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{{short description|Concept in physics}}
{{Distinguish|Action (physics)}}
{{For|the anti-pattern in computer science|Action at a distance (computer programming)}}
In [[physics]], '''action at a distance''' is the concept that an object's [[motion (physics)|motion]] can be affected by another object without being in physical contact with it (see [[Contact mechanics|mechanical contact]]); that is, the non-local interaction of objects that are separated in space. [[Coulomb's law]] and [[Newton's law of universal gravitation]] are based on action at a distance.

Historically, action at a distance was the earliest scientific model for [[gravity]] and [[electricity]] and it continues to be useful in many practical cases. In the 19th and 20th centuries, field models arose to explain these phenomena with more precision. The discovery of [[Electron|electrons]] and of [[special relativity]] led to new action at a distance models providing alternative to field theories. Under our modern understanding, the four [[fundamental interaction]]s (gravity, [[electromagnetism]], the [[strong interaction]] and the [[weak interaction]]) in all of physics are not described by action at a distance.

== Categories of action ==
In the study of [[mechanics]], action at a distance is one of three fundamental actions on matter that cause motion. The other two are direct impact ([[elastic collision |elastic]] or [[inelastic collisions]]) and actions in a [[Continuum mechanics|continuous medium]] as in [[fluid mechanics]] or [[solid mechanics]].<ref name=Hesse1955>{{Cite journal |last=Hesse |first=Mary B. |date=1955 |title=Action at a Distance in Classical Physics |url=https://www.jstor.org/stable/227576 |journal=Isis |volume=46 |issue=4 |pages=337–353 |doi=10.1086/348429 |jstor=227576 |s2cid=121166354 |issn=0021-1753}}</ref>{{rp|338}} 
Historically, physical explanations for particular phenomena have moved between these three categories over time as new models were developed.

Action-at-a-distance and actions in a continuous medium may be easily distinguished when the medium dynamics are visible, like waves in water or in an elastic solid. In the case of electricity or gravity, no medium is required. In the nineteenth century, criteria like the effect of actions on intervening matter, the observation of a time delay, the apparent storage of energy, or even the possibility of a plausible mechanical model for action transmission were all accepted as evidence against action at a distance.<ref name=Hesse2005>{{Cite book |last=Hesse |first=Mary B. |title=Forces and fields: the concept of action at a distance in the history of physics |date=2005 |publisher=Dover Publications |isbn=978-0-486-44240-2 |edition=Dover |location=Mineola, N.Y}}</ref>{{rp|198}} [[Aether theories]] were alternative proposals to replace apparent action-at-a-distance in gravity and electromagnetism, in terms of continuous action inside an (invisible) medium called "aether".<ref name=Hesse1955/>{{rp|338}}

Direct impact of macroscopic objects seems visually distinguishable from action at a distance. If however the objects are constructed of [[atoms]], and the volume of those atoms is not defined and atoms interact by electric and magnetic forces, the distinction is less clear.<ref name=Hesse2005/>

== Roles ==
The concept of action at a distance acts in multiple roles in physics and it can co-exist with other models according to the needs of each physical problem.

One role is as a summary of physical phenomena, independent of any understanding of the cause of such an action.<ref name=Hesse1955/> For example, astronomical tables of planetary positions can be compactly summarized using [[Newton's law of universal gravitation]], which assumes the planets interact without contact or an intervening medium. As a summary of data, the concept does not need to be evaluated as a plausible physical model. 

Action at a distance also acts as a model explaining physical phenomena even in the presence of other models. Again in the case of gravity, hypothesizing an instantaneous force between masses allows the return time of [[comets]] to be predicted as well as predicting the existence of previously unknown planets, like [[Neptune]].<ref>{{Cite book |last=Feather |first=Norman |title=An Introduction to the Physics of Mass Length and Time – Hardcover |publisher=Edinburgh University Press |year=1959}}</ref>{{rp|210}} These triumphs of physics predated the alternative more accurate model for gravity based on general relativity by many decades.

