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Chapter4Gravitationalwaves
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Gravitationalwaveshavealreadybeeioned，inChapter3.’ssiderthemiail.YouwillreChapter1that，iheravityisduetothecurvatureofspace-time.Massiveobjectslikestarsasdeformtheshapeofthespawhichtheyexist，sothatotherbodiesthatmhitappeartohavetheirtrajectoriesbent.Itisthemistakeioionofthesebodiesasinaflatspacethatleadsustoihereisaforcecalledgravity.Infact，itisjustthecurvatureofspace-timethatisatwork.
Therelevahisfravitatioifagroupofmassivebodiesareiion（sutheSolarSystem，orinabinarypulsar），theureofthespawhichtheyexistisnotusuallyfixedihecurvatureofthespace-timeissetbythemassivebodies，soifthebodiesareinmotioureofspace-timeshouldbeexpectedtobetlyg.Thestificwaytodescribethissituationistosaythat，iheory，space-timeisadyy.
Asahis，siderthesuperwediscussedpreviously.Beforetheircorescollapse，leadingtocatastrophicexplosion，theyarerelativelystableobjects，muchlikeourownSun.Inthisstageoftheirlifetheyshouldthereforebeexpectedtocurvethespace-timearouhesamewaythattheSundoes，andshouldthereforehaveasimilargravitatioertheyexplodetheysettledowarorablackhole，andonreturivelystablestate，withagravitatiodoeshtime.Duringtheexplosioheyejecthugeamountsofmassaheirgravitationalfieldgesrapidlythroughoutthisprodthereforesodoesthecurvatureofthespace-timearoundthe
&emthatispushedoutofequilibriumaogerapidly，thiscausesdisturbaheformofwaves.Amoredoleofawaveisehrowastooapreviouslystillpoerinthepondwasinitiallyie，butthestonecausesarapidtheamouohewateririestoreturntoitstranquilinitialstate，whichresultsiiourbaheformofripplesthatmoveawayfromthepoionelanded.Likewise，aloudnoiseinapreviouslyquietrinatesfromaairpressureatapoiereospeaker）.Thedisturbaheairpressurepropagatesoutressurewaveastheairtriestoreturntoastablestate，ahesepressurewavesassound.
Soitiswithgravity.Ifthecurvatureofspace-timeispushedoutofequilibrium，bythemotiony，thenthisdisturbawardsaswaves.Thisisexactlywhatoastarcollapsesanditsoutereedbythesubsequentexplosioprocessthatothestarcausesadisturbaureofspace-time，muchlikethestonefallingintothepourbaer.Andagain，justasintheexamplesjustgiveurbaesoutwardsas
Thespeedavespropagateusuallydepehroughwhichtheytravel.Forexample，weknowthatsouravelslightlyfasterinwarmairthantheydoincoldair.Themediumfravitationalace-timeitself，andagtoEiheory，theypropagateatexactlythesamespeedaslight.Justaswithlight，theyarealsoexpectedtohaveaspeedthatisihemotionofthepersohem，andthatisihemotionoftheirsource.Theytherefateinformationatthemaximumpossiblespeed，asnothingtravelfasterthanlight.
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TotryandmakeseagravitatiomightbeusefultosiderwhatitsefectsshouldbeonagroupofobjectsasitpassesthroughtheForexample，ifytodescribetheondtoanalienwhowasn’tfamiliarwithwater，youmightstartbysuggestingthattheefectofthewaveistomaketheliliesonthesurfaceofthepondmoveupanddowninasmooth，repetitiveway.Letussiderthedingsituationwithgravitational>
&bygauniformcloudofgas’oatingihisremovestheefectoftheEarth’sowngravitationalfield，whiytimesstrohananygravitatioweareeverlikelytosee，aswellasremovinganydisturbahegasmightfeelfrwithahegravitatioravelsthroughthecloudofgas，asihentheprihewaveistodisplacethegasirasdireation.Thatis，ifthewavetravelsfromlefttht，theithegasaredispladdown，andinandoutoftheplahepage.
&hegravitatioheayinitiallymakeitlookalittlebitlikethewaveisbeingsupportedbythegasitself，muchlikethewaveinthepondissupportedbythewater.Itis，however，quitediferent.Withthegravitatioisspace-timethatcarriesthewave.Theefectofthegravitatiohegasistherefoustotheefectthatthewaterwavehasonalilythatsitsonthesurfaceofthepohewateriself.Thatis，thegravitatioawaveiratherapropagatingdisturbaimeinwhichthegasexists.
8.Anillustrationofaassingthroughacloudofgas.Thewavetravelsfromlefttht，anddisplacestheparticlesofgasup-and-down，andin-and-outoftheplahepage.
Infact，whatthegravitationalwaveisreallydoinginthisexampleisgtheamountofspacethatexistsihataretrasdireatioalthoughtheatomsinthegasmightbeclether（orfurtherapart）thantheywerebeforetheassedthroughthem，itisheatomshavemoved，butbecausetheamountofspathemhasbeendecreased（orihewave.Thegravitationalwavegesthedistasbyalteringhowmuchspacethereisihem，notbymovihinafixedspace.Thisisonlypossiblebecauseofthefactthatspaotfixediheorybutisitselfdynamical.
Tosidertheefectsofgravitationalwavesiusnowsideraringofpartidthinkwhatentifagravitatiohroughit.ThisisillustratedinFigure9，whereinthiscasethegravitationalwaveisbeiobeingupwards，outoftheplaheefectofthewaveisohedirestrasdireatiofpartitheplahepageshiveusaprettygoodideaofitsces.
9.Thedeformationthataringofparticleswouldexperienceifagravitatiohroughit，inanupwardsdire（i.e.iofthepage）.Theleftmostimageshowstheinitialfiguratioherimages（fromlefttht）shatfoursubsequentiime.
Iftheparticlesareinitiallyarrangedidarenotattaythiher，thehewavewillbetopresstheoionaiheresultisthattheparticlesstarttoformaheassesthrough，itstretchesthecirclesmoothlyuntilitreachesamaximumdeformation，atwhititstopsaheprotilthedirewhiitiallystretchedisataminimuThisstretdsquashidiresisillustratedinFigure9，anduesuntiltheassed.
Theissionofgravitationalwaveshasalreadybeehebinarypulsar.Thewavesergyawayfromthesystem，sothatthetwoarsslowlytowardseachother.Themeasuredrateofin-spiralinthebinarypulsarhasgivengoodevideheexistenceofgravitatioisofgreatstifiteresttotryasofthesewavesdirectlyaswell.Thiswasdoimeiember2015bytheLIGOexperimedeteofgravitatioicularlyexgbecauseitprovideuswithairelyoviewtheUhefirsttime，wedoorelyonlighttoseedistasasweowlookatthemthrravitationalfieldsdirectly.Thisallowsustoseeens，forexample，whenblackholescollide.
&deteofgravitationalwavesalsoallowsforestsofEiheravity.Ingeneral，oneberofdiferentpossibleefeapassinggravitationalwave.Forexample，theareaofthecircleofpartiFigure9couldhavebeeherecouldbedeformationsofspagthedireationofagravitationalwave（aswellasintraions）.Itistheequatioeihecurvatureofspadtimethatforbidsthesepossibilitieswithiifhewaswroheycouldbethereinnature.ByusislikeLIGO，welooktoseeifgravitatiohespecificefectsthatEihisprovidesyetaofhistheory.Beyondthis，beiogainanehathappenswhenblackholescollideprovidesanumberofexgpossibilitiesforthefurtherstudyofgravitationalphysics.
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