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Chapter3Extrasolartestsofgravity
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TheSolarSystemallowsustoieanumberravitatios.Manyofthembemeasuredtohighaccuracy，becausewehaveeasyaccesstothenearbyplasaheyare，however，quiteweakgravitatiohisisbecausealloftheobjetheSolarSystemare，relativelyspeaking，ratherslowmovingandnotverydehtsalittlefurtherthough，wedobjectsthataremuethananythingwehaveavailablenearby.
&bygthelifeearsarethoughttoformwhenthecloudsasthatemergedfromtheBigBangcollapseuheirowngravitationalfield，aerauallyionsstart，aressurefromthesereaesstroobalaheinwardattraofgravity.
Thisresultsinastar:ahotballofcollapsedgasundergoiions.Thisprocessofggasaionsis，ofcourse，alshdesofensinourownSun.
Butthisishestory.AstarsuchastheSunlyeverhaveafiime.Eventuallythehydrogenrequiredfornuclearfusionwillrunout，aarwillhavetothermaterials.Thismakesitswelli.Ihesealtereventuallyrunout，aionalcollapseofthestarwillresume.sthecollapseatthispoihesizeofthestar.Asmallstarwillsettledowntobeeawhitedwarf.Thisisastateinwhichthequantummeicalpropertiesoftheelepreventitfrombeingahispointit’sjustnotpossibletofitarohatthestartakesup.
Ifastarisabitbigger，theninsteadofbeiwillendupasaar.Inastarofthistype，theendofnuclearfusiooacollapseofitscore.Thestarthencollapses，whicausesacolossalexplosion，knoernthisprocessthegravitatiorooforcetheeledprotonstobiheelepressureisthehestarcollapsesuronsaresodeintothesamespadresultisastarthathasadensitythat’sparabletothenu.Ironstarbethoughtofasagiantatomiucleus（butrotonsandwithoutelesit）.arsareverysmallaheyaredehananythisintheSolarSystem，aomoveatextremelyhighveloeutronstarisremeobjectthatresultfromagstarthough.Thattitlegoestoatypeofobjectcalledablackhole.Ifastarissethateventhepressurefromrons’tsupportit，thenitsetoablackhole.
Ablackholeisoremeobjectsthatexistihatisreallyleftafteracollapseofthistypeisthegravitatioself.Ablackholesistsioimeenclosedbyasurfaceeventhorizoionalfieldofablackholeissthatanythingthatfindsitswayihorizonevereslight.Hehehissewearegoingtosiderstarsystemsthateoftheobjectsjustdescribed.Astronomershavenowdiscehesesystems，aionsofthemhaveallowedustravityinwaysthataresimplyimpossibleinourownSolarSysteTheextremeheseobjectsamplifiestheefesteihat，eventhoughtheyareveryfaraway，theyoferusaingwindhwhichtoseetheefectsofgravity.
&aylorbinarypulsar
&aylorbinarypulsar，orPSRB1913+16，isastarsystemthatstwoarsinorbitarouher.Theremarkablethingaboutthissystemisthatoronstarsiswhatisknoulsar（aionof‘pulsatingstar’）.Thesearestarsthatappeartularpulsesofradiation，whenviewedfromEarth.Thisradiatiosurrouarandthatcausepowerfulbeamstobeprojectedoutwardsfretherwiththerapidrotatioars，thesebeamsappearasrapid’ashesofradiationtodistantastronomers，muchlikethesignalfromalighthouseappearsas’ashesoflighttonearbysailors.
Pulsarswerediscoveredforthefirsttimein1967，byJoBellBurnellandAheseastrohecharacteristic’ashesofradiationthatarenowknowntosigenceofapulsar，butatthetimewerequiteued.Iwasinallythoughtthatthese’ashescouldbesignalsfromanothercivilizatioroasfarastocallthesourceofthesignalLGM-1（LittleGreeer，similarsignalswerediscoveredfromotherpartsofthesky，anditwasrealizedthatthepulseswerefromarapidlyrotatiar.Atpresentwekehousandsofpulsars.Iurewearelikelytofindmanymore.
ThesighediscoveryofPSRB1913+16byJosephTaylorandRussellHulsein1974wasnotthatitulsar，butthatitulsarinorbitarouronstar.Thisinformationwasobtaiigthatthearrivaltimesofthepulsesvariedslightly.Thatis，thearrivaltimesofthepulsesweresometimesthreesedsearlierahreesedslater.Theperiodoverwhichthisgehappenedwasaboutsevenhoursandforty-fivemihepulsaremittedseventeenpulsesased，itossibletomakeasmoothchartthatshoattern.Theoionulsarwasinorbitarou，andthattheradiusoftheorbitwasabhtseds（thatis，thedistaravelsihreeseds，equaltoaboutamilliores）.
SotheHulse—TaylorpulsarwaskofabiwasstillnotpossibletoseetheotherobjethesysteThismeantit’tbearegularstar，butithadtobesomethingwithasimilarmasstoastar.Itwasdecidedthatthemostplausibleexplanationulsarartofabiheotherbodyiembeiarthatulsating（or，atleast，wasn’tsendinganypulsesofradiationiion）.Suchasystemisofpartiterestforthestudyofgravitybecausetheobjevolvedaresodenseaiheratsuchextremelyhighvelocities.ThismakesthesmallefectsthatareprediEiheorymuihatoronstarswasemittingasignalthatwasasaccurateasanatomicclockwastheitheatioedfromthissignalaboutthedetailsofthegravitatioion.
