Editing Speed of light (section)

== History ==
Until the [[early modern period]], it was not known whether light travelled instantaneously or at a very fast finite speed. The first extant recorded examination of this subject was in [[ancient Greece]]. The ancient Greeks, Arabic scholars, and classical European scientists long debated this until Rømer provided the first calculation of the speed of light. Einstein's theory of special relativity postulates that the speed of light is constant regardless of one's frame of reference. Since then, scientists have provided increasingly accurate measurements.

{| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"
|+ History of measurements of ''c'' (in m/s)
|-
! Year !! Experiment !! Value !! Deviation from 1983 value
|-
| <1638 || [[Galileo Galilei|Galileo]], covered lanterns ||colspan="2"| inconclusive<ref name=2newsciences /><ref name=boyer /><ref name=foschi&leone>
{{Citation
 |first1=Renato
 |last1=Foschi
 |first2=Matteo
 |last2=Leone
 |title=Galileo, measurement of the velocity of light, and the reaction times
 |journal=Perception
 |volume=38
 |issue=8
 |year=2009
 |pages=1251–1259
 |doi=10.1068/p6263
 |pmid=19817156
 |hdl=2318/132957
 |s2cid=11747908
 |hdl-access=free
}}</ref>{{rp|1252}}
|-
| <1667 || [[Accademia del Cimento]], covered lanterns ||colspan="2"| inconclusive<ref name=foschi&leone />{{rp|1253}}<ref name=magalotti>
{{Citation
 |first1=Lorenzo
 |last1=Magalotti
 |author-link=Lorenzo Magalotti
 |title=Saggi di Naturali Esperienze fatte nell' Accademia del Cimento
 |url=http://brunelleschi.imss.fi.it/cimentosite/indice.asp?xmlFile=Indice00.xml
 |edition=digital, online
 |publisher=Istituto e Museo di Storia delle Scienze
 |place=Florence
 |year=2001
 |orig-year=1667
 |access-date=25 September 2015
 |pages=[http://brunelleschi.imss.fi.it/cimentosite/ShowFullSize.asp?Image=FullSize/A0000283.JPG&Title=Pagina:%20265 265]–[http://brunelleschi.imss.fi.it/cimentosite/ShowFullSize.asp?Image=FullSize/A0000284.JPG&Title=Pagina:%20266 266]
}}</ref>
|-
| 1675 || [[Ole&nbsp;Rømer|Rømer]] and [[Christiaan&nbsp;Huygens|Huygens]], moons of Jupiter || {{val|220000000}}<ref name=roemer/><ref name="Huygens 1690 8–9"/> || −27%
|-
| 1729 || [[James&nbsp;Bradley]], aberration of light || {{val|301000000}}<ref name=How/> || +0.40%
|-
| 1849 || [[Hippolyte&nbsp;Fizeau]], toothed wheel || {{val|315000000}}<ref name=How/> || +5.1%
|-
| 1862 || [[Léon&nbsp;Foucault]], rotating mirror || {{val|298000000|500000}}<ref name=How/> || −0.60%
|-
| 1875 || Werner Siemens || 260 000 000<ref>{{Cite journal |last1=Buchwald |first1=Jed |last2=Yeang |first2=Chen-Pang |last3=Stemeroff |first3=Noah |last4=Barton |first4=Jenifer |last5=Harrington |first5=Quinn |date=2021-03-01 |title=What Heinrich Hertz discovered about electric waves in 1887–1888 |url=https://doi.org/10.1007/s00407-020-00260-1 |journal=Archive for History of Exact Sciences |language=en |volume=75 |issue=2 |pages=125–171 |doi=10.1007/s00407-020-00260-1 |s2cid=253895826 |issn=1432-0657}}</ref> ||
|-
| 1893 || [[Heinrich Hertz]] || 200 000 000<ref>{{Cite book |last=Hertz |first=Heinrich |title=Electric Waves |publisher=Macmillan and Co. |year=1893 |location=London}}</ref> ||
|-
| 1907 || Rosa and Dorsey, <abbr title="electromagnetic">EM</abbr> constants || {{val|299710000|30000}}<ref name="Essen1948"/><ref name="RosaDorsey"/> || −280 [[Parts-per notation|ppm]]
|-
| 1926 || [[Albert A. Michelson]], rotating mirror || {{val|299796000|4000}}<ref>{{Cite journal |doi = 10.1086/143021 |title = Measurement of the Velocity of Light Between Mount Wilson and Mount San Antonio |year = 1927 |last1 = Michelson |first1 = A. A. |journal = The Astrophysical Journal| volume = 65 |pages = 1 |bibcode=1927ApJ....65....1M}}</ref> || +12 ppm
|-
| 1950 || {{nowrap|Essen and Gordon-Smith}}, cavity resonator || {{val|299792500|3000}}<ref name="Essen1950"/> || +0.14 ppm
|-
| 1958 || K. D. Froome, radio interferometry || {{val|299792500|100}}<ref name="Froome1858"/> || +0.14 ppm
|-
| 1972 || Evenson ''et al.'', laser interferometry || {{val|299792456.2|1.1}}<ref name="NIST heterodyne"/> || −0.006 ppm
|-
| 1983 || 17th CGPM, definition of the metre ||colspan="2"| {{val|299792458}} (exact)<ref name=Resolution_1/>
|}

