Force

== Fundamental interactions ==
{{main|Fundamental interaction}}

All of the known forces of the universe are classified into four [[fundamental interaction]]s. The [[strong force|strong]] and the [[weak force|weak]] forces act only at very short distances, and are responsible for the interactions between [[subatomic particle]]s, including [[nucleon]]s and compound [[Atomic nucleus|nuclei]]. The [[electromagnetic force]] acts between [[electric charge]]s, and the [[gravitational force]] acts between [[mass]]es. All other forces in nature derive from these four fundamental interactions operating within [[quantum mechanics]], including the constraints introduced by the [[Schrödinger equation]] and the [[Pauli exclusion principle]].<ref name=Lieb-Bulletin/> For example, [[friction]] is a manifestation of the electromagnetic force acting between [[atoms]] of two surfaces. The forces in [[spring (device)|springs]], modeled by [[Hooke's law]], are also the result of electromagnetic forces. [[Centrifugal force (fictitious)|Centrifugal forces]] are [[acceleration]] forces that arise simply from the acceleration of [[rotation|rotating]] [[frames of reference]].<ref name=FeynmanVol1/>{{rp|((12-11))}}<ref name=Kleppner/>{{rp|359}}

The fundamental theories for forces developed from the [[Unified field theory|unification]] of different ideas. For example, Newton's universal theory of [[gravitation]] showed that the force responsible for objects falling near the surface of the [[Earth]] is also the force responsible for the falling of celestial bodies about the Earth (the [[Moon]]) and around the Sun (the planets). [[Michael Faraday]] and [[James Clerk Maxwell]] demonstrated that electric and magnetic forces were unified through a theory of electromagnetism. In the 20th century, the development of [[quantum mechanics]] led to a modern understanding that the first three fundamental forces (all except gravity) are manifestations of matter ([[fermion]]s) interacting by exchanging [[virtual particle]]s called [[gauge boson]]s.<ref>{{cite web |title=Fermions & Bosons |work=The Particle Adventure |url=http://particleadventure.org/frameless/fermibos.html |access-date=2008-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20071218074732/http://particleadventure.org/frameless/fermibos.html |archive-date=2007-12-18 }}</ref> This [[Standard Model]] of particle physics assumes a similarity between the forces and led scientists to predict the unification of the weak and electromagnetic forces in [[electroweak]] theory, which was subsequently confirmed by observation.<ref>{{cite web|url=https://www.nobelprize.org/prizes/physics/1999/advanced-information/ |title=Additional background material on the Nobel Prize in Physics 1999 |date=1999-10-12 |access-date=2023-07-26 |website=Nobel Prize |first=Cecilia |last=Jarlskog |author-link=Cecilia Jarlskog}}</ref>

{| class="wikitable" style="margin: 1em auto 1em auto;"
|+ '''The four fundamental forces of nature'''<ref>{{cite web |url=http://www.cpepphysics.org/cpep_sm_large.html |title=Standard model of particles and interactions |publisher=Contemporary Physics Education Project |date=2000 |access-date=2 January 2017 |archive-date=2 January 2017 |archive-url=https://web.archive.org/web/20170102180203/http://www.cpepphysics.org/cpep_sm_large.html |url-status=dead }}</ref>
!rowspan="2" style="text-align: center;"| Property/Interaction
!rowspan="2" style="text-align: center;background-color:#8585C2"|Gravitation
!style="background-color:#F012F0"|Weak
!style="background-color:#FF4D4D"|Electromagnetic
!colspan="2" style="text-align: center;background-color:#99B280"|Strong
|-
!colspan="2" style="text-align: center;background-color:#FF9999"| <small>(Electroweak)</small>
!style="background-color:#CCD8C0"|<small>Fundamental</small>
!style="background-color:#F0F3EC"|<small>Residual</small>
|-
|style="background-color:#FFFFF6"|Acts on:
|align="center"|Mass - Energy
|align="center"|Flavor
|align="center"|Electric charge
|align="center"|Color charge
|align="center"|Atomic nuclei
|-
|style="background-color:#FFFFF6"|Particles experiencing:
|align="center"|All
|align="center"|Quarks, leptons
|align="center"|Electrically charged
|align="center"|Quarks, Gluons
|align="center"|Hadrons
|-
|style="background-color:#FFFFF6"|Particles mediating:
|align="center"|Graviton <br /><small>(not yet observed)</small>
|align="center"|W<sup>+</sup> W<sup>−</sup> Z<sup>0</sup>
|align="center"|γ
|align="center"|Gluons
|align="center"|Mesons
|-
|style="background-color:#FFFFF6"|Strength in the scale of quarks:
|align="center"|{{val||e=-41}}
|align="center"|{{val||e=-4}}
|align="center"|1
|align="center"|60
|<small>Not applicable <br />to quarks</small>
|-
|style="background-color:#FFFFF6"|Strength in the scale of <br />protons/neutrons:
|align="center"|{{val||e=-36}}
|align="center"|{{val||e=-7}}
|align="center"|1
|align="center"|<small>Not applicable <br />to hadrons</small>
|align="center"|20
|}

