Editing Light (section)

== Speed of light ==
{{Main|Speed of light}}
[[File:Rocca dell'Abisso, Fondachelli Fantina, Sicilia.JPG|thumb|Beam of sun light inside the cavity of Rocca ill'Abissu at [[Fondachelli-Fantina]], Sicily]]
The speed of light in [[vacuum]] is defined to be exactly {{val|299792458|u=m/s}} (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum.

Different [[physicist]]s have attempted to measure the speed of light throughout history. [[Galileo Galilei|Galileo]] attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by [[Ole Rømer]], a Danish physicist, in 1676. Using a [[telescope]], Rømer observed the motions of [[Jupiter]] and one of its [[natural satellite|moons]], [[Io (moon)|Io]]. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.<ref>{{cite journal |url=http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.ss%2F1009212817 |title=Scientific Method, Statistical Method and the Speed of Light |journal=Statistical Science |year=2000 |volume=15 |pages=254–278 |issue=3 |doi=10.1214/ss/1009212817 |mr=1847825 |last1=Oldford |first1=R. W |last2=MacKay |first2=R. J |doi-access=free |access-date=21 August 2008 |archive-date=24 March 2017 |archive-url=https://web.archive.org/web/20170324201543/http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.ss%2F1009212817 |url-status=live }}</ref> However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of {{val|227000000|u=m/s}}.

Another more accurate measurement of the speed of light was performed in Europe by [[Hippolyte Fizeau]] in 1849.<ref name=EB1911>{{cite EB1911 |wstitle=Light |volume=16 |page=624 |first=Simon |last=Newcomb}}</ref> Fizeau directed a beam of light at a mirror several kilometers away. A rotating [[cog wheel]] was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel and the rate of rotation, Fizeau was able to calculate the speed of light as {{val|313000000|u=m/s}}.

[[Léon Foucault]] carried out an experiment which used rotating mirrors to obtain a value of {{val|298000000|u=m/s}}<ref name=EB1911/> in 1862. [[Albert A. Michelson]] conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from [[Mount Wilson (California)|Mount Wilson]] to [[Mount San Antonio]] in California. The precise measurements yielded a speed of {{val|299796000|u=m/s}}.<ref>{{cite journal |last=Michelson |first=A.A. |title=Measurements of the velocity of light between Mount Wilson and Mount San Antonio |journal=Astrophysical Journal |date=January 1927 |volume=65 |page=1 |doi=10.1086/143021 |bibcode=1927ApJ....65....1M}}</ref>

The effective velocity of light in various transparent substances containing ordinary [[matter]], is less than in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists were said to bring light to a "complete standstill" by passing it through a [[Bose–Einstein condensate]] of the element [[rubidium]], one team at [[Harvard University]] and the [[Rowland Institute for Science]] in Cambridge, Massachusetts and the other at the [[Harvard–Smithsonian Center for Astrophysics]], also in Cambridge.<ref>{{cite web |author=Harvard News Office |url=http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html |title=Harvard Gazette: Researchers now able to stop, restart light |publisher=News.harvard.edu |date=24 January 2001 |access-date=8 November 2011 |archive-url=https://web.archive.org/web/20111028041346/http://www.news.harvard.edu/gazette/2001/01.24/01-stoplight.html |archive-date=28 October 2011 }}</ref> However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped", it had ceased to be light.