=== Early history ===
[[Empedocles]] (c. 490–430 BCE) was the first to propose a theory of light<ref>
{{Cite book
 |title=Light-Matter Interaction: Physics and Engineering at the Nanoscale |edition=illustrated
 |first1=John
 |last1=Weiner
 |first2=Frederico
 |last2=Nunes
 |publisher=OUP Oxford
 |year=2013
 |isbn=978-0-19-856766-0
 |page=1
 |url=https://books.google.com/books?id=ctpG-kmmK8kC
}} [https://books.google.com/books?id=ctpG-kmmK8kC&pg=PA1 Extract of page 1].</ref> and claimed that light has a finite speed.<ref>
{{Cite book
 |last=Sarton |first=G. |author-link=George Sarton
 |year=1993
 |title=Ancient science through the golden age of Greece
 |url=https://books.google.com/books?id=VcoGIKlHuZcC&pg=PA248
 |page=248
 |publisher=[[Courier Dover]]
 |isbn=978-0-486-27495-9
}}</ref> He maintained that light was something in motion, and therefore must take some time to travel. [[Aristotle]] argued, to the contrary, that "light is due to the presence of something, but it is not a movement".<ref name=Statistics>
{{Cite journal
 |last1=MacKay |first1=R. H. |last2=Oldford |first2=R. W.
 |year=2000
 |title=Scientific Method, Statistical Method and the Speed of Light
 |url=http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/
 |journal=[[Statistical Science (journal)|Statistical Science]]
 |volume=15 |issue=3 |pages=254–278
 |doi=10.1214/ss/1009212817
 |doi-access=free
}} (click on "Historical background" in the table of contents)</ref> [[Euclid]] and [[Ptolemy]] advanced Empedocles' [[Emission theory (vision)|emission theory]] of vision, where light is emitted from the eye, thus enabling sight. Based on that theory, [[Heron of Alexandria]] argued that the speed of light must be [[Infinity|infinite]] because distant objects such as stars appear immediately upon opening the eyes.<ref>{{Cite book |title=Electronic Microwave Imaging with Planar Multistatic Arrays |first1=Sherif Sayed |last1=Ahmed |publisher=Logos Verlag Berlin |year=2014 |isbn=978-3-8325-3621-3 |page=1 |url=https://books.google.com/books?id=ob79AgAAQBAJ}} [https://books.google.com/books?id=ob79AgAAQBAJ&pg=PA1 Extract of page 1]</ref>

