# Strong interaction

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Binding of quarks in subatomic particles

"Color force" redirects here. For the company, see [Color Force](/source/Color_Force).

An animation of [color confinement](/source/Color_confinement), a property of the strong interaction. If energy is supplied to the quarks as shown, the [gluon](/source/Gluon) tube connecting [quarks](/source/Quark) elongates until it reaches a point where it "snaps" and the energy added to the system results in the formation of a quark–[antiquark](/source/Antiquark) pair. Thus single quarks are never seen in isolation.

An animation of the strong interaction between a proton and a neutron, mediated by [pions](/source/Pion). The colored small double circles inside are [gluons](/source/Gluon).

In [nuclear physics](/source/Nuclear_physics) and [particle physics](/source/Particle_physics), the **strong interaction**, also called the **strong force** or **strong nuclear force**, is one of the four known [fundamental interactions](/source/Fundamental_interaction). It confines [quarks](/source/Quark) into [protons](/source/Proton), [neutrons](/source/Neutron), and other [hadron](/source/Hadron) particles, and also binds neutrons and protons to create atomic nuclei, where it is called the [nuclear force](/source/Nuclear_force).

Most of the [mass](/source/Mass%E2%80%93energy_equivalence) of a [proton](/source/Proton) or [neutron](/source/Neutron) is the result of the strong interaction energy; the individual quarks provide only about 1% of the mass of a proton. At the range of 10−15 m (1 [femtometer](/source/Femtometer), slightly more than the radius of a [nucleon](/source/Nucleon)), the strong force is approximately 100 times as strong as [electromagnetism](/source/Electromagnetism), 106 times as strong as the [weak interaction](/source/Weak_interaction), and 1038 times as strong as [gravitation](/source/Gravity).[1]

In the context of atomic nuclei, the force binds protons and neutrons together to form a nucleus and is called the [nuclear force](/source/Nuclear_force) (or *residual strong force*).[2] Because the force is mediated by massive, short lived [mesons](/source/Mesons) on this scale, the residual strong interaction obeys a distance-dependent behavior between nucleons that is quite different from when it is acting to bind quarks within hadrons. There are also differences in the [binding energies](/source/Binding_energy) of the nuclear force with regard to [nuclear fusion](/source/Nuclear_fusion) versus [nuclear fission](/source/Nuclear_fission). Nuclear fusion accounts for most energy production in the [Sun](/source/Sun) and other [stars](/source/Star). Nuclear fission allows for decay of radioactive elements and [isotopes](/source/Isotope), although it is often mediated by the weak interaction. Artificially, the energy associated with the nuclear force is partially released in [nuclear power](/source/Nuclear_power) and [nuclear weapons](/source/Nuclear_weapons), both in [uranium](/source/Uranium) or [plutonium](/source/Plutonium)-based fission weapons and in fusion weapons like the [hydrogen bomb](/source/Hydrogen_bomb).[3][4]

## History

Before 1971, physicists were uncertain as to how the atomic nucleus was bound together. It was known that the nucleus was composed of [protons](/source/Proton) and [neutrons](/source/Neutron) and that protons possessed positive [electric charge](/source/Electric_charge), while neutrons were electrically neutral. Based on established principles of electromagnetism, positive charges would be expected to repel one another and the positively charged protons would be expected to cause the nucleus to fly apart. However, this was never observed. New discoveries in physics were needed to explain this phenomenon.

A stronger attractive force was postulated to explain how the atomic nucleus was bound despite the protons' mutual [electromagnetic repulsion](/source/Electromagnetic_repulsion). This hypothesized force was called the *strong force*, which was believed to be a fundamental force that acted on the [protons and neutrons](/source/Nucleons) that make up the nucleus.

In 1964, [Murray Gell-Mann](/source/Murray_Gell-Mann), and separately [George Zweig](/source/George_Zweig), proposed that [baryons](/source/Baryon), which include protons and neutrons, and [mesons](/source/Meson) were composed of elementary particles. Zweig called the elementary particles "aces" while Gell-Mann called them "quarks"; the theory came to be called the [quark model](/source/Quark_model).[5] The strong attraction between nucleons was the side-effect of a more fundamental force that bound the quarks together into protons and neutrons. The theory of [quantum chromodynamics](/source/Quantum_chromodynamics) explains that quarks carry what is called a [color charge](/source/Color_charge), although it has no relation to visible color.[6] Quarks with unlike color charge attract one another as a result of the strong interaction, and the particle that mediates this was called the [gluon](/source/Gluon).