Introductory physics textbooks discuss [[central forces]], like gravity, by models based on action-at-distance without discussing the cause of such forces or issues with it until the topics of [[Theory of relativity|relativity]] and [[Field (physics)|fields]] are discussed. For example, see ''[[The Feynman Lectures on Physics]]'' on gravity.<ref>Feynman, Richard P., Robert B. Leighton, and Matthew Sands. [https://www.feynmanlectures.caltech.edu/I_toc.html The Feynman lectures on physics, Vol. I: The new millennium edition: mainly mechanics, radiation, and heat.] Vol. 1. Basic books, 2011.</ref>

== History ==
=== Early inquiries into motion ===
{{Main | Scientific Revolution| History of gravitational theory }}
Action-at-a-distance as a physical concept requires identifying objects, distances, and their motion. In [[Science in classical antiquity |antiquity, ideas about the natural world]] were not organized in these terms. Objects in motion were modeled as living beings.<ref name=Hesse1955/> Around 1600, the scientific method began to take root. [[René Descartes]] held a more fundamental view, developing ideas of matter and action independent of theology. [[Galileo Galilei]] wrote about experimental measurements of falling and rolling objects. [[Johannes Kepler]]'s [[Kepler's laws of planetary motion|laws of planetary motion]] summarized [[Tycho Brahe]]'s astronomical observations.<ref name=Hesse2005/>{{rp|132}} Many experiments with electrical and magnetic materials led to new ideas about forces. These efforts set the stage for Newton's work on forces and gravity.

=== Newtonian gravity ===
{{Main| History of gravitational theory}}
In 1687 [[Isaac Newton]] published his ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'' which combined his [[Newton's laws of motion|laws of motion]] with a new mathematical analysis able to reproduce Kepler's empirical results.<ref name=Hesse2005/>{{rp|134}} His explanation was in the form of a [[law of universal gravitation]]: any two bodies are attracted by a force proportional to their mass and inversely proportional to the square of the distance between them.<ref name=Whittaker/>{{rp|28}} Thus the motions of planets were predicted by assuming forces working over great distances.

This mathematical expression of the force did not imply a cause. Newton considered action-at-a-distance to be an inadequate model for gravity.<ref>Ducheyne, S. "Newton on Action at a Distance". ''Journal of the History of Philosophy'' vol. 52.4 (2014): 675–702.</ref> Newton, in his words, considered action at a distance to be:

{{blockquote|so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it.<ref name="sep-qm-action-distance">{{cite encyclopedia |first=Joseph |last=Berkovitz |editor=Edward N. Zalta |year=2008 |title=Action at a Distance in Quantum Mechanics |encyclopedia=The Stanford Encyclopedia of Philosophy |edition=Winter 2008 |url=http://plato.stanford.edu/archives/win2008/entries/qm-action-distance/#ActDisCoExiNonSepHol}}</ref>|Isaac Newton|Letters to Bentley, 1692/3}}

Metaphysical scientists of the early 1700s strongly objected to the unexplained action-at-a-distance in Newton's theory. [[Gottfried Wilhelm Leibniz]] complained that the mechanism of gravity was "invisible, intangible, and not mechanical".<ref name=Hesse1955/>{{rp|339}} Moreover, initial comparisons with astronomical data were not favorable. As mathematical techniques improved throughout the 1700s, the theory showed increasing success, predicting the date of the return of [[Halley's comet]]<ref>{{Cite journal |date=1987-09-30 |title=The history of Halley's Comet |url=https://royalsocietypublishing.org/doi/10.1098/rsta.1987.0091 |journal=Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences |language=en |volume=323 |issue=1572 |pages=349–367 |doi=10.1098/rsta.1987.0091 |bibcode=1987RSPTA.323..349H |issn=0080-4614 |last1=Hughes |first1=D. W. |s2cid=123592786 }}</ref> and aiding the discovery of planet [[Neptune]] in 1846.<ref>{{cite web | url=http://www-history.mcs.st-and.ac.uk/HistTopics/Neptune_and_Pluto.html | title=Mathematical discovery of planets |author1=J J O'Connor |author2=E F Robertson | date=September 1996 | access-date=2009-09-11}}</ref> These successes and the increasingly empirical focus of science towards the 19th century led to acceptance of Newton's theory of gravity despite distaste for action-at-a-distance.<ref name=Hesse1955/>