[[Early Islamic philosophy|Early Islamic philosophers]] initially agreed with the [[Aristotelian physics|Aristotelian view]] that light had no speed of travel. In 1021, [[Alhazen]] (Ibn al-Haytham) published the ''[[Book of Optics]]'', in which he presented a series of arguments dismissing the emission theory of [[Visual perception|vision]] in favour of the now accepted intromission theory, in which light moves from an object into the eye.<ref>
{{Cite journal
 | last1 = Gross | first1 = C. G.
 | title = The Fire That Comes from the Eye
 | journal = Neuroscientist
 | volume = 5
 | pages = 58–64
 | year = 1999
 | doi = 10.1177/107385849900500108
 | s2cid = 84148912
}}</ref> This led Alhazen to propose that light must have a finite speed,<ref name=Statistics/><ref name=Hamarneh>
{{Cite journal
 |last=Hamarneh |first=S.
 |year=1972
 |title=Review: Hakim Mohammed Said, ''Ibn al-Haitham''
 |journal=[[Isis (journal)|Isis]]
 |volume=63 |issue=1 |page=119
 |doi=10.1086/350861
}}</ref><ref name=Lester>
{{Cite book
 |last=Lester |first=P. M.
 |year=2005
 |title=Visual Communication: Images With Messages
 |pages=10–11
 |publisher=[[Thomson Wadsworth]]
 |isbn=978-0-534-63720-0
}}</ref> and that the speed of light is variable, decreasing in denser bodies.<ref name=Lester/><ref>
{{Cite web
 |first1=J. J.
 |last1=O'Connor
 |author-link1=John J. O'Connor (mathematician)
 |first2=E. F.
 |last2=Robertson
 |author-link2=Edmund F. Robertson
 |url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Al-Haytham.html
 |title=Abu Ali al-Hasan ibn al-Haytham
 |work=[[MacTutor History of Mathematics archive]]
 |publisher=[[University of St Andrews]]
 |access-date=12 January 2010
}}</ref> He argued that light is substantial matter, the propagation of which requires time, even if this is hidden from the senses.<ref>
{{Cite conference
 |last         = Lauginie
 |first        = P.
 |year         = 2004
 |title        = Measuring Speed of Light: Why? Speed of what?
 |url          = http://sci-ed.org/documents/Lauginie-M.pdf
 |conference   = Fifth International Conference for History of Science in Science Education
 |location     = Keszthely, Hungary
 |pages        = 75–84
 |access-date  = 12 August 2017
 |archive-url  = https://web.archive.org/web/20150704043700/http://sci-ed.org/documents/Lauginie-M.pdf
 |archive-date = 4 July 2015
 |url-status=dead
}}</ref> Also in the 11th century, [[Al-Biruni|Abū Rayhān al-Bīrūnī]] agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.<ref>
{{Cite web
 |first1=J. J.
 |last1=O'Connor
 |first2=E. F.
 |last2=Robertson
 |url=http://www-history.mcs.st-andrews.ac.uk/Biographies/Al-Biruni.html
 |title=Abu han Muhammad ibn Ahmad al-Biruni
 |work=MacTutor History of Mathematics archive
 |publisher=University of St Andrews
 |access-date=12 January 2010
}}</ref>

In the 13th century, [[Roger Bacon]] argued that the speed of light in air was not infinite, using philosophical arguments backed by the writing of Alhazen and Aristotle.<ref name=Lindberg>
{{Cite book
 |last=Lindberg |first=D. C.
 |year=1996
 |title=Roger Bacon and the origins of Perspectiva in the Middle Ages: a critical edition and English translation of Bacon's Perspectiva, with introduction and notes
 |url=https://books.google.com/books?id=jSPHMKbjYkQC&pg=PA143
 |page=143
 |isbn=978-0-19-823992-5
 |publisher=Oxford University Press
}}</ref><ref>
{{Cite book
 |last=Lindberg |first=D. C.
 |year=1974
 |chapter=Late Thirteenth-Century Synthesis in Optics
 |editor=Edward Grant
 |title=A source book in medieval science
 |chapter-url=https://books.google.com/books?id=fAPN_3w4hAUC&q=roger-bacon%20speed-of-light&pg=RA1-PA395
 |page=396
 |publisher=Harvard University Press
 |isbn=978-0-674-82360-0
}}</ref> In the 1270s, [[Witelo]] considered the possibility of light travelling at infinite speed in vacuum, but slowing down in denser bodies.<ref name=Marshall>
{{Cite journal
 |last=Marshall |first=P.
 |year=1981
 |title=Nicole Oresme on the Nature, Reflection, and Speed of Light
 |journal=[[Isis (journal)|Isis]]
 |volume=72 |issue=3 |pages=357–374 [367–374]
 |doi=10.1086/352787
 |s2cid=144035661
}}</ref>