## Behavior of the strong interaction

The strong interaction is observable at two ranges, and mediated by different force carriers in each one. On a scale less than about 0.8 [fm](/source/Femtometre) (roughly the radius of a nucleon), the force is carried by [gluons](/source/Gluon) and holds [quarks](/source/Quark) together to form protons, neutrons, and other hadrons. On a larger scale, up to about 3 fm, the force is carried by [mesons](/source/Meson) and binds nucleons ([protons](/source/Proton) and [neutrons](/source/Neutron)) together to form the [nucleus](/source/Atomic_nucleus) of an [atom](/source/Atom).[2] In the former context, it is often known as the **color force**, and is so strong that if hadrons are struck by high-energy particles, they produce [jets](/source/Jet_(particle_physics)) of massive particles instead of emitting their constituents (quarks and gluons) as freely moving particles. This property of the strong force is called [color confinement](/source/Color_confinement).

Two layers of strong interaction Interaction range held carrier result Strong < 0.8 fm quark gluon hadron Residual Strong 1–3 fm hadron meson nucleus

### Within hadrons

The fundamental [couplings](/source/Coupling_(physics)) of the strong interaction, from left to right: (a) gluon radiation, (b) gluon splitting and (c,d) gluon self-coupling.

The word *strong* is used since the strong interaction is the "strongest" of the four fundamental forces. At a distance of 10−15 m, its strength is around 100 times that of the [electromagnetic force](/source/Electromagnetic_force), some 106 times as great as that of the weak force, and about 1038 times that of [gravitation](/source/Gravitation).

The strong force is described by [quantum chromodynamics](/source/Quantum_chromodynamics) (QCD), a part of the [Standard Model](/source/Standard_Model) of particle physics. Mathematically, QCD is a non-abelian [gauge theory](/source/Gauge_theory) based on a local (gauge) [symmetry group](/source/Symmetry_group) called [SU(3)](/source/SU(3)).

The force carrier particle of the strong interaction is the gluon, a massless [gauge boson](/source/Gauge_boson). Gluons are thought to interact with quarks and other gluons by way of a type of charge called [color charge](/source/Color_charge). Color charge is analogous to electromagnetic charge, but it comes in three types (±red, ±green, and ±blue) rather than one, which results in different rules of behavior. These rules are described by [quantum chromodynamics](/source/Quantum_chromodynamics) (QCD), the theory of quark–gluon interactions. Unlike the [photon](/source/Photon) in electromagnetism, which is neutral, the gluon carries a color charge. Quarks and gluons are the only fundamental particles that carry non-vanishing color charge, and hence they participate in strong interactions only with each other. The strong force is the expression of the gluon interaction with other quark and gluon particles.

All quarks and gluons in QCD interact with each other through the strong force. The strength of interaction is parameterized by the strong [coupling constant](/source/Coupling_constant). This strength is modified by the gauge color charge of the particle, a [group-theoretical](/source/Group_theory) property.

The strong force acts between quarks. Unlike all other forces (electromagnetic, weak, and gravitational), the strong force does not diminish in strength with increasing distance between pairs of quarks. After a limiting distance (about the size of a [hadron](/source/Hadron)) has been reached, it remains at a strength of about 10000 [N](/source/Newton_(unit)), no matter how much farther the distance between the quarks.[7]: 164 As the separation between the quarks grows, the energy added to the pair creates new pairs of matching quarks between the original two; hence it is impossible to isolate quarks. The explanation is that the amount of work done against a force of 10000 N is enough to create particle–antiparticle pairs within a very short distance. The energy added to the system by pulling two quarks apart would create a pair of new quarks that will pair up with the original ones. In QCD, this phenomenon is called [color confinement](/source/Color_confinement); as a result, only hadrons, not individual free quarks, can be observed. The failure of all experiments that have searched for [free quarks](/source/Free_quark) is considered to be evidence of this phenomenon.

The elementary quark and gluon particles involved in a high energy collision are not directly observable. The interaction produces jets of newly created hadrons that are observable. Those hadrons are created, as a manifestation of mass–energy equivalence, when sufficient energy is deposited into a quark–quark bond, as when a quark in one proton is struck by a very fast quark of another impacting proton during a [particle accelerator](/source/Particle_accelerator) experiment. However, [quark–gluon plasmas](/source/Quark%E2%80%93gluon_plasma) have been observed.[8]

### Between hadrons

Main article: [Nuclear force](/source/Nuclear_force)

A [Feynman diagram](/source/Feynman_diagram) (shown by the animation in the lead) with the individual [quark](/source/Quark) constituents shown, to illustrate how the fundamental strong interaction gives rise to the [nuclear force](/source/Nuclear_force). Straight lines are quarks, while multi-colored loops are [gluons](/source/Gluon) (the carriers of the fundamental force).

While color confinement implies that the strong force acts without distance-diminishment between pairs of quarks in compact collections of bound quarks (hadrons), at distances approaching or greater than the radius of a proton, a residual force (described below) remains. It manifests as a force between the "colorless" hadrons, and is known as the *[nuclear force](/source/Nuclear_force)* or *residual strong force* (and historically as the *strong nuclear force*).