=== Electrical action at a distance ===
{{ Main |History of electromagnetic theory}}
[[File:RGNb10360943.01 Nollet The Electric Boy.tif|thumb| [[Jean-Antoine Nollet]] reproducing Stephan Gray's “electric boy” experiment, in which a boy hanging from insulating silk ropes is given an electric charge. A group are gathered around. A woman is encouraged to bend forward and poke the boy's nose, to get an electric shock.<ref name=Park1898>Benjamin, Park (1898). ''A History of Electricity (the Intellectual Rise in Electricity) from Antiquity to the Days of Benjamin Franklin'', New York: John Wiley & Sons.</ref>{{rp|489}}]]
Electrical and magnetic phenomena also began to be explored systematically in the early 1600s. In [[William Gilbert (physicist)|William Gilbert]]'s early theory of "electric effluvia," a kind of electric atmosphere, he rules out action-at-a-distance on the grounds that "no action can be performed by matter save by contact".<ref name=Sanford>{{Cite journal |last=Sanford |first=Fernando |date=1921 |title=Some Early Theories Regarding Electrical Forces—The Electric Emanation Theory |url=https://www.jstor.org/stable/6312 |journal=The Scientific Monthly |volume=12 |issue=6 |pages=544–550 |bibcode=1921SciMo..12..544S |issn=0096-3771}}</ref> 
However subsequent experiments, especially those by [[Stephen Gray (scientist)|Stephen Gray]] showed electrical effects over distance. Gray developed an impressive experiment call the "electric boy" demonstrating electric transfer without direct contact.<ref name=Park1898/>
[[Franz Aepinus]] was the first to show, in 1759, that a theory of action at a distance for electricity provides a simpler replacement for the electric effluvia theory.<ref name="Whittaker">{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 1: The classical theories |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |location=New York}}</ref>{{rp|42}} Despite this success, Aepinus himself considered the nature of the forces to be unexplained: he did "not approve of the doctrine which assumes the possibility of action at a distance", setting the stage for a shift to theories based on aether.<ref name=Sanford/>{{rp|549}}

By 1785 [[Charles-Augustin de Coulomb]] showed that two electric charges at rest experience a force inversely proportional to the square of the distance between them, a result now called [[Coulomb's law]]. The striking similarity to gravity strengthened the case for action at a distance, at least as a mathematical model.<ref>Woodruff, A. E. “Action at a Distance in Nineteenth Century Electrodynamics.” Isis, vol. 53, no. 4, 1962, pp. 439–59. JSTOR, http://www.jstor.org/stable/227719. Accessed 24 Oct. 2023.</ref>

As mathematical methods improved, especially through the work of [[Pierre-Simon Laplace]], [[Joseph-Louis Lagrange]], and [[Siméon Denis Poisson]], more sophisticated mathematical methods began to influence the thinking of scientists. The concept of [[potential energy]] applied to small [[test particle|test particles]] led to the concept of a [[scalar field]], a mathematical model representing the forces throughout space. While this mathematical model is not a mechanical medium, the mental picture of such a field resembles a medium.<ref name=Hesse2005/>{{rp|197}}

=== Fields as an alternative  ===
{{Main|History of field theory}}
[[File:10.2307 community.26233914-1.jpg|thumb|Glazed frame, containing "Delineation of Lines of Magnetic Force by Iron filings" prepared by Michael Faraday]]
It was [[Michael Faraday]] who first suggested that action at a distance, even in the form of a (mathematical) potential field, was inadequate as an account of electric and magnetic forces.<ref name=Hesse1955/>{{rp|341}} Faraday, an empirical experimentalist, cited three reasons in support of some medium transmitting electrical force: 1) [[electrostatic induction]] across an insulator depends on the nature of the insulator, 2) cutting a charged insulator causes opposite charges to appear on each half, and 3) electric discharge sparks are curved at an insulator. From these reasons he concluded that the particles of an insulator must be [[Dielectric polarization |polarized]], with each particle contributing to continuous action. He also experimented with magnets, demonstrating lines of force made visible by iron filings. However, in both cases his field-like model depends on particles that interact through an action-at-a-distance: his mechanical field-like model has no more fundamental physical cause than the long-range central field model.<ref name=Hesse1955/>{{rp|348}}