In the early 17th century, [[Johannes Kepler]] believed that the speed of light was infinite since empty space presents no obstacle to it. [[René Descartes]] argued that if the speed of light were to be finite, the Sun, Earth, and Moon would be noticeably out of alignment during a [[lunar eclipse]]. Although this argument fails when aberration of light is taken into account, the latter was not recognized until the following century.<ref>{{Cite journal |last=Sakellariadis |first=Spyros |date=1982 |title=Descartes' Experimental Proof of the Infinite Velocity of Light and Huygens' Rejoinder |url=https://www.jstor.org/stable/41133639 |journal=[[Archive for History of Exact Sciences]] |volume=26 |issue=1 |pages=1–12 |doi=10.1007/BF00348308 |jstor=41133639 |s2cid=118187860 |issn=0003-9519}}</ref> Since such misalignment had not been observed, Descartes concluded the speed of light was infinite. Descartes speculated that if the speed of light were found to be finite, his whole system of philosophy might be demolished.<ref name=Statistics /> Despite this, in his derivation of [[Snell's law]], Descartes assumed that some kind of motion associated with light was faster in denser media.<ref>{{Cite book |last=Cajori |first=Florian |url=https://books.google.com/books?id=XNtUAAAAYAAJ |title=A History of Physics in Its Elementary Branches: Including the Evolution of Physical Laboratories |date=1922 |publisher=Macmillan |pages=76 |language=en}}</ref><ref>{{Cite journal |last=Smith |first=A. Mark |date=1987 |title=Descartes's Theory of Light and Refraction: A Discourse on Method |url=https://www.jstor.org/stable/1006537 |journal=[[Transactions of the American Philosophical Society]] |volume=77 |issue=3 |pages=i–92 |doi=10.2307/1006537 |jstor=1006537 |issn=0065-9746}}</ref> [[Pierre de Fermat]] derived Snell's law using the opposing assumption, the denser the medium the slower light travelled. Fermat also argued in support of a finite speed of light.<ref>{{Cite book|author-link=Carl Benjamin Boyer |first=Carl Benjamin |last=Boyer |title=The Rainbow: From Myth to Mathematics |year=1959 |pages=205–206 |publisher=Thomas Yoseloff |oclc=763848561}}</ref>

