Speed of lightWord文档格式.docx

Speed of lightWord文档格式.docx

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Exactvalues

Metrespersecond

299,792,458

Planckunits

1

Approximatevalues

kilometrespersecond

300,000

kilometresperhour

1,080million

milespersecond

186,000

milesperhour

671million

astronomicalunitsperday

173

Approximatelightsignaltraveltimes

Distance

Time

onefoot

1.0ns

onemetre

3.3ns

fromgeostationaryorbittoEarth

119ms

thelengthofEarth'

sequator

134ms

fromMoontoEarth

1.3s

fromSuntoEarth（1AU）

8.3min

fromneareststartoSun（1.3pc）

4.24years

fromthenearestgalaxy（theCanisMajorDwarfGalaxy）toEarth

25,000years

acrosstheMilkyWay

100,000years

fromtheAndromedaGalaxytoEarth

2.5millionyears

Thespeedoflightinvacuum,usuallydenotedbyc,isaphysicalconstantimportantinmanyareasofphysics.Itsvalueis299,792,458metrespersecond,afigurethatisexactsincethelengthofthemetreisdefinedfromthisconstantandtheinternationalstandardfortime.[1]Inimperialunitsthisspeedisapproximately186,282milespersecond.

Accordingtospecialrelativity,cisthemaximumspeedatwhichallenergy,matter,andinformationintheuniversecantravel.Itisthespeedofallmasslessparticlesandassociatedfields—includingelectromagneticradiationsuchaslight—invacuum,anditispredictedbythecurrenttheorytobethespeedofgravity（thatis,gravitationalwaves）.Suchparticlesandwavestravelatcregardlessofthemotionofthesourceortheinertialframeofreferenceoftheobserver.Inthetheoryofrelativity,cinterrelatesspaceandtime,andappearsinthefamousequationofmass–energyequivalenceE 

= 

mc2.[2]

Thespeedatwhichlightpropagatesthroughtransparentmaterials,suchasglassorair,islessthanc.Theratiobetweencandthespeedvatwhichlighttravelsinamaterialiscalledtherefractiveindexnofthematerial（n 

c 

/ 

v）.Forexample,forvisiblelighttherefractiveindexofglassistypicallyaround1.5,meaningthatlightinglasstravelsatc/1.5≈200,000 

km/s;

therefractiveindexofairforvisiblelightisabout1.0003,sothespeedoflightinairisabout90 

km/sslowerthanc.

Inmostpracticalcases,lightcanbethoughtofasmovinginstantaneously,butforlongdistancesandverysensitivemeasurementsthefinitespeedoflighthasnoticeableeffects.Incommunicatingwithdistantspaceprobes,itcantakeminutestohoursforamessagetogetfromEarthtothespacecraftorviceversa.Thelightweseefromstarsleftthemmanyyearsago,allowingustostudythehistoryoftheuniversebylookingatdistantobjects.Thefinitespeedoflightalsolimitsthetheoreticalmaximumspeedofcomputers,sinceinformationmustbesentwithinthecomputerfromchiptochip.Finally,thespeedoflightcanbeusedwithtimeofflightmeasurementstomeasurelargedistancestohighprecision.

OleRø

merfirstdemonstratedin1676thatlighttravelledatafinitespeed（asopposedtoinstantaneously）bystudyingtheapparentmotionofJupiter'

smoonIo.In1865,JamesClerkMaxwellproposedthatlightwasanelectromagneticwave,andthereforetraveledatthespeedcappearinginhistheoryofelectromagnetism.In1905,AlbertEinsteinpostulatedthatthespeedoflightwithrespecttoanyinertialframeisindependentofthemotionofthelightsource,[3]andexploredtheconsequencesofthatpostulatebyderivingthespecialtheoryofrelativityandshowingthattheparameterchadrelevanceoutsideofthecontextoflightandelectromagnetism.Aftercenturiesofincreasinglyprecisemeasurements,in1975thespeedoflightwasknowntobe299,792,458 

m/swithameasurementuncertaintyof4partsperbillion.In1983,themetrewasredefinedintheInternationalSystemofUnits（SI）asthedistancetravelledbylightinvacuumin1⁄299,792,458ofasecond.Asaresult,thenumericalvalueofcinmetrespersecondisnowfixedexactlybythedefinitionofthemetre.[4]

[edit]Numericalvalue,notation,andunits

Thespeedoflightinvacuumisusuallydenotedbyc,for"

constant"

ortheLatinceleritas（meaning"

swiftness"

）.Originally,thesymbolVwasused,introducedbyJamesClerkMaxwellin1865.In1856,WilhelmEduardWeberandRudolfKohlrauschusedcforaconstantlatershowntoequal√2timesthespeedoflightinvacuum.In1894,PaulDruderedefinedcwithitsmodernmeaning.EinsteinusedVinhisoriginalGerman-languagepapersonspecialrelativityin1905,butin1907heswitchedtoc,whichbythenhadbecomethestandardsymbol.[5][6]

Sometimescisusedforthespeedofwavesinanymaterialmedium,andc0forthespeedoflightinvacuum.[7]Thissubscriptednotation,whichisendorsedinofficialSIliterature,[4]hasthesameformasotherrelatedconstants:

namely,μ0forthevacuumpermeabilityormagneticconstant,ε0forthevacuumpermittivityorelectricconstant,andZ0fortheimpedanceoffreespace.Thisarticleusescexclusivelyforthespeedoflightinvacuum.

