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Gravitatioore
&enceofgravitationalredictedbyEinsteinin1916，and，afternearlyaturyofefort，theyweredetectedforthefirsttimein2015.Thereasonittooksolongtodothisislargelyaceoftheextremelylolitudeofthesignal.Inthediagramshowniheefectofthegravitationalwaveswasenormouslyexaggeratediryaeasiertovisualizetheirefereality，the’usintheshapeshouldoobeatthelevelofaboutoinahundredmilliontrillion.Thatis，ifwemadearingofparticlesthatwasa1，000kmacross，weshouldsshapetogebymorethanatrillionthofatimetre.Thisisobviouslyverydiculttodetect.
&hediculty，orperhapsbecauseofit，ahugeamouhasbeeryiatioly.MuchoftheearlyhistoryofthisworkiarenowcalledWeberbars.Theseis，erJosephWeberoftheUyofMaryland，sistededersofmetal.Theywereaboutametrele.TheideawasthatifagravitatiohroughtheEarth，ahroughthedeteitmightcausethebart，likeabellhitbyahammer.Forthistohappeioobeofjusttherightfrequency.Ifsuchawaveweretoexist，itshouldbeexpeallvibrationsinthebar.
&orsthatwereusedtoseeifthebarwasvibratiiveenoughtodetegesihof1partin1，000trillion.Remarkablysehisis，it’sravitatiotoexist.Thisdidn’tstopafewfalsealarmsbeihough.In1968，WeberclaimedhehadevideheexistenceofgravitationalouldhavemadehimaprimederforaNobelPrize.Uheclaimeddete’tbereproduditisnowwidelybelievedthatitwaserroneous.
SomemodernversionsoftheWeberbararestillbeioday.AheMiisistsofa1，150kgmetalsphere，andisabout1，000timesmoreseheonesusedbyWeberhimself.However，mostmoderdeteggravitationalwavesuseadiferentteology，knowry.Detectorsbasedonthisteologyaresimilar，inpriheMiorleydevicedesChapter2.Themoderndevicesare，however，much，muchlarger.Thelargestofall，atthetime，istheLIGOdetetheUSA.
LIGO，ortheLaserIerGravitatioory，hasteration;oneisinLivingston，Louisiana，aherisinRid，Washiesagiaer.Theierssistoftwo‘arms’，whicharebuilttlestoeachother，asillustratedinFigure6.Eacharmisseveralkil，andsatubeofnearvaetrewide.Lasersarefireddowhesetubes，aedfromhangiheendofeathelaserlightmakesitswaybacktothepoihearmsmeetitbestudied，ahofeasihepatterhetwolaserbeamsi.Whenagravitatiohroughthem，thearmlengthsdthepatterheiialsoge.
&ectorisexquisitelyacdhasasensitivitythatisaboutamillioerthaheteicalgesrequiredtoreachthisaccuracyarepheheexperimentalistshavehadtoovereallkindsnalsthatpollutethedata.Theserangefromseismioise，inwhiyiiohcausethere’egobjectstovibrate，tothevibratiorongwi，thedetectorsarenowbeioingfactorsistlythefactthatlaserlightismadeupfromphotons，sothatauoussignalfrobjeotpossible.
&hesegreatproblems，aheenormousfinancial，politidengineeringdicultiesithastakehem，theLIGOdetectorhasultimatelybeesu14September2015itdetected，forthefirsttimeinhumanhistory，direcefortheexistenceofgravitationalwavesfromgblackholes.It’salmostimpossibletooverstatethesighisevent，whichwillprobablygodowninhistoryasostificatsoftheme.Solet’ssideritiail.
&ionofgravitationalwavesbyLIGO
Atprecisely45sedspast9:50a.（Greeime），oember2015，theLIGOgravitatiooratLivingston，Louisiaedafluierferometer.Thissignallastedforonly0.2sedappearedtomakethe4km-longarmsoftheiriergthbyabout11，000thewidthofproton.About0.007sedslater，asimilarsigedbythedeteHanford，Washingtoalarmbellsringing，aistserimentweresooniheyhaddetectedagravitationalassingthroughtheEarth.
&hinkabouttheeventthatcausedthewave，let’ssiderthesignalitself.IfyouweretolookatthedatafrOsite，itearaslittlemorethanabliponthebackoftheoisethatexistswithihesignal，atitspeak，wasolitudeoftherapollutesthedetectors’output.Thismakesitdiculttosee，anddiitasatruedete.Thereasoistsweresoceditreallywasagravitatiowofold.First，thewobbleiectors，atthetwodiferentlos，wereverysimilar.Itmightbepossibletohavearaoioethingthatlookedlikeagravitatioohaveexactlythesametremoratalmostthesametimeattwraphisishighlyunlikely.Sedjustasimportantly，theLIGOstistskshapethewaveshouldtake.Thisallowedthemtosthedata，inaproatchfiltering，tolookfnal.TakisintoaeansthattheLIGOstistsaremorethan99.999perfidentthatthewobblestheyobservedreallywereduetoapassinggravitatiotoanyspurioussouroise.
