Editing Force carrier

{{Short description|In quantum physics, type of particle that gives rise to forces between other particles}}
{{For|the song|Force Carrier}}

In [[quantum field theory]], a '''force carrier''' is a type of [[particle]] that gives rise to [[force]]s between other particles. They serve as the [[Quantum|quanta]] of a particular kind of [[field (physics)|physical field]].<ref>{{cite journal | last1 = Jaeger | first1 = Gregg | year = 2021 | title = Exchange Forces in Particle Physics | journal = Foundations of Physics | volume = 51 | issue =1 | page = 13 |doi=10.1007/s10701-021-00425-0| bibcode = 2021FoPh...51...13J | s2cid = 231811425 }}</ref><ref>[[Steven Weinberg]], ''Dreams of a Final Theory'', Hutchinson, 1993.</ref> Force carriers are also known as '''messenger particles''', '''intermediate particles''', or '''exchange particles'''.<ref>{{cite web | url=http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/expar.html | title=Exchange Particles }}</ref>

== Particle and field viewpoints ==
{{main|Wave–particle duality}}
[[Quantum field theories]] describe nature in terms of [[field (physics)|field]]s. Each field has a [[Complementarity (physics)|complementary]] description as the set of particles of a particular type.  A [[force]] between two particles can be described either as the action of a [[force field (physics)|force field]] generated by one particle on the other, or in terms of the exchange of [[virtual particles|virtual]] force-carrier particles between them.<ref>{{cite journal|last1=Jaeger|first1=Gregg|title=Are virtual particles less real?|journal=Entropy |volume=21 |issue=2|page=141|date=2019|doi=10.3390/e21020141|pmid=33266857|pmc=7514619 |bibcode=2019Entrp..21..141J|url=http://philsci-archive.pitt.edu/15858/1/Jaeger%20Are%20Virtual%20Particles%20Less%20Real_%20entropy-21-00141-v3.pdf|doi-access=free}}</ref>

The energy of a wave in a field (for example, an [[electromagnetic wave]] in the [[electromagnetic field]]) is quantized, and the [[excited state|quantum excitation]]s of the field can be interpreted as particles.  The [[Standard Model]] contains the following force-carrier particles, each of which is an excitation of a particular force field:
*[[Gluon]]s, excitations of the [[strong interaction|strong]] [[gauge field]].
*[[Photon]]s, [[W and Z bosons|W bosons, and Z bosons]], excitations of the [[electroweak interaction|electroweak]] gauge fields.
*[[Higgs boson]]s, excitations of one component of the [[Higgs field]], which gives mass to fundamental particles.
In addition, composite particles such as [[meson]]s, as well as [[quasiparticle]]s, can be described as excitations of an [[effective field theory|effective field]].

Gravity is not a part of the Standard Model, but it is thought that there may be particles called [[gravitons]] which are the excitations of [[gravitational waves]].  The status of this particle is still tentative, because the theory is incomplete and because the interactions of ''single'' gravitons may be too weak to be detected.<ref name="Rothman">{{cite journal |last=Rothman |first=Tony |author2=Stephen Boughn |date=November 2006 |title=Can Gravitons be Detected? |journal=Foundations of Physics |volume=36 |issue=12 |pages=1801–1825 |arxiv=gr-qc/0601043 |doi=10.1007/s10701-006-9081-9 |bibcode = 2006FoPh...36.1801R |s2cid=14008778 }}</ref>

== Forces from the particle viewpoint ==
{{main|Static forces and virtual-particle exchange}}
[[Image:Electron-scattering.png|thumb|220px|A [[Feynman diagram]] of scattering between two electrons by emission of a virtual [[photon]].]]
When one particle [[scattering|scatters]] off another, altering its trajectory, there are two ways to think about the process.  In the field picture, we imagine that the field generated by one particle caused a force on the other.  Alternatively, we can imagine one particle emitting a [[virtual particle]] which is absorbed by the other.  The virtual particle transfers [[momentum]] from one particle to the other.  This particle viewpoint is especially helpful when there are a large number of complicated quantum corrections to the calculation since these corrections can be visualized as [[Feynman diagrams]] containing additional virtual particles.

Another example involving virtual particles is [[beta decay]] where a virtual [[W&nbsp;boson]] is emitted by a [[nucleon]] and then decays to e<sup>±</sup> and (anti)neutrino.

The description of forces in terms of virtual particles is limited by the applicability of the [[perturbation theory (quantum mechanics)|perturbation theory]] from which it is derived.  In certain situations, such as low-energy [[quantum chromodynamics|QCD]] and the description of [[bound states]], perturbation theory breaks down.

== History ==
The concept of messenger particles dates back to the 18th century when the French physicist [[Charles-Augustin de Coulomb|Charles Coulomb]] showed that the electrostatic force between electrically charged objects follows a law similar to [[Newton's Law of Gravitation]]. In time, this relationship became known as [[Coulomb's law]]. By 1862, [[Hermann von Helmholtz]] had described a ray of light as the "quickest of all the messengers". In 1905, [[Albert Einstein]] proposed the existence of a light-particle in answer to the question: "what are light quanta?"

In 1923, at the [[Washington University in St. Louis]], [[Arthur Holly Compton]] demonstrated an effect now known as [[Compton scattering]]. This effect is only explainable if light can behave as a stream of particles, and it convinced the physics community of the existence of Einstein's light-particle. Lastly, in 1926, one year before the theory of quantum mechanics was published, [[Gilbert N. Lewis]] introduced the term "[[photon]]", which later became the name for Einstein's light particle.<ref>{{cite arXiv |last1=Kragh |first1=Helge |year=2014|title=Photon: New light on an old name |class=physics.hist-ph |eprint=1401.0293}}</ref> From there, the concept of messenger particles developed further, notably to [[rest mass|massive]] force carriers (e.g. for the [[Yukawa potential]]).

== See also ==
{{Portal|Physics}}
*[[Virtual particle]]
*[[Fundamental interaction]]
*[[Exciton]]

==References==
{{reflist}}

{{DEFAULTSORT:Force Carrier}}
[[Category:Particle physics]]