=== First measurement attempts ===

In 1629, [[Isaac Beeckman]] proposed an experiment in which a person observes the flash of a cannon reflecting off a mirror about one mile (1.6&nbsp;km) away. In 1638, [[Galileo Galilei]] proposed an experiment, with an apparent claim to having performed it some years earlier, to measure the speed of light by observing the delay between uncovering a lantern and its perception some distance away. He was unable to distinguish whether light travel was instantaneous or not, but concluded that if it were not, it must nevertheless be extraordinarily rapid.<ref name=2newsciences>
{{Cite book
 |last=Galilei
 |first=G.
 |year=1954
 |orig-year=1638
 |title=Dialogues Concerning Two New Sciences
 |url=https://www.questia.com/read/88951396/dialogues-concerning-two-new-sciences
 |page=43
 |others=Crew, H.; de Salvio A. (trans.)
 |publisher=[[Dover Publications]]
 |isbn=978-0-486-60099-4
 |ref=Reference-Galileo-1954
 |access-date=29 January 2019
 |archive-date=30 January 2019
 |archive-url=https://web.archive.org/web/20190130053744/https://www.questia.com/read/88951396/dialogues-concerning-two-new-sciences
 |url-status=dead
}}</ref><ref name=boyer>
{{Cite journal
 |last=Boyer |first=C. B.
 |year=1941
 |title=Early Estimates of the Velocity of Light
 |journal=[[Isis (journal)|Isis]]
 |volume=33 |issue=1 |page=24
 |doi=10.1086/358523
 |s2cid=145400212
 |ref=boyer-1941
}}</ref> According to Galileo, the lanterns he used were "at a short distance, less than a mile". Assuming the distance was not too much shorter than a mile, and that "about a thirtieth of a second is the minimum time interval distinguishable by the unaided eye", Boyer notes that Galileo's experiment could at best be said to have established a lower limit of about 60 miles per second for the velocity of light.<ref name="boyer"/> In 1667, the [[Accademia del Cimento]] of Florence reported that it had performed Galileo's experiment, with the lanterns separated by about one&nbsp;mile, but no delay was observed.<ref>{{Cite journal|last1=Foschi|first1=Renato|last2=Leone|first2=Matteo|date=August 2009|title=Galileo, Measurement of the Velocity of Light, and the Reaction Times|url=http://journals.sagepub.com/doi/10.1068/p6263|journal=Perception|language=en|volume=38|issue=8|pages=1251–1259|doi=10.1068/p6263|pmid=19817156|hdl=2318/132957 |s2cid=11747908|issn=0301-0066|hdl-access=free}}</ref> The actual delay in this experiment would have been about 11 [[microsecond]]s.

[[File:Illustration from 1676 article on Ole Rømer's measurement of the speed of light.png|thumb|upright=0.8|Rømer's observations of the occultations of Io from Earth|alt=A diagram of a planet's orbit around the Sun and of a moon's orbit around another planet. The shadow of the latter planet is shaded.]]
[[Rømer's determination of the speed of light|The first quantitative estimate of the speed of light]] was made in 1676 by Ole Rømer.<ref name="cohen"/><ref name="roemer"/> From the observation that the periods of Jupiter's innermost moon [[Io (moon)|Io]] appeared to be shorter when the Earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. [[Christiaan Huygens]] combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of speed of light of {{val|220000|u=km/s}}, which is 27% lower than the actual value.<ref name="Huygens 1690 8–9">
{{Cite book
 |last=Huygens |first=C.
 |year=1690
 |title=Traitée de la Lumière |language=fr
 |url=https://archive.org/details/bub_gb_kVxsaYdZaaoC
 |pages=[https://archive.org/details/bub_gb_kVxsaYdZaaoC/page/n19 8]–9
 |publisher=Pierre van der Aa
}}</ref>

In his 1704 book ''[[Opticks]]'', [[Isaac Newton]] reported Rømer's calculations of the finite speed of light and gave a value of "seven or eight minutes" for the time taken for light to travel from the Sun to the Earth (the modern value is 8&nbsp;minutes 19&nbsp;seconds).<ref>
{{Cite book
 |last=Newton |first=I.
 |year=1704
 |contribution=Prop. XI
 |title=Optiks
 |url=http://gallica.bnf.fr/ark:/12148/bpt6k3362k.image.f235.vignettesnaviguer
}} The text of Prop.&nbsp;XI is identical between the first (1704) and second (1719) editions.</ref> Newton queried whether Rømer's eclipse shadows were coloured. Hearing that they were not, he concluded the different colours travelled at the same speed. In 1729, [[James Bradley]] discovered [[aberration of light|stellar aberration]].<ref name="Bradley1729"/> From this effect he determined that light must travel 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8&nbsp;minutes 12&nbsp;seconds to travel from the Sun to the Earth.<ref name="Bradley1729"/>

