Gravitational Wave Detection by Interferometry.docx
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Gravitational Wave Detection by Interferometry.docx
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GravitationalWaveDetectionbyInterferometry
http:
//relativity.livingreviews.org/Articles/lrr-2000-3/
GravitationalWaveDetectionbyInterferometry
(GroundandSpace)
SheilaRowan
GinztonLaboratory
StanfordUniversity
StanfordCA94305-4085
srowan@loki.stanford.edu
JimHough
DepartmentofPhysicsandAstronomy
UniversityofGlasgow
GlasgowG128QQ,UK
J.hough@physics.gla.ac.uk
(Accepted24January2000)
Abstract:
Significantprogresshasbeenmadeinrecentyearsonthedevelopmentofgravitationalwavedetectors.Sourcessuchascoalescingcompactbinarysystems,low-massX-raybinaries,stellarcollapsesandpulsarsareallpossiblecandidatesfordetection.ThemostpromisingdesignofgravitationalwavedetectorusestestmassesalongdistanceapartandfreelysuspendedaspendulumsonEarthorindrag-freecraftinspace.Themainthemeofthisreviewisadiscussionofthemechanicalandopticalprinciplesusedinthevariouslongbaselinesystemsbeingbuiltaroundtheworld-LIGO(USA),VIRGO(Italy/France),TAMA 300(Japan)andGEO 600(Germany/UK)-andinLISA,aproposedspace-borneinterferometer.
1Introduction
Gravitationalwaves,oneofthemoreexoticpredictionsofEinstein'sGeneralTheoryofRelativitymay,afterdecadesofcontroversyovertheirexistence,bedetectedwithinthenexttenyears.
Sourcessuchasinteractingblackholes,coalescingcompactbinarysystems,stellarcollapsesandpulsarsareallpossiblecandidatesfordetection;observingsignalsfromthemwillsignificantlyboostourunderstandingoftheUniverse.Newunexpectedsourceswillalmostcertainlybefoundandtimewilltellwhatnewinformationsuchdiscoverieswillbring.Gravitationalwavesareripplesinthecurvatureofspace-timeandmanifestthemselvesasfluctuatingtidalforcesonmassesinthepathofthewave.Thefirstgravitationalwavedetectorswerebasedontheeffectoftheseforcesonthefundamentalresonantmodeofaluminiumbarsatroomtemperature.InitialinstrumentswereconstructedbyJosephWeber [104
105
]andsubsequentlydevelopedbyothers.Reviewsofthisearlyworkaregivenin [102,29].Followingthelackofconfirmeddetectionofsignals,aluminiumbarsystemsoperatedatandbelowthetemperatureofliquidheliumweredevelopedandworkinthisareaisstillunderway [74
77
9
47
].Howeverthemostpromisingdesignofgravitationalwavedetectors,offeringthepossibilityofveryhighsensitivitiesoverawiderangeoffrequency,useswidelyseparatedtestmassesfreelysuspendedaspendulumsonearthorinadragfreecraftinspace;laserinterferometryprovidesameansofsensingthemotionofthemassesproducedastheyinteractwithagravitationalwave.
Groundbaseddetectorsofthistype,basedonthepioneeringworkofBobForwardandcolleagues(HughesAircraft) [71],RaiWeissandcolleagues(MIT)[107
],RonDreverandcolleagues(Glasgow/Caltech)[31,30
]andHeinzBillingandcolleagues(MPQGarching) [13
],willbeusedtoobservesourceswhoseradiationisemittedatfrequenciesaboveafewHz,andspacebornedetectors,asoriginallyenvisagedbyPeterBenderandJimFaller [27,35]atJILAwillbedevelopedforimplementationatlowerfrequencies.
Alreadygravitationalwavedetectorsoflongbaselinearebeingbuiltinanumberofplacesaroundtheworld;intheUSA(LIGOprojectledbyaCaltech/MITconsortium) [11,1],inItaly(VIRGOproject,ajointItalian/Frenchventure) [20,2],inGermany(GEO 600projectbeingbuiltbyacollaborationcentredontheUniversityofGlasgow,theUniversityofHannover,theMaxPlanckInstituteforQuantumOptics,theMaxPlanckInstituteforGravitationalPhysics(AlbertEinsteinInstitute),GolmandtheUniversityofWales,Cardiff) [52,3]andinJapan(TAMA 300project) [100,4].Aspace-bornedetector,LISA, [26
5
6
]-proposedbyacollaborationofEuropeanandUSresearchgroups-hasbeenadoptedbyESAasafutureCornerstoneMission.Whencompleted,thisdetectorarrayshouldhavethecapabilityofdetectinggravitationalwavesignalsfromviolentastrophysicaleventsintheUniverse,providinguniqueinformationontestingaspectsofGeneralRelativityandopeningupanewfieldofastronomy.
Werecommendanumberofexcellentbooksforreference.ForapopularaccountofthedevelopmentofthegravitationalwavefieldthereadershouldconsultChapter10of`BlackHolesandTimeWarps'byKipS.Thorne [98].Acomprehensivereviewofdevelopmentstowardlaserinterferometerdetectorsisfoundin`FundamentalsofInterferometricGravitationalWaveDetectors'byPeterSaulson [88],anddiscussionsrelevanttothetechnologyofbothbarandinterferometricdetectorsarefoundin`TheDetectionofGravitationalWaves'editedbyDavidBlair [14].InadditiontothehomesiteofthisjournalandthesiteslistedabovethereisaveryinformativegeneralsitemaintainedbytheNationalCentreforSupercomputingApplications [7].