=== Gravitational ===
[[File:GRAVITY A powerful new probe of black holes.jpg|thumb|Instruments like GRAVITY provide a powerful probe for gravity force detection.<ref>{{cite web |title=Powerful New Black Hole Probe Arrives at Paranal |url=http://www.eso.org/public/announcements/ann15061/ |access-date=13 August 2015}}</ref>]]
Newton's law of gravitation is an example of ''action at a distance'': one body, like the Sun, exerts an influence upon any other body, like the Earth, no matter how far apart they are. Moreover, this action at a distance is ''instantaneous.'' According to Newton's theory, the one body shifting position changes the gravitational pulls felt by all other bodies, all at the same instant of time. [[Albert Einstein]] recognized that this was inconsistent with special relativity and its prediction that influences cannot travel faster than the [[speed of light]]. So, he sought a new theory of gravitation that would be relativistically consistent.<ref>
{{cite book
 | last1=Misner |first1=Charles W. |author-link1=Charles W. Misner
 | last2=Thorne |first2=Kip S. |author-link2=Kip Thorne
 | last3=Wheeler |first3=John Archibald |author-link3=John Archibald Wheeler
 | year=1973
 | title=Gravitation
 | title-link=Gravitation (book)
 | publisher=[[W. H. Freeman]]
 | location=San Francisco
 | isbn=978-0-7167-0344-0
 | pages=3–5
}}</ref><ref>
{{Cite book
 |last=Choquet-Bruhat |first=Yvonne |author-link=Yvonne Choquet-Bruhat
 |url=https://www.worldcat.org/oclc/317496332
 |title=General Relativity and the Einstein Equations
 |date=2009
 |publisher=Oxford University Press
 |isbn=978-0-19-155226-7 |location=Oxford |oclc=317496332
 |pages=37–39
}}</ref> [[Mercury (planet)|Mercury]]'s orbit did not match that predicted by Newton's law of gravitation. Some astrophysicists predicted the existence of an undiscovered planet ([[Vulcan (hypothetical planet)|Vulcan]]) that could explain the discrepancies. When Einstein formulated his theory of [[general relativity]] (GR) he focused on Mercury's problematic orbit and found that his theory added [[Perihelion precession of Mercury|a correction, which could account for the discrepancy]]. This was the first time that Newton's theory of gravity had been shown to be inexact.<ref>{{cite news |last1=Siegel |first1=Ethan |title=When Did Isaac Newton Finally Fail? |url=https://www.forbes.com/sites/startswithabang/2016/05/20/when-did-isaac-newton-finally-fail/#6fdc279675f5 |access-date=3 January 2017 |work=Forbes |date=20 May 2016}}</ref>

Since then, general relativity has been acknowledged as the theory that best explains gravity. In GR, gravitation is not viewed as a force, but rather, objects moving freely in gravitational fields travel under their own inertia in [[geodesic|straight lines]] through [[curved spacetime]] – defined as the shortest spacetime path between two spacetime events. From the perspective of the object, all motion occurs as if there were no gravitation whatsoever. It is only when observing the motion in a global sense that the curvature of spacetime can be observed and the force is inferred from the object's curved path. Thus, the straight line path in spacetime is seen as a curved line in space, and it is called the ''[[external ballistics|ballistic]] [[trajectory]]'' of the object. For example, a [[Basketball (ball)|basketball]] thrown from the ground moves in a [[parabola]], as it is in a uniform gravitational field. Its spacetime trajectory is almost a straight line, slightly curved (with the [[radius of curvature (applications)|radius of curvature]] of the order of few [[light-year]]s). The time derivative of the changing momentum of the object is what we label as "gravitational force".<ref name=Kleppner />

