Strong interaction
The strong interaction, one of the four fundamental forces of nature alongside gravity, electromagnetism, and the weak interaction, is responsible for binding quarks together to form hadrons such as protons and neutrons, and for holding atomic nuclei together through its residual effects.[1][2] Described by quantum chromodynamics (QCD), a non-Abelian gauge theory based on the SU(3)c color symmetry group, it acts exclusively on particles carrying "color charge," a property analogous to electric charge but with three types (red, green, blue) and their anticolors.[1] Quarks, the fundamental constituents of matter, interact via the exchange of gluons—eight massless gauge bosons that themselves carry color charge, leading to self-interactions among gluons unlike in electromagnetism.[1] This force dominates at subnuclear scales, with its intrinsic strength characterized by the running coupling constant αs, which has a value of approximately 0.118 at the Z boson mass scale.[1] A defining feature of the strong interaction is asymptotic freedom, where the coupling strength decreases at short distances or high energies (below ~10-15 m or above ~200 MeV), allowing perturbative quantum field theory calculations for processes like deep inelastic scattering.[1] In contrast, at larger distances or low energies, the interaction exhibits confinement, preventing free quarks or gluons from existing in isolation; instead, they are perpetually bound into color-neutral hadrons, explaining why quarks are never observed singly in experiments.[1] QCD, formulated in the early 1970s, successfully predicts these behaviors through its Lagrangian, which includes quark kinetic terms, gluon field strengths, and Yukawa-like quark-gluon couplings, with no additional free parameters beyond αs and quark masses (the latter arising from electroweak interactions).[1] The residual strong interaction between composite hadrons, such as protons and neutrons (collectively nucleons), arises from the underlying quark-gluon dynamics and is mediated primarily by the exchange of light mesons like pions.[2] This nuclear force has a limited range of about 1.5 × 10-15 m (1.5 femtometers), comparable to nuclear diameters, due to the finite mass of the pion (~135 MeV/c²), and is attractive at distances of 0.5–2 fm while becoming repulsive at closer ranges to prevent nucleon overlap.[3] Relative to other forces, its coupling between protons is roughly 10 times stronger than the electromagnetic repulsion (coupling constant ~2.5 × 10-27 J·m versus 2.31 × 10-28 J·m), enabling stable nuclei despite positive charges, and vastly exceeds the weak force (~104 times stronger) and gravity (~1037 times stronger).[3] Charge independence approximately holds, treating protons and neutrons similarly, though small differences arise from electromagnetic corrections and quark mass asymmetries.[2] Beyond atomic nuclei, the strong interaction governs high-energy phenomena like jet production in particle collisions and the quark-gluon plasma state achieved in heavy-ion experiments at facilities such as the LHC.[1] Its non-perturbative aspects, including hadronization and nuclear structure, are studied via lattice QCD simulations on supercomputers, confirming predictions like the proton's mass (~938 MeV) being almost entirely due to QCD binding energy rather than quark masses.[1] Ongoing research explores connections to chiral symmetry breaking, where the near-masslessness of up and down quarks leads to Goldstone bosons (pions) as effective degrees of freedom at low energies.[1]Fundamentals
Definition and characteristics
The strong interaction, also known as the strong nuclear force, is one of the four fundamental forces of nature, governing the behavior of subatomic particles at the smallest scales. It is the mechanism that binds quarks together to form hadrons, such as protons and neutrons, and is mediated by massless gauge bosons called gluons. Unlike the electromagnetic force, which acts on electrically charged particles, the strong interaction exclusively affects particles that carry a property known as color charge—quarks possess one of three types (red, green, or blue), while antiquarks carry anticolors, and gluons carry both a color and an anticolor, enabling self-interactions.[4][5] Key characteristics of the strong interaction include its extreme strength and limited range. It is the most powerful of the fundamental forces, approximately 100 times stronger than the electromagnetic interaction at short interquark distances. The force operates over a very short range of about $10^{-15} meters (1 femtometer), comparable to the size of atomic nuclei, beyond which it drops off rapidly due to the massive effective mediators in the residual form and confinement effects at the quark level. The interaction is always attractive between color-charged particles in configurations that form color-neutral bound states, such as quark-antiquark pairs or three-quark combinations, but it exhibits non-central (tensor and spin-dependent) components and velocity-dependent behavior arising from relativistic effects and gluon exchanges.[6][7][8] In nature, the strong interaction plays a crucial role in the structure of matter by confining quarks within hadrons, preventing free quarks from existing in isolation, and thus ensuring the stability of protons and neutrons. Additionally, a residual strong interaction emerges between these color-neutral hadrons, manifesting as the nuclear force that binds protons and neutrons together in atomic nuclei, overcoming electromagnetic repulsion despite its much weaker effective strength at that scale. At short interquark distances, the potential energy between a quark and an antiquark can be approximated perturbatively as a Coulomb-like form: V(r) \approx -\frac{4}{3} \frac{\alpha_s}{r}, where r is the separation, and \alpha_s is the strong coupling constant, analogous to the fine-structure constant in electromagnetism but running with energy scale.[2][4]Comparison with other fundamental forces
The strong interaction, also known as the strong nuclear force, is the most powerful of the four fundamental forces, dominating interactions at subatomic scales within atomic nuclei, while the electromagnetic, weak, and gravitational forces govern phenomena at larger distances or different particle properties.[3] Unlike the infinite-range electromagnetic and gravitational forces, the strong force operates over extremely short distances, approximately 1 femtometer (fm), which is the typical size of an atomic nucleus, and effectively vanishes beyond this range due to color confinement.[3] In contrast, the weak force has an even shorter range of about $10^{-18} meters, mediating processes like beta decay, while gravity, though universal, is negligible at nuclear scales.[9] To illustrate these differences, the following table summarizes key properties of the four forces, based on their relative strengths:| Force | Relative Strength | Range | Mediator(s) | Charge Type |
|---|---|---|---|---|
| Strong | 1 | ~1 fm ($10^{-15} m) | Gluons (8 types) | Color (red, green, blue and anticolors) |
| Electromagnetic | $10^{-2} | Infinite | Photon | Electric (±) |
| Weak | $10^{-6} | ~$10^{-18} m | W^+, W^-, Z bosons | Flavor (weak isospin/hypercharge) |
| Gravitational | $10^{-38} | Infinite | Graviton (hypothetical) | Mass-energy |