Faraday's observations, as well as others, led [[James Clerk Maxwell]] to a breakthrough formulation in 1865, a set of [[Maxwell's equations |equations]] that combined electricity and magnetism, both static and dynamic, and which included electromagnetic radiation – light.<ref name=Whittaker/>{{rp|253}} Maxwell started with elaborate mechanical models but ultimately produced a purely mathematical treatment using dynamical [[vector fields]]. The sense that these fields must be set to vibrate to propagate light set off a search of a medium of propagation; the medium was called the [[luminiferous aether]] or the [[Aether theories|aether]].<ref name=Whittaker/>{{rp|279}}

In 1873 Maxwell addressed action at a distance explicitly.<ref>Maxwell, J. C., On action at a distance, Proceedings of the Royal Institution of Great
Britain, VII, 48–49, 1873–1875. Reproduced in Kuehn, K. (2016). Action-at-a-Distance. In: A Student's Guide Through the Great Physics Texts. Undergraduate Lecture Notes in Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-21816-8_30</ref> He reviews Faraday's lines of force, carefully pointing out that Faraday himself did not provide a mechanical model of these lines in terms of a medium. Nevertheless the many properties of these lines of force imply these "lines must not be regarded as mere mathematical abstractions". Faraday himself viewed these lines of force as a model, a "valuable aid" to the experimentalist, a means to suggest further experiments. 

In distinguishing between different kinds of action Faraday suggests three criteria: 1) do additional material objects alter the action?, 2) does the action take time, and 3) does it depend upon the receiving end? For electricity, Faraday knew that all three criteria were met for electric action, but gravity was thought to only meet the third one. After Maxwell's time a fourth criteria, the transmission of energy, was added, thought to also apply to electricity but not gravity. With the advent of new theories of gravity, the modern account would give gravity all of the criteria except dependence on additional objects.

=== Fields fade into spacetime ===
{{Main | Luminiferous ether}}
The success of Maxwell's field equations led to numerous efforts in the later decades of the 19th century to represent electrical, magnetic, and gravitational fields, primarily with mechanical models.<ref name=Whittaker/>{{rp|279}} No model emerged that explained the existing phenomena. In particular no good model for [[stellar aberration]], the shift in the position of stars with the Earth's relative velocity. The best models required the ether to be stationary while the Earth moved, but [[Michelson–Morley experiment|experimental efforts]] to measure the effect of Earth's motion through the aether found no effect.  

In 1892 [[Hendrik Lorentz]] proposed a modified aether based on the emerging microscopic molecular model rather than the strictly macroscopic continuous theory of Maxwell.<ref name=Darrigol >{{Cite book |last=Darrigol |first=Olivier |title=Electrodynamics from Ampère to Einstein |date=2005 |publisher=Oxford Univ. Press |isbn=978-0-19-850593-8 |edition=Reprinted |location=Oxford}}</ref>{{rp|326}} Lorentz investigated the mutual interaction of a moving solitary electrons within a stationary aether.<ref name=Whittaker/>{{rp|393}} He rederived Maxwell's equations in this way but, critically, in the process he changed to represent the wave in the coordinates moving electrons. He showed that the wave equations had the same form if they were transformed using a particular scaling [[Lorentz factor|factor]],
<math display=block>\gamma = \frac{1}{\sqrt{1-(u^2/c^2)}}.</math> 
where <math>u</math> is the velocity of the moving electrons and <math>c</math> is the speed of light. Lorentz noted that if this factor were applied as a [[length contraction]] to moving matter in a stationary ether, it would eliminate any effect of motion through the ether, in agreement with experiment. 

In 1899, [[Henri Poincaré]] questioned the existence of an aether, showing that the [[principle of relativity]] prohibits the absolute motion assumed by proponents of the aether model. He named the transformation used by Lorentz the [[Lorentz transformation]] but interpreted it as a transformation between two inertial frames with relative velocity <math>u</math>. This transformation makes the electromagnetic equations look the same in every uniformly moving inertial frame. Then, in 1905, [[Albert Einstein]] demonstrated that the principle of relativity, applied to the simultaneity of time and the constant [[speed of light|speed of]] light, precisely predicts the Lorentz transformation. This theory of [[special relativity]] quickly became the modern concept of [[spacetime]].