The nuclear force acts between hadrons, known as [mesons](/source/Meson) and [baryons](/source/Baryon). This "residual strong force", acting indirectly, transmits gluons that form part of the virtual [π](/source/Pion) and [ρ mesons](/source/Rho_meson), which, in turn, transmit the force between nucleons that holds the nucleus (beyond [hydrogen-1](/source/Hydrogen-1) nucleus) together.[9]

The residual strong force is thus a minor residuum of the strong force that binds quarks together into protons and neutrons. This same force is much weaker *between* neutrons and protons, because it is mostly neutralized *within* them, in the same way that electromagnetic forces between neutral atoms ([van der Waals forces](/source/Van_der_Waals_force)) are much weaker than the electromagnetic forces that hold electrons in association with the nucleus, forming the atoms.[7]

Unlike the strong force, the residual strong force diminishes with distance, and does so rapidly. The decrease is approximately as a negative exponential power of distance, though there is no simple expression known for this; see *[Yukawa potential](/source/Yukawa_potential)*. The rapid decrease with distance of the attractive residual force and the less rapid decrease of the repulsive electromagnetic force acting between protons within a nucleus, causes the instability of larger atomic nuclei, such as all those with [atomic numbers](/source/Atomic_number) larger than 82 (the element lead).

Although the nuclear force is weaker than the strong interaction itself, it is still highly energetic: transitions produce [gamma rays](/source/Gamma_ray).[*[clarification needed](https://en.wikipedia.org/wiki/Wikipedia:Please_clarify)*] The mass of a nucleus is significantly different from the summed masses of the individual nucleons. This [mass defect](/source/Mass_defect) is due to the potential energy associated with the nuclear force. Differences between mass defects power [nuclear fusion](/source/Nuclear_fusion) and [nuclear fission](/source/Nuclear_fission).

## Unification

The so-called [Grand Unified Theories](/source/Grand_Unified_Theory) (GUT) aim to describe the strong interaction and the electroweak interaction as aspects of a single force, similarly to how the electromagnetic and weak interactions were unified by the Glashow–Weinberg–Salam model into [electroweak interaction](/source/Electroweak_interaction). The strong interaction has a property called [asymptotic freedom](/source/Asymptotic_freedom), wherein the strength of the strong force diminishes at higher energies (or temperatures). The theorized energy where its strength becomes equal to the electroweak interaction is the [grand unification energy](/source/Grand_unification_energy). However, no Grand Unified Theory has yet been successfully formulated to describe this process, and Grand Unification remains an [unsolved problem in physics](/source/List_of_unsolved_problems_in_physics).

If GUT is correct, after the [Big Bang](/source/Big_Bang) and during the [electroweak epoch](/source/Electroweak_epoch) of the universe, the [electroweak force](/source/Electroweak_interaction) separated from the strong force. Accordingly, a [grand unification epoch](/source/Grand_unification_epoch) is hypothesized to have existed prior to this.

## See also

- [Physics portal](https://en.wikipedia.org/wiki/Portal:Physics)

- [Mathematical formulation of quantum mechanics](/source/Mathematical_formulation_of_quantum_mechanics)

- [Mathematical formulation of the Standard Model](/source/Mathematical_formulation_of_the_Standard_Model)

- [Nuclear binding energy](/source/Nuclear_binding_energy)

- [QCD matter](/source/QCD_matter)

- [Quantum field theory](/source/Quantum_field_theory)

- [Yukawa interaction](/source/Yukawa_interaction)

## References

1. **[^](#cite_ref-1)** Relative strength of interaction varies with distance. See for instance [Matt Strassler](/source/Matt_Strassler)'s essay, ["The strength of the known forces"](http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/).

1. ^ [***a***](#cite_ref-auto_2-0) [***b***](#cite_ref-auto_2-1) ["The four forces: the strong interaction Duke University Astrophysics Dept website"](http://webhome.phy.duke.edu/~kolena/modern/forces.html#005).

1. **[^](#cite_ref-3)** Ragheb, Magdi. ["Chapter 4 Nuclear Processes, The Strong Force"](https://web.archive.org/web/20121218195010/https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Nuclear%20Processes%20The%20Strong%20Force.pdf) (PDF). University of Illinois. Archived from [the original](https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Nuclear%20Processes%20The%20Strong%20Force.pdf) (PDF) on 2012-12-18. Retrieved 2023-10-03.

1. **[^](#cite_ref-4)** ["Lesson 13: Binding energy and mass defect"](https://web.archive.org/web/20230528145648/http://www.furryelephant.com/content/radioactivity/binding-energy-mass-defect/). *Furry Elephant physics educational site*. Archived from [the original](http://www.furryelephant.com/content/radioactivity/binding-energy-mass-defect/) on 2023-05-28. Retrieved 2023-10-03.

