Editing Fundamental interaction (section)

== Overview of the fundamental interactions ==
[[Image:Particle overview.svg|thumb|400px|An overview of the various families of elementary and composite particles, and the theories describing their interactions. Fermions are on the left, and bosons are on the right.]]

In the [[Model (abstract)|conceptual model]] of fundamental interactions, [[matter]] consists of [[fermion]]s, which carry [[Physical property|properties]] called [[charge (physics)|charge]]s and [[Spin (physics)|spin]] ±{{Frac|1|2}} (intrinsic [[angular momentum]] ±{{frac|''ħ''|2}}, where ħ is the [[reduced Planck constant]]). They attract or repel each other by exchanging [[boson]]s.

The interaction of any pair of fermions in perturbation theory can then be modelled thus:
: Two fermions go in → ''interaction'' by boson exchange → two changed fermions go out.

The exchange of bosons always carries [[energy]] and [[momentum]] between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process (e.g., turn them from one type of fermion to another). Since bosons carry one unit of angular momentum, the fermion's spin direction will flip from +{{Frac|1|2}} to −{{Frac|1|2}} (or vice versa) during such an exchange (in units of the [[reduced Planck constant]]). Since such interactions result in a change in momentum, they can give rise to classical Newtonian [[force]]s. In quantum mechanics, physicists often use the terms "force" and "interaction" interchangeably; for example, the weak interaction is sometimes referred to as the "weak force".

According to the present understanding, there are four fundamental interactions or forces: [[gravitation]], electromagnetism, the [[weak interaction]], and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observed [[natural phenomenon|physical phenomenon]] by these fundamental interactions. Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are the [[unified field theory|unification]] of:
* [[Electric force|Electric]] and [[magnetic force]] into electromagnetism;
* The [[electromagnetic interaction]] and the weak interaction into the electroweak interaction; see below.

Both magnitude ("relative strength") and "range" of the associated potential, as given in the table, are meaningful only within a rather complex theoretical framework. The table below lists properties of a conceptual scheme that remains the subject of ongoing research.
{| class="wikitable" style="margin:1em auto;"
|-
! Interaction !! Current theory !! Mediators !! Relative strength<ref>Approximate. See [[Coupling constant]] for more exact strengths, depending on the particles and energies involved.</ref>!! Long-distance behavior (potential) !! Range (m)<ref>{{Cite journal |last=Salam |first=Abdus |date=2020 |title=Fundamental interaction |url=https://www.accessscience.com/content/fundamental-interaction/275600 |journal=Access Science |language=en |doi=10.1036/1097-8542.275600}}</ref>
|-
| Weak || [[Electroweak theory]] (EWT) || [[W and Z bosons]] || 10<sup>33</sup> || <math> \frac{1}{r} \ e^{-m_{\rm W,Z} \ r}</math>|| 10<sup>−18</sup>
|-
| Strong || [[Quantum chromodynamics]] <br />(QCD) || [[gluon]]s || 10<sup>38</sup> || <math>{\sim r}</math><br /> ([[Color confinement]], [[#Strong interaction|see discussion below]]) || 10<sup>−15</sup>
|-
| [[Gravitation]] || [[General relativity]]<br />(GR) || [[graviton]]s (hypothetical) || 1 || <math>\frac{1}{r^2}</math>|| {{math|∞|size=150%}}
|-
| Electromagnetic || [[Quantum electrodynamics]] <br />(QED) || [[photon]]s || 10<sup>36</sup> || <math>\frac{1}{r^2}</math>|| {{math|∞|size=150%}}
|}

The modern (perturbative) [[quantum mechanics|quantum mechanical]] view of the fundamental forces other than gravity is that particles of matter ([[fermion]]s) do not directly interact with each other, but rather carry a charge, and exchange [[virtual particles]] ([[gauge boson]]s), which are the interaction carriers or force mediators. For example, photons mediate the interaction of [[electric charge]]s, and gluons mediate the interaction of [[color charge]]s. The full theory includes perturbations beyond simply fermions exchanging bosons; these additional perturbations can involve bosons that exchange fermions, as well as the creation or destruction of particles: see [[Feynman diagrams]] for examples.