Editing Dynamical friction (section)

==Applications==
Dynamical friction is particularly important in the formation of planetary systems and interactions between galaxies.

===Protoplanets===
During the formation of planetary systems, dynamical friction between the [[protoplanet]] and the [[protoplanetary disk]] causes energy to be transferred from the protoplanet to the disk.  This results in the inward migration of the protoplanet.

===Galaxies===
When galaxies interact through collisions, dynamical friction between stars causes matter to sink toward the center of the galaxy and for the orbits of stars to be randomized.  This process is called violent relaxation and can change two [[spiral galaxies]] into one larger [[elliptical galaxy]].<ref name="Struck_1999">{{cite journal |last1=Struck  |first1=Curtis |title=Galaxy Collisions |journal=Phys. Rep. |date=1999 |volume=321|issue=1–3 |pages=1–137|doi=10.1016/S0370-1573(99)00030-7|arxiv=astro-ph/9908269 |bibcode=1999PhR...321....1S |s2cid=119369136 }}</ref>

===Galaxy clusters===
The effect of dynamical friction explains why the brightest (more massive) galaxy tends to be found near the center of a galaxy cluster. The effect of the two body collisions slows down the galaxy, and the drag effect is greater the larger the galaxy mass. When the galaxy loses kinetic energy, it moves towards the center of the cluster.
However the observed velocity dispersion of galaxies within a galaxy cluster does not depend on the mass of the galaxies. The explanation is that a galaxy cluster relaxes by violent relaxation, which sets the velocity dispersion to a value independent of the galaxy's mass.

===Star clusters===
The effect of dynamical friction explains why the most massive stars of SCs tend to be found near the center of star cluster. This concentration of more massive stars in the cluster's cores tend to favor collisions between stars, which may trigger the runaway collision mechanism to form intermediate mass black holes.{{cn|date=June 2024}} [[Globular cluster]]s orbiting through the stellar field of a galaxy experience dynamic friction. This drag force causes the cluster to lose energy and spiral in toward the galactic center.<ref>{{cite journal
 | title=Chandrasekhar's dynamical friction and non-extensive statistics
 | last1=Silva | first1=J. M. | last2=Lima | first2=J. A. S. | last3=de Souza | first3=R. E. | last4=Del Popolo | first4=A. | last5=Le Delliou | first5=Morgan | last6=Lee | first6=Xi-Guo
 | journal=Journal of Cosmology and Astroparticle Physics
 | issue=5 | at=id. 021 | date=May 2016
 | volume=2016 | doi=10.1088/1475-7516/2016/05/021 | arxiv=1604.02034
 | bibcode=2016JCAP...05..021S }}</ref>

===Photons===

[[Fritz Zwicky]] proposed in 1929 that a gravitational drag effect on photons could be used to explain [[cosmological redshift]] as a form of [[tired light]].<ref name=Zwicky1929>{{Citation | first = F. | last = Zwicky | title = On the Redshift of Spectral Lines Through Interstellar Space | authorlink = Fritz Zwicky
| journal = Proceedings of the National Academy of Sciences
| volume = 15
|date=October 1929 | issue = 10 
| pages = 773&ndash;779
| bibcode = 1929PNAS...15..773Z | doi = 10.1073/pnas.15.10.773 | pmc = 522555 | pmid=16577237
| doi-access = free }}.</ref> However, his analysis had a mathematical error, and his approximation to the magnitude of the effect should actually have been zero, as pointed out in the same year by [[Arthur Stanley Eddington]]. Zwicky promptly acknowledged the correction,<ref name=Zwicky1929B>{{Citation | title = On the Possibilities of a Gravitational Drag of Light | bibcode = 1929PhRv...34.1623Z | journal = Physical Review | volume = 34 | issue = 12 | pages = 1623&ndash;1624| date = 1929 | first = F. | last = Zwicky | authorlink = Fritz Zwicky | doi = 10.1103/PhysRev.34.1623.2 | postscript = . | url = https://authors.library.caltech.edu/5559/1/ZWIpr29.pdf }}</ref> although he continued to hope that a full treatment would be able to show the effect.

It is now known that the effect of dynamical friction on photons or other particles moving at relativistic speeds is negligible, since the magnitude of the drag is inversely proportional to the square of velocity. Cosmological redshift is conventionally understood to be a consequence of the [[expansion of the universe]].