Editing Fundamental interaction (section)

=== Strong interaction ===
{{Main|Strong interaction}}
The ''strong interaction'', or ''strong nuclear force'', is the most complicated interaction, mainly because of the way it varies with distance. The nuclear force is powerfully attractive between nucleons at distances of about 1 femtometre (fm, or 10<sup>−15</sup> metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm, the nuclear force becomes repulsive. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows.

After the nucleus was discovered in 1908, it was clear that a new force, today known as the nuclear force, was needed to overcome the [[Electrostatics|electrostatic repulsion]], a manifestation of electromagnetism, of the positively charged protons. Otherwise, the nucleus could not exist. Moreover, the force had to be strong enough to squeeze the protons into a volume whose diameter is about 10<sup>−15</sup> [[metre|m]], much smaller than that of the entire atom. From the short range of this force, [[Hideki Yukawa]] predicted that it was associated with a massive force particle, whose mass is approximately 100 MeV.

The 1947 discovery of the [[pion]] ushered in the modern era of particle physics. Hundreds of hadrons were discovered from the 1940s to 1960s, and an [[Regge theory|extremely complicated theory]] of hadrons as strongly interacting particles was developed. Most notably:
* The pions were understood to be oscillations of [[Vacuum expectation value|vacuum condensates]];
* [[Jun John Sakurai]] proposed the rho and omega [[vector boson]]s to be [[Yang–Mills theory|force carrying particles]] for approximate symmetries of [[isospin]] and [[hypercharge]];
* [[Geoffrey Chew]], Edward K. Burdett and [[Steven Frautschi]] grouped the heavier hadrons into families that could be understood as vibrational and rotational excitations of [[string theory|strings]].

While each of these approaches offered insights, no approach led directly to a fundamental theory.

[[Murray Gell-Mann]] along with [[George Zweig]] first proposed fractionally charged quarks in 1961. Throughout the 1960s, different authors considered theories similar to the modern fundamental theory of [[quantum chromodynamics|quantum chromodynamics (QCD)]] as simple models for the interactions of quarks. The first to hypothesize the gluons of QCD were [[Moo-Young Han]] and [[Yoichiro Nambu]], who introduced the [[quark color]] charge. Han and Nambu hypothesized that it might be associated with a force-carrying field. At that time, however, it was difficult to see how such a model could permanently confine quarks. Han and Nambu also assigned each quark color an integer electrical charge, so that the quarks were fractionally charged only on average, and they did not expect the quarks in their model to be permanently confined.

In 1971, Murray Gell-Mann and [[Harald Fritzsch]] proposed that the Han/Nambu color gauge field was the correct theory of the short-distance interactions of fractionally charged quarks. A little later, [[David Gross]], [[Frank Wilczek]], and [[David Politzer]] discovered that this theory had the property of [[asymptotic freedom]], allowing them to make contact with [[deep inelastic scattering|experimental evidence]]. They concluded that QCD was the complete theory of the strong interactions, correct at all distance scales. The discovery of asymptotic freedom led most physicists to accept QCD since it became clear that even the long-distance properties of the strong interactions could be consistent with experiment if the quarks are permanently [[color confinement|confined]]: the strong force increases indefinitely with distance, trapping quarks inside the hadrons.

Assuming that quarks are confined, [[Mikhail Shifman]], [[Arkady Vainshtein]] and [[Valentine Zakharov]] were able to compute the properties of many low-lying hadrons directly from QCD, with only a few extra parameters to describe the vacuum. In 1980, [[Kenneth G. Wilson]] published computer calculations based on the first principles of QCD, establishing, to a level of confidence tantamount to certainty, that QCD will confine quarks. Since then, QCD has been the established theory of strong interactions.

QCD is a theory of fractionally charged quarks interacting by means of 8 bosonic particles called gluons. The gluons also interact with each other, not just with the quarks, and at long distances the lines of force collimate into strings, loosely modeled by a linear potential, a constant attractive force. In this way, the mathematical theory of QCD not only explains how quarks interact over short distances but also the string-like behavior, discovered by Chew and Frautschi, which they manifest over longer distances.