=== Connections with electromagnetism ===
{{See also|History of electromagnetic theory|History of special relativity}}
In the 19th century [[Hippolyte Fizeau]] developed a method to determine the speed of light based on time-of-flight measurements on Earth and reported a value of {{val|315000|u=km/s}}.<ref name="guarnieri 7-1">{{Cite journal|last=Guarnieri|first=M.|year=2015|title=Two Millennia of Light: The Long Path to Maxwell's Waves|journal=IEEE Industrial Electronics Magazine|volume=9|issue=2|pages=54–56, 60|doi=10.1109/MIE.2015.2421754|s2cid=20759821}}</ref> His method was improved upon by [[Léon Foucault]] who obtained a value of {{val|298000|u=km/s}} in 1862.<ref name="How"/> In the year 1856, [[Wilhelm Eduard Weber]] and [[Rudolf Kohlrausch]] measured the ratio of the electromagnetic and electrostatic units of charge, 1/{{radic|''ε''<sub>0</sub>''μ''<sub>0</sub>}}, by discharging a [[Leyden jar]], and found that its numerical value was very close to the speed of light as measured directly by Fizeau. The following year [[Gustav Kirchhoff]] calculated that an electric signal in a [[electrical resistance|resistanceless]] wire travels along the wire at this speed.<ref>
{{Cite journal
 |last1=Kirchhoff |first1=G.
 |title=Über die Bewegung der Elektricität
 |url=http://gallica.bnf.fr/ark:/12148/bpt6k15187j/f549.item.r=
 |journal=[[Annalen der Physik]]
 |volume=178
 |issue=12
 |year=1857
 |pages=529–244
 |doi=10.1002/andp.18571781203
 |bibcode=1857AnP...178..529K
}}</ref>

In the early 1860s, Maxwell showed that, according to the theory of electromagnetism he was working on, electromagnetic waves propagate in empty space<ref>See, for example:
* {{Cite book
 |title=College physics: reasoning and relationships
 |first1=Nicholas J.
 |last1=Giordano
 |publisher=Cengage Learning
 |year=2009
 |isbn=978-0-534-42471-8
 |page=787
 |url=https://books.google.com/books?id=BwistUlpZ7cC
}} [https://books.google.com/books?id=BwistUlpZ7cC&pg=PA787 Extract of page 787]
* {{Cite book
 |title=The riddle of gravitation
 |first1=Peter Gabriel
 |last1=Bergmann
 |publisher=Courier Dover Publications
 |year=1992
 |isbn=978-0-486-27378-5
 |page=17
 |url=https://books.google.com/books?id=WYxkrwMidp0C
}} [https://books.google.com/books?id=WYxkrwMidp0C&pg=PA17 Extract of page 17]
* {{Cite book
 |title=The equations: icons of knowledge
 |first1=Sander
 |last1=Bais
 |publisher=Harvard University Press
 |year=2005
 |isbn=978-0-674-01967-6
 |page=[https://archive.org/details/equationsiconsof0000bais/page/40 40]
 |url=https://archive.org/details/equationsiconsof0000bais|url-access=registration
}} [https://archive.org/details/equationsiconsof0000bais/page/40 Extract of page 40]
</ref> at a speed equal to the above Weber/Kohlrausch ratio, and drawing attention to the numerical proximity of this value to the speed of light as measured by Fizeau, he proposed that light is in fact an electromagnetic wave.<ref name=maxwellbio>
{{Cite web
 |last1=O'Connor
 |first1=J. J.
 |last2=Robertson
 |first2=E. F.
 |date=November 1997
 |title=James Clerk Maxwell
 |url=http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Maxwell.html
 |publisher=School of Mathematics and Statistics, [[University of St Andrews]]
 |access-date=13 October 2010
 |url-status=dead
 |archive-url=https://web.archive.org/web/20110128034939/http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Maxwell.html
 |archive-date=28 January 2011
}}</ref> Maxwell backed up his claim with his own experiment published in the 1868 Philosophical Transactions which determined the ratio of the electrostatic and electromagnetic units of electricity.<ref>Campbell, Lewis; Garnett, William; Rautio, James C. "The Life of James Clerk Maxwell", p.&nbsp;544, {{ISBN|978-1-77375-139-9}}.</ref>

=== "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}}</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>