ThisresultisenormousatfortheexperimeLIGO，whoprodudoperatedtwoofthemostsensitivepiecesofstifitthathaveeverexisted.Itis，however，alsoahugeatfeheoreticalphysicists.Thisisbecausethematchfilteriionedbefore，andwhichiscrucialforthedete，requiresthedetailedmodellihemravitationalfieldsinnature:mergingblackholes.ItisonlybyuaailsofenswhenblackholescollidethatstistsroducefiltersfortheexpectedgravitationalwavesignalsthatLIGOmightdetedthisisalotmorediitsounds.Thereareanumberofdiferentwaysthattwoblaerge，andanawfullotofmathematiputationalworkhasbeeoualywhatEiheorypredictsfravitatioshouldbeemittedfromsuchsystems.
Iwon’tgoiailsofaics，butweshouldsidertheactualastrophysitthatledtotheemissionofthewaves.Fromtheshapeofthesigors，aailedmodellingblackholesdescribedearlier，itisthoughtthatthewaveswerecreatedbytwoblackholesthatspiralledtowardseachotherbeforeeventuallymergilingdowntoformeblackhole.Thetwoblackholesthatcametogetherinthissariowerearouy-hirty-sixtimesthemassoftheSuleddowntoafieinwhichtheresultingblackholewasaboutsixty-twotimesthemassoftheSun.Thismightsoundratherpeaceful，butit’saeofthemostvioleoature.
&ereaderwillwehirty-sixdoesy-two.Thisisbecausethegravitatioedduringthemergercarriedaway3solarmassesy（rememberthatE=msteihisisaraordinaryamountyforasystemtolose，andgoessomewaytwhyitshouldbethoughtofasavioleogivesomeber，siderthefollowiwoblackholeswereemittinggravitatioheyweregivingythaarsintheobservableuogether.Theywerealsoobservablefrom1.3billionlightyearsaway，whichisasizablefraoftheentireobservableuhen，wasaveryextremeevent，andahighlyexthepointofviewofgravitationalphysicists.
Futureprospects
Althoughgravitationalwaveshaveed，there’snoreasonforustorestonourlaurels.ThedetebyLIGO，aswellasbeingtheendofalo，isalsothestartofaroofthereasonforthishopefulstatementisthatLIGOwill，withanyluuetodeteoreoftheseevents.Another，however，isduetotheionofgravitatioorsthatisbeingplannedandbuilt.ThisihepossibilityofbuildingaLIGOsiteoutsideoftheUSA，withIhetfavouredloextrasiteofthistypewouldiheabilityofLIGOtodetermiionontheskyfromwhichagravitatioed，andhenceimprovetheprospeggravitationalwavesasaastronomywithoutlight.
BeyondLIGO，anotherpossibilityforthefuturedeteofgravitationalrojectcalledeLISA（theEvolvedLaserIerSpaa）.TheeLISAmissionisaEuropeanSpacyproposaltocreateadetespace.ThisbriaisoverEarth-basedgravitatioors，theforemostofwhichisthatitwouldbeimmuoseismioise.Thismeansitwouldbeseeoffrequeisextremelyhardtodetetheground.Ituchlargerthah-baseddetectors，asthelaserssimplybefiredbetweees，rotectivehousing.TheproposalforeLISAistocreateatriangleoflaserbeamsbetweesatellites，eachofwhichisseparatedfromtheothersbyafewmilliores.Wheogravitatioerisbetter’soeLISAisagreatprospectforfuturedetes.
ButwhilethearmlengthsofeLISAbehuge，andtheseismioiseispletelyabsesowheeinspaceis，butisy.
ThechargedmaterialthattheSunthrowsofshouldbeexpeterfereace-baseddetectorofthiskind，asshouldtheicraysthatbardtheEarth.OnthegroundweareshieldedfromthisiheEarth’satmagravitatioorinspaotbe.And，ofcourse，itismuchmorediicultteandmaiwhenit’siheless，therearehighhopesthateLISAwillbebuilt，andthatitwilldetectgravitationalace.
&herwayinwhichgravitatiobedetethefutureisusingologicalobservables.ologyisthestudyofthestateaionoftheUniverseasawhole，andtherearesiderableprospectsfthefisofgravitationalwavesinanumberofupissions.WewilldiscusstheseiailinChapter5.
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