IntheInternationalSystemofUnits（SI）,themetreisdefinedasthedistancelighttravelsinvacuumin1⁄299,792,458ofasecond.Thisdefinitionfixesthespeedoflightinvacuumatexactly299,792,458 

m/s.[8][9][10]Asadimensionalphysicalconstant,thenumericalvalueofcisdifferentfordifferentunitsystems.[Note1]Inbranchesofphysicsinwhichcappearsoften,suchasinrelativity,itiscommontousesystemsofnaturalunitsofmeasurementinwhichc=1.[12][13]Usingtheseunits,cdoesnotappearexplicitlybecausemultiplicationordivisionby1doesnotaffecttheresult.

[edit]Fundamentalroleinphysics

Seealso:

Introductiontospecialrelativity,Specialrelativity, 

andOne-wayspeedoflight

Thespeedatwhichlightwavespropagateinvacuumisindependentbothofthemotionofthewavesourceandoftheinertialframeofreferenceoftheobserver.[Note2]ThisinvarianceofthespeedoflightwaspostulatedbyEinsteinin1905,[3]afterbeingmotivatedbyMaxwell'

stheoryofelectromagnetismandthelackofevidencefortheluminiferousaether;

[14]ithassincebeenconsistentlyconfirmedbymanyexperiments.Itisonlypossibletoverifyexperimentallythatthetwo-wayspeedoflight（forexample,fromasourcetoamirrorandbackagain）isframe-independent,becauseitisimpossibletomeasuretheone-wayspeedoflight（forexample,fromasourcetoadistantdetector）withoutsomeconventionastohowclocksatthesourceandatthedetectorshouldbesynchronized.However,byadoptingEinsteinsynchronizationfortheclocks,theone-wayspeedoflightbecomesequaltothetwo-wayspeedoflightbydefinition.[13][15]Thespecialtheoryofrelativityexplorestheconsequencesofthisinvarianceofcwiththeassumptionthatthelawsofphysicsarethesameinallinertialframesofreference.[16][17]Oneconsequenceisthatcisthespeedatwhichallmasslessparticlesandwaves,includinglight,musttravelinvacuum.

TheLorentzfactorγasafunctionofvelocity.Itstartsat1andapproachesinfinityasvapproaches 

c.

Specialrelativityhasmanycounterintuitiveandexperimentallyverifiedimplications.[18]Theseincludetheequivalenceofmassandenergy（E=mc2）,lengthcontraction（movingobjectsshorten）,[Note3]andtimedilation（movingclocksrunslower）.Thefactor 

γbywhichlengthscontractandtimesdilate,isknownastheLorentzfactorandisgivenbyγ=（1−v2/c2）−1/2,wherevisthespeedoftheobject.Thedifferenceofγfrom1isnegligibleforspeedsmuchslowerthan 

c,suchasmosteverydayspeeds—inwhichcasespecialrelativityiscloselyapproximatedbyGalileanrelativity—butitincreasesatrelativisticspeedsanddivergestoinfinityasvapproachesc.

Theresultsofspecialrelativitycanbesummarizedbytreatingspaceandtimeasaunifiedstructureknownasspacetime（with 

crelatingtheunitsofspaceandtime）,andrequiringthatphysicaltheoriessatisfyaspecialsymmetrycalledLorentzinvariance,whosemathematicalformulationcontainstheparameter 

c.[21]Lorentzinvarianceisanalmostuniversalassumptionformodernphysicaltheories,suchasquantumelectrodynamics,quantumchromodynamics,theStandardModelofparticlephysics,andgeneralrelativity.Assuch,theparameter 

cisubiquitousinmodernphysics,appearinginmanycontextsthatareunrelatedtolight.Forexample,generalrelativitypredictsthat 

cisalsothespeedofgravityandofgravitationalwaves.[22][23]Innon-inertialframesofreference（gravitationallycurvedspaceoracceleratedreferenceframes）,thelocalspeedoflightisconstantandequalto 

c,butthespeedoflightalongatrajectoryoffinitelengthcandifferfrom 

c,dependingonhowdistancesandtimesaredefined.[24]

Itisgenerallyassumedthatfundamentalconstantssuchas 

chavethesamevaluethroughoutspacetime,meaningthattheydonotdependonlocationanddonotvarywithtime.However,ithasbeensuggestedinvarioustheoriesthatthespeedoflightmayhavechangedovertime.[25][26]Noconclusiveevidenceforsuchchangeshasbeenfound,buttheyremainthesubjectofongoingresearch.[27][28]

Italsoisgenerallyassumedthatthespeedoflightisisotropic,meaningthatithasthesamevalueregardlessofthedirectioninwhichitismeasured.Observationsoftheemissionsfromnuclearenergylevelsasafunctionoftheorientationoftheemittingnucleiinamagneticfield（seeHughes–Dreverexperiment）,andofrotatingopticalresonators（seeResonat