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Chapter6Light，spadtime
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&Grosseteste，BishopofLindthefirstcelloroftheUyofOxfordihtury，wasoheleadingthinkersofhisday，andapropoheworksoftheaGreeks.Forhim，asformanyphilosophers，thegelight’siderstandingthewrosseteste’streatiseo，entitledDeLuce，heextolstheprimaryimporta:‘Thefirst…isinmyopinionlight.Fhtofitsverynaturediffusesitselfiioninsuchaointoflightwillprodutaneouslyasphereoflightofasoever’.
Frosseteste，lightdefispropagationinstantlythroughouttheulight，thereishereforenoforumistakeplace.Matter，andthusthespatialextensios，arecoupledtolight，butotbeseparatelydefihisiio，spadmatter—ie’shaoquaiohedevelopmentologyinthesubsequeuries.
&ime
Foroheradmittednordemaioofspaceasapre-existiy，asortoftheatreisplayedthemselvese-scalemotioheheavenswasiohisideaofasetofuniversallaws.Einstein，bytrast，placeslightatthetreofspa，itdefiimebyvirtueofsettiffromooftheuher.Thefactthatthereisafinitemaximumspeedturnsouttomakespadtimeiein’stheoryofrelativityteachesusthatweotthithinkihisisbecauseourperofspadtimeisbasedosofdistaimeihesemeasuresappeardifferentlytothosemovious，becauseofthespeedlimitimposedbylight.
Howdoesthisstrawiningofspadtimebylightarise?Let’sstartwithoionofspathinkofthisasasortofscaffold—aaginaryrodsoffixedlengthallectedtogetherinathree-dimensionalframework，asshownionthoughtthatthissortofstructurepre-existeda，aalleventstookplaewhereinthisstructure.Eventsthereforebespecifiedbytheirdistanafixedpointintheframebygthenumberofrodstoreachthelooftheevent.Ofcourseeventsoccuratatime，too，sothesustbeequippedwitheasurethetime.Ifaclockisplacedatthejunofeachoftherods，theyallshowthesametimeeverywhereinspadweeasilydefinea‘uime’.
askseveralquestions.First，howshouldwebuildaclod，howshouldweeheyareallsynizedacrossspace?Third，howshouldwebuildaruler?Thesequestionsallhaveaareiedtothepropertiesoflight.Iothelastisthis:ohelengthofthepathtravelledbylightinavagatimeintervalof1299，792，458ofased.Itisthereforeliheahefirstquestion:howaccurateabebuilt.
28.Ascafspacemadeupofa3Dlattieasuringrods.Ateaterseisaclock，allsynized.
Clocks
Theprimarycharacteristicofaclockisthatit‘ticks’.Thatis，itsigervalsoftime.Bygtheicksbetwees，weatimeiheThemorepreciseaclockis，themulartheiweeiagrandfatherclock，the‘ticks’areprovidedbytheregularsenduluIronicwristwatch，the‘ticks’aretheossofapieceofquartzcrystal.Thesearemuchmularthanthesendulum，whibeaffectedbythetemperatureandhumidityoftheplacewherethegrandfatherclockislocated.Thereforethequartzwatchissaidtokeepbettertimethaherclock.
&accuratetheworldarebasedonthehighlyregularclicksofeleovingaroundinatoms.We’veseenthatatomisjumpbetweeableehinanatoForsomeatoms，thediffereweentheselevelsisextremelywelldefiherateatwhipbetweewothereforebeusedtodefiicks，basedonthefrequeusedtopushtheelesupahetwystates.
Theideaissimpleenough:illumiomwithmicrowaves（recallthatmicrowavesarejustlikelight，butwithamuchlowerfrequensofossoftheelectricfieldpersedratherthanmillionsofbillionsasinthecaseofvisiblelight）.Thenslowlygethefrequenicrowavesuroheatylevelsmosteffitly.Thisdefihenumberofcyclesperseicrowaveradiatioicksofthetermsoftheenergylevelspagofeleaparti.
Therearemanyteicalgestobuildingsuicclogtheatthemiinitialstate，illuminatingwithmicrowavesiomaximizethesensitivity，detegthattheelesareihehigherstateatagiveclocksbasedoomsareaccuratewaytomeasuretime，witharateoftigthatdeviatesfrompleteandutterregularitybyohofaseehundredmillionyears.
TheseclocksprovideatimestandardthatisagreediionallyandmaintainedbygoverlaboratoriessuchastheNationalIandardsandTeology（NIST）iheNationalPhysicalLaboratory（NPL）intheUK，andPhysikalisisdesanstalt（PTB）iheyareposofmanyteologiesthatunderpinourdailylives.Foriheyarevitalflobalpositiohatisthebasisfation，ingthesatnavonlyusedincars.
ization
&geistosywoclocks，sothattheyarecalibrated.Oodothisisbysendingasignalfromooaartthefirstditotheotherclogitwhenyourfirst‘tick’occurred.Theperseofthatknowswhatfraoftickherclockisbehindyours（siheclocksarethesamestru，weassumetheytickatthesamerate）ahisinformatiohee.
&erestiothis.ImagitosynizeyourEarthtothatonadista，inafar-awaygalaxy.Yousendyourpulseoflightoffiio，andthenyoulaisfaraway，itmighttakeaverylohelighttiventhehighvelocityoflight.Meanwhileyolderahepersonwhoreceivesyoursynizationmessagewillhavereceiveditfromtheyoungyou—shewillseeyouasyouwerewhehemessage.
Likeelookatthenightsky，astars，weareseeihembyreglightthatlefttheirsurfag，longtimeago.Aalaxiesthatareevenfartheraway，sowelookintothedeepera，seeingtheuwasbillionso.Ihelightthatreachesusisalsobillionsofyearsold—ithasbeentravellingacrossspaagesihi.Lightistheoldestthingwetheuniverse.
Ourclocks，though，areabitclether.Aingfactisthatwhenyouputoheseanaeroplanemovingat800kphorso，youfindthatitticksslowerthaheground.Thatis，ifyousettheclockstotellthesametime，youwillfindthattheoheaeroplaneappearsfromthegrouigataslowerratethanthegrouhisisaaximumspeedatwhialsveyedbetweewoclocks—thespeedoflight.
Youseethisbylookiionihere，personAisonthegroundandpersonBisinanaeroplanemovingathighspeed.BwatchesasAsendsalightpulsetoamirrorsuspeHabovetheground.FromA’spoihelightpulsetravelsdistanceH.However，fromB’sperspectivethatsignalwilltravelaslightlyloahanHsiobemovingbackeedrelativetoB.Therefore，becausethesiglightspeedagtobothAandB，ahesameicksbetweensendihesigheoionisthatB’sclockistigslowerthanA’sseenfromA，andA’sclockistigslowerthanB’swhenseenfromB.Thiseffectiscalledtimedilation.
29.Timedilatioivemotion.ObserversAandBbothmeasurethetimeforapulsetoarriveatthesuspendedmirror.Theymeasuredifferenttimesbecausetheyaremoviooher.
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