Editing Dynamical friction (section)

===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]].