1. **[^](#cite_ref-5)** Wilczek, Frank (1982). "Quantum chromodynamics: The modern theory of the strong interaction". *Annual Review of Nuclear and Particle Science*. **32** (1): 177–209. [Bibcode](/source/Bibcode_(identifier)):[1982ARNPS..32..177W](https://ui.adsabs.harvard.edu/abs/1982ARNPS..32..177W). [doi](/source/Doi_(identifier)):[10.1146/annurev.ns.32.120182.001141](https://doi.org/10.1146%2Fannurev.ns.32.120182.001141).

1. **[^](#cite_ref-6)** [Feynman, R.P.](/source/Richard_Feynman) (1985). [*QED: The Strange Theory of Light and Matter*](/source/QED%3A_The_Strange_Theory_of_Light_and_Matter). Princeton University Press. p. 136. [ISBN](/source/ISBN_(identifier)) [978-0-691-08388-9](https://en.wikipedia.org/wiki/Special:BookSources/978-0-691-08388-9). The idiot physicists, unable to come up with any wonderful Greek words anymore, call this type of polarization by the unfortunate name of 'color', which has nothing to do with color in the normal sense.

1. ^ [***a***](#cite_ref-Fritzsch1983_7-0) [***b***](#cite_ref-Fritzsch1983_7-1) Fritzsch, H. (1983). [*Quarks: The Stuff of Matter*](https://archive.org/details/quarksstuffofmat00frit). Basic Books. pp. [167–168](https://archive.org/details/quarksstuffofmat00frit/page/167). [ISBN](/source/ISBN_(identifier)) [978-0-465-06781-7](https://en.wikipedia.org/wiki/Special:BookSources/978-0-465-06781-7).

1. **[^](#cite_ref-8)** ["Quark–gluon plasma is the most primordial state of matter"](https://web.archive.org/web/20170118030724/http://physics.about.com/od/physicsqtot/fl/Quark-Gluon-Plasma.htm). *About.com Education*. Archived from [the original](http://physics.about.com/od/physicsqtot/fl/Quark-Gluon-Plasma.htm) on 2017-01-18. Retrieved 2017-01-16.

1. **[^](#cite_ref-9)** ["3. The Strong Force"](https://web.archive.org/web/20211022113145/http://www.damtp.cam.ac.uk/user/tong/pp/pp3.pdf) (PDF). Department of Applied Mathematics and Theoretical Physics, University of Cambridge. Archived from [the original](http://www.damtp.cam.ac.uk/user/tong/pp/pp3.pdf) (PDF) on 22 October 2021. Retrieved 10 January 2023.

## Further reading

- Christman, J.R. (2001). ["MISN-0-280: *The Strong Interaction*"](https://physnet.org/modules/pdf_modules/m280.pdf) (PDF).

- Ding, Minghui; Roberts, Craig; Schmidt, Sebastian, eds. (2024). [*Strong Interactions in the Standard Model: Massless Bosons to Compact Stars*](https://www.mdpi.com/books/reprint/9524-strong-interactions-in-the-standard-model-massless-bosons-to-compact-stars). [MDPI](/source/MDPI). [ISBN](/source/ISBN_(identifier)) [978-3-7258-1502-9](https://en.wikipedia.org/wiki/Special:BookSources/978-3-7258-1502-9). {{[cite book](https://en.wikipedia.org/wiki/Template:Cite_book)}}: |journal= ignored ([help](https://en.wikipedia.org/wiki/Help:CS1_errors#periodical_ignored))

- [Halzen, F.](/source/Francis_Halzen); [Martin, A.D.](/source/Alan_Martin_(physicist)) (1984). [*Quarks and Leptons: An Introductory Course in Modern Particle Physics*](https://archive.org/details/quarksleptonsint0000halz). John Wiley & Sons. [ISBN](/source/ISBN_(identifier)) [978-0-471-88741-6](https://en.wikipedia.org/wiki/Special:BookSources/978-0-471-88741-6).

- [Kane, G.L.](/source/Gordon_L._Kane) (1987). *Modern Elementary Particle Physics*. Perseus Books. [ISBN](/source/ISBN_(identifier)) [978-0-201-11749-3](https://en.wikipedia.org/wiki/Special:BookSources/978-0-201-11749-3).

- Morris, R. (2003). [*The Last Sorcerers: The Path from Alchemy to the Periodic Table*](https://archive.org/details/lastsorcererspat0000morr). Joseph Henry Press. [ISBN](/source/ISBN_(identifier)) [978-0-309-50593-2](https://en.wikipedia.org/wiki/Special:BookSources/978-0-309-50593-2).

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