Editing Speed of light (section)

=== "Luminiferous aether" ===
{{main|Luminiferous aether}}
The wave properties of light were well known since [[Thomas Young (scientist)|Thomas Young]]. In the 19th century, physicists believed light was propagating in a medium called aether (or ether). But for electric force, it looks more like the gravitational force in Newton's law. A transmitting medium was not required. After Maxwell theory unified light and electric and magnetic waves, it was favored that both light and electric magnetic waves propagate in the same aether medium (or called the [[luminiferous aether]]).<ref>{{Cite journal |last=Watson |first=E. C. |date=1957-09-01 |title=On the Relations between Light and Electricity |url=https://doi.org/10.1119/1.1934460 |journal=American Journal of Physics |volume=25 |issue=6 |pages=335–343 |doi=10.1119/1.1934460 |bibcode=1957AmJPh..25..335W |issn=0002-9505|url-access=subscription }}</ref>
[[File:Einstein en Lorentz.jpg|thumb|upright|Hendrik Lorentz (right) with Albert Einstein (1921)]]
It was thought at the time that empty space was filled with a background medium called the luminiferous aether in which the electromagnetic field existed. Some physicists thought that this aether acted as a [[preferred frame]] of reference for the propagation of light and therefore it should be possible to measure the motion of the Earth with respect to this medium, by measuring the [[isotropy]] of the speed of light. Beginning in the 1880s several experiments were performed to try to detect this motion, the most famous of which is [[Michelson–Morley experiment|the experiment]] performed by [[Albert A. Michelson]] and [[Edward W. Morley]] in 1887.<ref>{{Cite book |last1=Consoli |first1=Maurizio |last2=Pluchino |first2=Alessandro |date=2018 |title=Michelson-Morley Experiments: An Enigma for Physics & The History of Science |url=https://books.google.com/books?id=VdWEDwAAQBAJ&pg=PA118 |publisher=World Scientific |pages=118–119 |isbn=978-9-813-27818-9 |access-date=4 May 2020}}</ref><ref>
{{Cite journal
 |last1=Michelson |first1=A. A. |last2=Morley |first2=E. W.
 |year=1887
 |title=On the Relative Motion of the Earth and the Luminiferous Ether
 |journal=[[American Journal of Science]]
 |volume=34 |issue=203 |pages=333–345
 |doi=10.1366/0003702874447824
 |s2cid=98374065
 |url=https://zenodo.org/record/1450078
}}</ref> The detected motion was found to always be nil (within observational error). Modern experiments indicate that the two-way speed of light is [[isotropic]] (the same in every direction) to within 6 nanometres per second.<ref>
{{Cite book
 | last = French | first = A. P.
 | year = 1983
 | title = Special relativity
 | pages = 51–57
 | publisher = Van Nostrand Reinhold
 | isbn = 978-0-442-30782-0
}}</ref>

Because of this experiment [[Hendrik Lorentz]] proposed that the motion of the apparatus through the aether may cause the apparatus to [[Lorentz contraction|contract]] along its length in the direction of motion, and he further assumed that the time variable for moving systems must also be changed accordingly ("local time"), which led to the formulation of the [[Lorentz transformation]]. Based on [[Lorentz ether theory|Lorentz's aether theory]], [[Henri Poincaré]] (1900) showed that this local time (to first order in ''v''/''c'') is indicated by clocks moving in the aether, which are synchronized under the assumption of constant light speed. In 1904, he speculated that the speed of light could be a limiting velocity in dynamics, provided that the assumptions of Lorentz's theory are all confirmed. In 1905, Poincaré brought Lorentz's aether theory into full observational agreement with the [[principle of relativity]].<ref>
{{Cite book
 |last=Darrigol
 |first=O.
 |year=2000
 |title=Electrodynamics from Ampére to Einstein
 |publisher=Clarendon Press
 |isbn=978-0-19-850594-5
 |url-access=registration
 |url=https://archive.org/details/electrodynamicsf0000darr
}}</ref><ref>
{{Cite book
 |last=Galison |first=P.
 |author-link=Peter Galison
 |year=2003
 |title= Einstein's Clocks, Poincaré's Maps: Empires of Time
 |publisher=W. W. Norton
 |isbn=978-0-393-32604-8
}}</ref>