=== Electromagnetic ===

[[Maxwell's equations]] and the set of techniques built around them adequately describe a wide range of physics involving force in electricity and magnetism. This classical theory already includes relativity effects.<ref>{{Cite book |last1=Panofsky |first1=Wolfgang K. |title=Classical electricity and magnetism |last2=Phillips |first2=Melba |date=2005 |publisher=Dover Publ |isbn=978-0-486-43924-2 |edition=2 |location=Mineola, NY}}</ref>  Understanding quantized electromagnetic interactions between elementary particles requires [[quantum electrodynamics]] (or QED). In QED, photons are fundamental exchange particles, describing all interactions relating to electromagnetism including the electromagnetic force.<ref>{{cite book|first=Anthony |last=Zee |author-link=Anthony Zee |title=Quantum Field Theory in a Nutshell |title-link=Quantum Field Theory in a Nutshell |edition=2nd |isbn=978-0-691-14034-6 |year=2010 |page=29 |publisher=Princeton University Press}}</ref>

=== Strong nuclear ===
{{main|Strong interaction}}
There are two "nuclear forces", which today are usually described as interactions that take place in quantum theories of particle physics. The [[strong nuclear force]] is the force responsible for the structural integrity of [[atomic nuclei]], and gains its name from its ability to overpower the electromagnetic repulsion between protons.<ref name=Cutnell/>{{rp|940}}<ref>{{cite OED|strong, 7.g ''physics'' |1058721983}}</ref>

The strong force is today understood to represent the [[Fundamental interaction|interaction]]s between [[quark]]s and [[gluon]]s as detailed by the theory of [[quantum chromodynamics]] (QCD).<ref>{{cite web |last=Stevens |first=Tab |title=Quantum-Chromodynamics: A Definition – Science Articles |date=10 July 2003 |url=http://www.physicspost.com/science-article-168.html |archive-url=https://web.archive.org/web/20111016103116/http://www.physicspost.com/science-article-168.html |archive-date=2011-10-16 |access-date=2008-01-04}}</ref> The strong force is the [[fundamental force]] mediated by gluons, acting upon quarks, [[antiparticle|antiquarks]], and the gluons themselves. The strong force only acts ''directly'' upon elementary particles. A residual is observed between [[hadron]]s (notably, the [[nucleon]]s in atomic nuclei), known as the [[nuclear force]]. Here the strong force acts indirectly, transmitted as gluons that form part of the virtual pi and rho [[meson]]s, the classical transmitters of the nuclear force. The failure of many searches for [[free quark]]s has shown that the elementary particles affected are not directly observable. This phenomenon is called [[color confinement]].<ref>{{Cite book |last=Goldberg |first=Dave |title=The Standard Model in a Nutshell |date=2017 |publisher=Princeton University Press |isbn=978-0-691-16759-6}}</ref>{{Rp|page=232}}

=== Weak nuclear ===
{{Main|Weak interaction}}
Unique among the fundamental interactions, the weak nuclear force creates no bound states.<ref name=GreinerMuller>{{Cite book |last1=Greiner |first1=Walter |title=Gauge theory of weak interactions: with 75 worked examples and exercises |last2=Müller |first2=Berndt |last3=Greiner |first3=Walter |date=2009 |publisher=Springer |isbn=978-3-540-87842-1 |edition=4|location=Heidelberg}}</ref> The weak force is due to the exchange of the heavy [[W and Z bosons]]. Since the weak force is mediated by two types of bosons, it can be divided into two types of interaction or "[[Feynman diagram|vertices]]" — [[charged current]], involving the electrically charged W<sup>+</sup> and W<sup>−</sup> bosons, and [[neutral current]], involving electrically neutral Z<sup>0</sup> bosons. The most familiar effect of weak interaction is [[beta decay]] (of neutrons in atomic nuclei) and the associated [[radioactivity]].<ref name=Cutnell/>{{rp|951}} This is a type of charged-current interaction. The word "weak" derives from the fact that the field strength is some 10<sup>13</sup> times less than that of the [[strong force]]. Still, it is stronger than gravity over short distances. A consistent electroweak theory has also been developed, which shows that electromagnetic forces and the weak force are indistinguishable at a temperatures in excess of approximately {{val|e=15|ul=K}}.<ref>{{Cite book |last=Durrer |first=Ruth |title=The Cosmic Microwave Background |date=2008 |publisher=Cambridge Pniversity Press |isbn=978-0-521-84704-9 |pages=41–42 |author-link=Ruth Durrer}}</ref> Such temperatures occurred in the plasma collisions in the early moments of the [[Big Bang]].<ref name=GreinerMuller/>{{rp|201}}