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[[Fil:Speed of light from Earth to Moon 400px.gif|thumb|Lysets færd fra Jorden til Månen i realtid]]
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STOP MED AT BANDE MIG
'''Lysets hastighed''' eller '''lysets fart''' er den [[fart]], hvormed elektromagnetiske svingninger udbreder sig i et medium. I [[vakuum]] er [[lys]]ets hastighed maksimal, og siden 1983 er [[meter]]en defineret som den [[længde]], lyset tilbagelægger i vakuum på 1/299.792.458 [[sekund]]er<ref>[http://www.bipm.org/en/CGPM/db/17/1/ www.bipm.org/en/CGPM/db/17/1/]</ref>. Det vil sige, at lysets hastighed er 299.792.458 meter per sekund eller tæt på 300.000 km/s. I andre medier end vakuum er lysets hastighed mindre, se [[brydningsindeks]]. I [[partikel (fysik)|partikelmodellen]] opfattes lys som en strøm af masseløse partikler, såkaldte [[foton]]er, og lysets hastighed bliver så den hastighed, hvormed fotoner bevæger sig. Fotoner er masseløse men kan overføre impuls (bevægelsesmængde).
En stribe [[interferens|interferometriske]] eksperimenter udført af Michelson og Morley omkring år 1900 viste, at lysets hastighed ikke afhænger af den relative [[hastighed]] mellem kilde og observatør. Det blev dødsstødet for [[æterteorien]], der gik ud på, at lyset skulle udbrede sig i et særligt medium, æteren, som udfyldte tomrummet mellem materielle legemer, og i forhold til hvilken lysets hastighed skulle måles.
Lysets vakuumhastighed spiller en central rolle i Bo [[Albert Einstein|Einsteins]] [[specielle relativitetsteori]], der bl.a. forudsiger, at <math>c</math> er den øvre grænse for, hvor hurtigt noget kan bevæge sig. Forsøg på at falsificere denne forudsigelse, f.eks. de af Günther Nimtz i 1990'erne udførte forsøg på at realisere [[overlyshastighed]] ved [[kvantemekanisk tunnelering]] under udnyttelse af Hartmann-effekten, er indtil videre slået fejl.<ref>{{Cite web |url=http://theory.gsi.de/~vanhees/faq/nimtz/nimtz.html |title=www.theory.gsi.de |access-date=11. august 2006 |archive-date= 4. august 2003 |archive-url=https://web.archive.org/web/20030804060608/http://theory.gsi.de/~vanhees/faq/nimtz/nimtz.html |url-status=dead }}</ref>
I dag er lysets vakuumhastighed fastsat ved definition og kan derfor ikke bestemmes eksperimentielt eller mere præcist. [[Meter]]en er derimod defineret som den strækning lyset bevæger sig i vakuum på 1/299.792.458 sekund, og kan derfor bestemmes eksperimentielt og med større og større nøjagtighed. Lysets hastighed danner endvidere udgangspunkt for den astronomiske afstandsenhed [[lysår]].
Selvom det ikke længere giver mening at måle lysets hastighed i vakuum, udføres der fortsat forsøg med lysets hastighed. I 2001 lykkedes det eksempelvis [[Lene Vestergaard Hau]] og hendes gruppe at bremse lyset til gangfart uden tab af fasekohærens.
== Se også Rick Roll ==
* [[Cherenkovstråling]]
* [[Hesitation of Light]]
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