=== Special relativity ===
In 1905 Einstein postulated from the outset that the speed of light in vacuum, measured by a non-accelerating observer, is independent of the motion of the source or observer. Using this and the principle of relativity as a basis he derived the [[special theory of relativity]], in which the speed of light in vacuum ''c'' featured as a fundamental constant, also appearing in contexts unrelated to light. This made the concept of the stationary aether (to which Lorentz and Poincaré still adhered) useless and revolutionized the concepts of space and time.<ref>
{{Cite book
 |last=Miller
 |first=A. I.
 |year=1981
 |title=Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911)
 |publisher=Addison–Wesley
 |isbn=978-0-201-04679-3
 |url-access=registration
 |url=https://archive.org/details/alberteinsteinss0000mill
}}</ref><ref>
{{Cite book
 |last=Pais |first=A.
 |author-link=Abraham Pais
 |year=1982
 |title= [[Subtle is the Lord: The Science and the Life of Albert Einstein]]
 |publisher=Oxford University Press
 |isbn=978-0-19-520438-4
}}</ref>

=== Increased accuracy of ''c'' and redefinition of the metre and second ===
{{See also|History of the metre}}

In the second half of the 20th century, much progress was made in increasing the accuracy of measurements of the speed of light, first by cavity resonance techniques and later by laser interferometer techniques. These were aided by new, more precise, definitions of the metre and second. In 1950, [[Louis Essen]] determined the speed as {{val|299792.5|3.0|u=km/s}}, using cavity resonance.<ref name="Essen1950"/> This value was adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. In 1960, the [[history of the metre#Krypton standard|metre was redefined]] in terms of the wavelength of a particular spectral line of [[krypton-86]], and, in 1967, the second was redefined in terms of the hyperfine transition frequency of the ground state of [[caesium-133]].<ref name="13thCGPMr1">
{{Cite web
 |year=1967
 |title=Resolution 1 of the 15th CGPM
 |url=https://www.bipm.org/en/CGPM/db/13/1/
 |publisher=[[International Bureau of Weights and Measures|BIPM]]
 |access-date=14 March 2021
 |archive-date=11 April 2021
 |archive-url=https://web.archive.org/web/20210411132806/https://www.bipm.org/en/CGPM/db/13/1/
 |url-status=dead
}}</ref>

In 1972, using the laser interferometer method and the new definitions, a group at the US [[National Institute of Standards and Technology|National Bureau of Standards]] in [[Boulder, Colorado]] determined the speed of light in vacuum to be ''c''&nbsp;=&nbsp;{{val|299792456.2|1.1|u=m/s}}. This was 100 times less uncertain than the previously accepted value. The remaining uncertainty was mainly related to the definition of the metre.<ref name="11thCGPM">
{{Cite web
 |year=1967
 |title=Resolution 6 of the 15th CGPM
 |url=http://www.bipm.org/en/CGPM/db/11/6/
 |publisher=[[International Bureau of Weights and Measures|BIPM]]
 |access-date=13 October 2010
}}</ref><ref name="NIST heterodyne"/> As similar experiments found comparable results for ''c'', the 15th [[General Conference on Weights and Measures]] in 1975 recommended using the value {{val|299792458|u=m/s}} for the speed of light.<ref name="15thCGPM">
{{Cite web
 |year=1975
 |title=Resolution 2 of the 15th CGPM
 |url=http://www.bipm.org/en/CGPM/db/15/2/
 |publisher=BIPM
 |access-date=9 September 2009
}}</ref>

=== Defined as an explicit constant ===
In 1983 the 17th meeting of the General Conference on Weights and Measures (CGPM) found that wavelengths from frequency measurements and a given value for the speed of light are more [[reproducibility|reproducible]] than the previous standard. They kept the 1967 definition of second, so the [[caesium]] [[Hyperfine structure#Use in defining the SI second and meter|hyperfine frequency]] would now determine both the second and the metre. To do this, they redefined the metre as "the length of the path traveled by light in vacuum during a time interval of 1/{{val|299792458}} of a second".<ref name=Resolution_1>
{{Cite web
 |year=1983
 |title=Resolution 1 of the 17th CGPM
 |url=https://www.bipm.org/en/committees/cg/cgpm/17-1983/resolution-1
 |publisher=BIPM
 |access-date=23 August 2009
}}</ref>