Thus the aether model, initially so very different from action at a distance, slowly changed to 
resemble simple empty space.<ref name=Whittaker/>{{rp|393}} 

In 1905, Poincaré proposed [[gravitational wave]]s, emanating from a body and propagating at the speed of light, as being required by the Lorentz transformations<ref>{{Cite journal |last=Poincaré |first=Henri |date=1905 |title=Sur la dynamique de l'électron |url=https://www.academie-sciences.fr/pdf/dossiers/Poincare/Poincare_pdf/Poincare_CR1905.pdf |journal=Académie des sciences, Note de H. Poincaré. C.R. T. |volume=140 |pages=1504–1508}}</ref> and suggested that, in analogy to an accelerating [[electrical charge]] producing [[electromagnetic wave]]s, accelerated masses in a relativistic field theory of gravity should produce gravitational waves.<ref name="CGShistory">
{{cite journal
|first1=J.L. |last1=Cervantes-Cota
|first2=S. |last2=Galindo-Uribarri
|first3=G.F. |last3=Smoot
|title=A Brief History of Gravitational Waves
|journal=Universe
|volume=2
|number=3
|year=2016
|pages=22
|doi=10.3390/universe2030022|bibcode=2016Univ....2...22C
|arxiv = 1609.09400 |s2cid=2187981
|doi-access=free
}}</ref> However, until 1915 gravity stood apart as a force still described by action-at-a-distance. In that year Einstein showed that a field theory of spacetime, [[general relativity]], consistent with relativity can explain gravity. New effects resulting from this theory were dramatic for [[cosmology]] but minor for planetary motion and physics on Earth.
Einstein himself noted Newton's "enormous practical success".<ref>{{Cite journal |last=Pais |first=A. |date=1979-10-01 |title=Einstein and the quantum theory |url=https://link.aps.org/doi/10.1103/RevModPhys.51.863 |journal=Reviews of Modern Physics |language=en |volume=51 |issue=4 |pages=863–914 |doi=10.1103/RevModPhys.51.863 |bibcode=1979RvMP...51..863P |issn=0034-6861}}</ref>

== Modern action at a distance ==
{{Main| Wheeler-Feynman absorber theory}}
In the early decades of the 20th century [[Karl Schwarzschild]],<ref name= "schwarzschil1903b">K. Schwarzschild, Nachr. ges. Wiss. Gottingen (1903) 128,132</ref> [[Hugo Tetrode]],<ref>H. Tetrode, Zeitschrift für Physik 10:137, 1922</ref> and [[Adriaan Fokker]].<ref>A. D. Fokker, Zeitschrift für Physik 58:386, 1929</ref> independently developed non-instantaneous models for action at a distance consistent with special relativity. In 1949 [[John Archibald Wheeler]] and [[Richard Feynman]] built on these models to develop a new field-free theory of electromagnetism.
While Maxwell's field equations are generally successful, the Lorentz model of a moving electron interacting with the field encounters mathematical difficulties: the self-energy of the moving point charge within the field is infinite.<ref name=Barut>{{Cite book |last=Barut |first=Asim O. |title=Electrodynamics and classical theory of fields & particles |date=1980 |publisher=Dover Publ |isbn=978-0-486-64038-9 |edition=Unabridged and corr. republ. of the ed. New York, Macmillan, 1964 |series=Dover books on physics and chemistry |location=New York}}</ref>{{rp|187}} The [[Wheeler-Feynman absorber theory]] of electromagnetism avoids the self-energy issue.<ref name=Barut/>{{rp|213}} They interpret [[Abraham–Lorentz force]], the apparent force resisting electron acceleration, as a real force returning from all the other existing charges in the universe.

