force.

  • quantum chromodynamics: a quantum field theory that describes the interactions of quarks and gluons, subatomic particles that are responsible for the strong interaction.

  • quantum field theory: a theory that explains interactions between subatomic particles as the result of a field extending between them.

  • quark: an elementary fermion that combines with other quarks to form a baryon, such as a proton or neutron, or with an antiquark to form a particle called a meson.

  • strong interaction: the fundamental process of particle interaction that binds quarks into hadrons and hadrons into nuclei.

The Strong Force and Quarks

Before scientists understood quantum physics, they still understood that atoms were made of smaller particles. Back in the 1800s, electrons were noted as a part of an atom. Because electrons were negatively charged and removing an electron from an atom left it with a positive charge, scientists believed that atoms were held together with electromagnetic forces. Then experiments showed that atoms had a positively charged nucleus that contained both positively charged protons and neutrally charged neutrons and were surrounded by negatively charged electrons. Clearly, electrons and nuclei were held together electromagnetically, but what was holding the protons so closely together when, logically, their charges should make them repulse each other?

Scientists first thought that neutrons were functioning as some kind of glue. But, since neutrons have no charge, they theorized that there must be another force strong enough to overcome electromagnetic force. In the 1930s, Hideki Yukawa theorized that a new type of quantum particle existed that provided this strong interaction. Then in the 1960s, when Murray Gell-Mann and George Zweig began to promote their theory of fundamental particles called quarks, the strong force began to come into focus. In the Gell-Mann/Zweig model, quarks have electrical charges of 1/3 or 2/3 as well as possessing a color force. This theory sparked a new field of study they called quantum chromodynamics, part of the quantum field theory which explains the behavior of subatomic particles. The strong force is about 100 times stronger than electromagnetism. It acts directly only on quarks and gluons, the basic exchange particle that acts as a mediator force between quarks (and called gluons because they act like glue).

“Color” in this sense doesn’t refer to an actual color that one can see, but is a way to help one understand the way the strong force works. The strong force has three types of charges: Red, blue, and green, and three opposite charges: Anti-red, anti-blue, and anti-green. To make a strong neutral particle, such as a hadron, the “colors” must combine together to form a neutral, such as a particle made of a red, a green, and a blue quark or a particle made of a red quark and an anti-red quark. Because quarks are always exchanging gluons, they have color, but don’t have a specific color at any one time. This extremely complex dance of color between quarks means that one never sees a quark on its own—the strong force keeps quarks together in groups of two or three that are always color neutral. This is called “color confinement.” Color force only directly affects particles inside a hadron.

Even after decades of study, scientists are still learning things about the strong force, and any calculations involving them must be solved by supercomputers.

What’s Important about the Strong Force?

The strong force holds quarks, one of the basic particles that make up our universe, together. By understanding this force, we can have a better understanding of our universe as a whole. The breaking of the strong force bond is what releases energy when heat is generated in a nuclear power plant or when a nuclear weapon is detonated.

Citation Types

Type
Format
MLA 9th
Madsen, Marianne Moss. "Strong Force." Principles of Physics, edited by Donald R. Franceschetti, Salem Press, 2016. Salem Online, online.salempress.com/articleDetails.do?articleName=POP_0112.
APA 7th
Madsen, M. M. (2016). Strong force. In D. R. Franceschetti (Ed.), Principles of Physics. Salem Press. online.salempress.com.
CMOS 17th
Madsen, Marianne Moss. "Strong Force." Edited by Donald R. Franceschetti. Principles of Physics. Hackensack: Salem Press, 2016. Accessed December 14, 2025. online.salempress.com.