As a result of this definition, the value of the speed of light in vacuum is exactly {{val|299792458|u=m/s}}<ref name=Wheeler>
{{Cite book
 |last1=Taylor |first1=E. F. |author-link1=Edwin F. Taylor |last2=Wheeler |first2=J. A. |author-link2=John Archibald Wheeler
 |year=1992
 |title=Spacetime Physics: Introduction to Special Relativity
 |url=https://books.google.com/books?id=PDA8YcvMc_QC&pg=PA59
 |edition=2
 |publisher=Macmillan
 |isbn=978-0-7167-2327-1
 |page=59
}}</ref><ref name=timeline>
{{Cite web
 |last=Penzes |first=W. B.
 |year=2009
 |title=Time Line for the Definition of the Meter
 |url=https://www.nist.gov/pml/div683/upload/museum-timeline.pdf
 |publisher=[[National Institute of Standards and Technology|NIST]]
 |access-date=11 January 2010
}}</ref> and has become a defined constant in the SI system of units.<ref name="fixes"/> Improved experimental techniques that, prior to 1983, would have measured the speed of light no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of krypton-86 and other light sources.<ref name=Adams>
{{Cite book
 |last=Adams |first=S.
 |year=1997
 |title=Relativity: An Introduction to Space–Time Physics
 |url=https://books.google.com/books?id=1RV0AysEN4oC&pg=PA140
 |page=140
 |publisher=CRC Press
 |isbn=978-0-7484-0621-0
 |quote=One peculiar consequence of this system of definitions is that any future refinement in our ability to measure&nbsp;''c'' will not change the speed of light (which is a defined number), but will change the length of the meter!
}}</ref><ref name=W_Rindler>
{{Cite book
 |last=Rindler |first=W.
 |year=2006
 |title=Relativity: Special, General, and Cosmological
 |url=https://books.google.com/books?id=MuuaG5HXOGEC&pg=PT41
 |page=41
 |edition=2
 |publisher=Oxford University Press
 |isbn=978-0-19-856731-8
 |quote=Note that [...] improvements in experimental accuracy will modify the meter relative to atomic wavelengths, but not the value of the speed of light!
}}</ref>

In 2011, the CGPM stated its intention to redefine all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. It proposed a new, but completely equivalent, wording of the metre's definition: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly {{val|299792458}} when it is expressed in the SI unit {{nowrap|m s<sup>−1</sup>}}."<ref>{{Cite web |url=http://www.bipm.org/en/si/new_si/explicit_constant.html |title=The "explicit-constant" formulation |archive-url=https://web.archive.org/web/20140811195806/http://www.bipm.org/en/si/new_si/explicit_constant.html |archive-date=11 August 2014 |website=BIPM |date=2011}}</ref> This was one of the changes that was incorporated in the [[2019 revision of the SI]], also termed the ''New SI''.<ref>See, for example:
* {{Cite web |last=Conover |first=Emily |author-link=Emily Conover |date=2 November 2016 |title=Units of measure are getting a fundamental upgrade |url=https://www.sciencenews.org/article/units-measure-are-getting-fundamental-upgrade |access-date=6 February 2022 |website=[[Science News]] |language=en-US}}
* {{Cite journal |last1=Knotts |first1=Sandra |last2=Mohr |first2=Peter J. |last3=Phillips |first3=William D. |date=January 2017 |title=An Introduction to the New SI |url=http://scitation.aip.org/content/aapt/journal/tpt/55/1/10.1119/1.4972491 |journal=[[The Physics Teacher]] |language=en |volume=55 |issue=1 |pages=16–21 |doi=10.1119/1.4972491 |bibcode=2017PhTea..55...16K |s2cid=117581000 |issn=0031-921X}}
* {{Cite journal |date=11 May 2018 |title=SI Redefinition |url=https://www.nist.gov/si-redefinition |access-date=6 February 2022 |journal=[[National Institute of Standards and Technology]] |language=en}}
</ref>