The Wheeler-Feynman theory has inspired new thinking about the [[arrow of time]] and about the nature of [[quantum non-locality]].<ref>{{Cite book |last=Price |first=Huw |title=Time's arrow and Archimedes' point: new directions for the physics of time |date=1996 |publisher=Oxford university press |isbn=978-0-19-510095-2 |location=New York Oxford}}</ref> The theory has implications for cosmology; it has been extended to [[quantum mechanics]].<ref>{{Cite journal |last1=Hoyle |first1=F. |last2=Narlikar |first2=J. V. |date=1995-01-01 |title=Cosmology and action-at-a-distance electrodynamics |url=https://link.aps.org/doi/10.1103/RevModPhys.67.113 |journal=Reviews of Modern Physics |language=en |volume=67 |issue=1 |pages=113–155 |doi=10.1103/RevModPhys.67.113 |bibcode=1995RvMP...67..113H |issn=0034-6861}}</ref> A similar approach has been applied to develop an alternative theory of gravity consistent with general relativity.<ref>{{Cite journal |last=Narlikar |first=J.V. |date=2003 |title=Action at a Distance and Cosmology: A Historical Perspective |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.41.112202.151716 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=41 |issue=1 |pages=169–189 |doi=10.1146/annurev.astro.41.112202.151716 |bibcode=2003ARA&A..41..169N |issn=0066-4146}}</ref> [[John G. Cramer]] has extended the Wheeler-Feynman ideas to create the [[transactional interpretation]] of quantum mechanics.

== "Spooky action at a distance" ==
{{anchor|spooky action at a distance}}
Einstein wrote to [[Max Born]] about issues in quantum mechanics in 1947 and 
used a phrase translated as "spooky action at a distance". The phrase has been picked up and used as a description for the cause of small non-classical correlations between physically separated measurement of [[quantum entanglement |entangled quantum states]]. The correlations are [[Bell theorem |predicted by quantum mechanics]] and [[Bell test |verified by experiments]]. Rather than a postulate like Newton's gravitational force, this use of "action-at-a-distance" concerns observed correlations which are not easy to explain within simple [[interpretations of quantum mechanics]].<ref>{{Cite journal |date=2023-10-30 |title=Entanglement is spooky, but not action at a distance |journal= Science News |url=https://www.sciencenews.org/blog/context/entanglement-spooky-not-action-distance |language=en-US}}</ref><ref>{{Cite journal |last=Boughn |first=Stephen |date=2022-04-16 |title=There Is No Spooky Action at a Distance in Quantum Mechanics |journal=Entropy |language=en |volume=24 |issue=4 |pages=560 |doi=10.3390/e24040560 |issn=1099-4300 |pmc=9029371 |pmid=35455223 |bibcode=2022Entrp..24..560B |doi-access=free }}</ref><ref>{{Cite web|last=Berkovitz |first=Joseph |date=2008 |editor-last=Zalta |editor-first=Edward N. |title=Action at a Distance in Quantum Mechanics |work=Stanford Encyclopedia of Philosophy.|url=https://plato.stanford.edu/archives/win2008/entries/qm-action-distance/ |publisher=Metaphysics Research Lab, Stanford University}}</ref>

== Force in quantum field theory ==
{{Main|Force carriers}}
[[Quantum field theory]] does not need action at a distance. At the most fundamental level only four forces are needed and each are described as resulting from the exchange of specific [[bosons]]. Two are short range: the [[strong interaction]] mediated by [[mesons]] and the [[weak interaction]] mediated by the [[weak boson]]; two are long range: [[electromagnetism]] mediated by the [[photon]] and [[gravity]] hypothesized to be mediated by the [[graviton]].<ref name=SimplyZee>{{Cite book |last=Zee |first=Anthony |title=Quantum field theory, as simply as possible |date=2023 |publisher=Princeton University Press |isbn=978-0-691-17429-7 |location=Princeton Oxford}}</ref>{{rp|132}} However, the entire concept of force is of secondary concern in advanced modern particle physics.  Energy forms the basis of physical models and the word [[action (physics)|action]] has shifted away from implying a force to a specific technical meaning, an integral over the difference between [[potential energy]] and [[kinetic energy]].<ref name=SimplyZee/>{{rp|173}}

== See also ==	
* {{Annotated link |Central force}}
* {{Annotated link |Principle of locality}}
* {{Annotated link |Quantum nonlocality}}

==References==

{{Reflist}}

== External links ==

* {{ free-content attribution | title = Newton’s action at a distance – Different views | author = Nicolae Sfetcu | documentURL = https://www.telework.ro/en/newtons-action-at-a-distance-different-views/ | license = CC-BY-SA }}

[[Category:Force|Action at a distance]]
[[Category:Concepts in physics]]