2GravitationalWaves
Someearlyrelativistswerescepticalabouttheexistenceofgravitationalwaves;however,the1993NobelPrizeinPhysicswasawardedtoHulseandTaylorfortheirexperimentalobservationsandsubsequentinterpretationsoftheevolutionoftheorbitofthebinarypulsarPSR1913+16 [54,97],thedecayofthebinaryorbitbeingconsistentwithangularmomentumandenergybeingcarriedawayfromthissystembygravitationalwaves [108].
Gravitationalwavesareproducedwhenmatterisacceleratedinanasymmetricalway;butduetothenatureofthegravitationalinteraction,detectablelevelsofradiationareproducedonlywhenverylargemassesareacceleratedinverystronggravitationalfields.Suchasituationcannotbefoundonearthbutisfoundinavarietyofastrophysicalsystems.Gravitationalwavesignalsareexpectedoverawiderangeoffrequencies;from
Hzinthecaseofripplesinthecosmologicalbackgroundto
Hzfromtheformationofneutronstarsinsupernovaexplosions.Themostpredictablesourcesarebinarystarsystems.Howevertherearemanysourcesofmuchgreaterastrophysicalinterestassociatedwithblackholeinteractionsandcoalescences,neutronstarcoalescences,low-massX-raybinariessuchasSco-X1,stellarcollapsestoneutronstarsandblackholes(supernovaexplosions),pulsars,andthephysicsoftheearlyUniverse.Forafulldiscussionofsourcesrefertothematerialcontainedinreferences [55
24,69].
Whyistherecurrentlysuchinterestworldwideinthedetectionofgravitationalwaves?
PartlybecauseobservationofthevelocityandpolarisationstatesofthesignalswillallowadirectexperimentalcheckofthewavepredictionsofGeneralRelativity;butmoreimportantlybecausethedetectionofthesignalsshouldprovideobserverswithnewanduniqueinformationaboutastrophysicalprocesses.Itisinterestingtonotethatthegravitationalwavesignalfromacoalescingcompactbinarystarsystemhasarelativelysimpleformandthedistancetothesourcecanbeobtainedfromacombinationofitssignalstrengthanditsevolutionintime.Iftheredshiftatthatdistanceisfound,Hubble'sConstant-thevalueforwhichhasbeenasourceoflivelydebateformanyyears-maythenbedeterminedwith,potentially,ahighdegreeofaccuracy [90].
Onlynowtechnologyhasreachedastagewherethebuildingofdetectorsofthesensitivityrequiredtoobservesuchinterestingsourcescanbeenvisaged.
3DetectionofGravitationalWaves
Gravitationalwavesaremostsimplythoughtofasripplesinthecurvatureofspace-time,theireffectbeingtochangetheseparationofadjacentmassesonearthorinspace;thistidaleffectisthebasisofallpresentdetectors.Gravitationalwavestrengthsarecharacterisedbythegravitationalwaveamplitudeh,givenby
where
isthechangeinseparationoftwomassesadistanceLapart;forthestrongestallowedcomponentofgravitationalradiationthevalueofhisproportionaltothethirdtimederivativeofthequadrupolemomentofthesourceoftheradiationandinverselyproportionaltothedistancetothesource.Theradiationfielditselfisquadrupoleinnatureandthisshowsupinthepatternoftheinteractionofthewaveswithmatter.
Theproblemfortheexperimentalphysicististhatthepredictedmagnitudesoftheamplitudesorstrainsinspaceinthevicinityoftheearthcausedbygravitationalwavesevenfromthemostviolentastrophysicaleventsareextremelysmall,oftheorderof
orlower [55
].Indeedcurrenttheoreticalmodelsontheeventrateandstrengthofsucheventssuggestthatinordertodetectafeweventsperyear-fromcoalescingneutronstarbinarysystemsforexample-anamplitudesensitivitycloseto
overtimescalesasshortasamillisecondisrequired.IftheFouriertransformofalikelysignalisconsidereditisfoundthattheenergyofthesignalisdistributedoverafrequencyrangeorbandwidthwhichisapproximatelyequalto1/timescale.Fortimescalesofamillisecondthebandwidthisapproximately1000 Hz,andinthiscasethespectraldensityoftheamplitudesensitivityisobtainedbydividing
bythesquarerootof1000.Thusdetectornoiselevelsmusthaveanamplitudespectraldensitylowerthan
overthefrequencyrangeofthesignal.Signalstrengthsattheearth,integratedoverappropriatetimeintervals,foranumberofsourcesareshowninFig. 1.
Figure1:
Somepossiblesourcesforgroundbasedandspace-bornedetectors.
Theweaknessofthesignalmeansthatlimitingnoisesourceslikethethermalmotionofmoleculesinthedetector(thermalnoise),seismicorothermechanicaldisturbances,andnoiseassociatedwiththedetectorreadout,whetherelectronicoroptical,mustbereducedtoaverylowlevel.Forsignalsabove
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