DatahasbeeheHulse—Taylorbinarypulsarsisdis1974.ThishasbeendoneprimarilywiththeArecibotelesPuertoRico，whietre-wideradioantenna（andwhichwillbefamiliartoanyonewhohasseenthefilmGoldeisthislargeofdatathatmakesthispartiarypulsarsystemspecial.Later，IwilldiseotherbinarypulsarsystemsthatarenowkhesehavebeenobservedfastheHulse—Taylorpulsar.Thelargedatabasethathasbeenstructedforthissystemallowsforsomeveryprecisetestsofgravitytobeperformed.
&’snowsiderthespecifiationaboutgravityisehepulsar’ssigthishappensisthroughtimedelaysaefectsthatthesignalexperietravelsthroughthegravitatiohepaar.YouwillrecallthatbothoftheseefectshavebeeheSolarSysteNowtheybemeasuredintwodistaars，whichorbiteachotheratadistaissimilartotheradiusofourownSuhatwillbefamiliar，andthatisvisibleiaylorbihepreoftheorbit.JustastheorbitofMercuryprecessesarouoothearsiaylorbinarysystempredeachother.ToparewithsimilarefeourSolarSystem，theorbitoftheHulse—TaylorpulsarprecessesasmuadayasMercurydoesiury.
Afiwhichitispossibletomeasurewiththebinarypulsar，andwhichisimpossibletomeasureintheSolarSystem，istheperiodofanorbitduetotheemissionofgravitatioheexistenceofgravitatioyetbeeisadefiioeihereshouldexistripplesihatergyoutofasysteThisisaphehasiheory，andisthereforeofpartiterestfEinstein’sgravity.IwillexplaingravitationalwavesiailinChapter4，ingattemptstomeasurethemdirectly.Fornow，IjustwantyoutokeepinmindthattheywerepredictedbyEihattheyshouldayfromthebinarypulsarsystem，astheyareemittedfrbodies.
SotherearethreerelativisticefectsthatbemeasuredinthearrivaltimesofthepulsesfromtheHulse—TaylorsysteThesearethetimedelayefethegravitatiohepaar;thepreoftheorbitofeachstar;andthedeorbitalperiodduetothelosshgravitatiowoofthesepieatiooihemassesofthetwoars，whitilthispoibeenmeasuredinanyway.ThethirdpieceofdatabeusedtoseeifEiheoryiscorrect.
Usihemassesofthetwoarsiaylorbinarysystemeachbedetermi1.4timesthatoftheSun.Thisisihesizeofthetimedelayefect，whibemeasuredtoabout0.02pertacdtheamountofpreoftheorbit，whibemeasuredtoanaccuracyofabout0.0001perassesofthetwostars，wecalculatewhatEiheoryprediouthroughgravitationalwaves，andwhatthismeaalperiod.ItturnsoutthatEiheorypredictstheorbitsshoulddecreasebyabout3.5metresperyear.Thisisexactlywhatismeasured，uptotheaccuracyoftheobservations，whichistlyarouhisisaaatioein’stheravity，andoneforwhichHulseandTaylorwereawardedtheNobelPrizeinPhysi1993.
&efectthatisexpectedtootheHulse—Tayleodeticpre（thedireoftheaxisofaspinningtop）.Thisefectwillbeeheshapeofthesignalsthatarrivefromthebinarypulsar，becausethepulsaritselfactsasaspinningtopinitsorbitarounditspanion.AlthoughithasprobablybeeheiionofthisdataistlyogiveaestofEiheory.Thisismaiheuailsiohepulsesofradiationonthesurfaceofthear.
&aylorbinarypulsaroferssomeexttestsofgravity，butwhileitisuorically，itisnowheonlybinarypulsarthatweknowabout.LetusnowewlydiscoveredsystemsthatinsomealreadyrivalthetestsofgravityerformwiththeHulse—Taylorsystem，andwhiisetooutstripitiure.
Otherbinarypulsarsystems
&shistheHulse—Taylorbinarypulsarhastheprivilegeofbeieritsdiscoverers，aswellashaviifiame（PSRB1913+16）.Otherbinarypulsarsystemsareusuallyjustreferredtobytheirstifiames.Thethat’sbeehisistocallthesystemPSR，for’PulsatingSourceofRadiation’，aeitspositioermshtasdde（theseareatesthatiioheletter‘B’or‘J’isalsousedtodehepulsarwasdiscoveredbeforeorafter1993（theonesdiscoveredafterthisdateusuallyhavetheirpositioohigheraccuracy）.
Upuntil2006，weoheexisteherbinarypulsarsystemsthathadorbitalperiodsoflessthanaday.Someofthesesystemshavespecialpropertiesthatmakethempartiterestingfgravity，andalthoughtheyhaven’tbeenobservedfastheHulse—Taylorpulsar，theystillofersintohowgravityworks.Ihisse，Iwillgiveaverybriefsummaryofsomeofthemofthesesystems，beforefinishingofwithalookforwardatwhatthefutureholdsforextrasolartestsofgravity.
&withPSRB1534+12.Asthehisbinarypulsarsystemwasdiscoveredbefore1993.Theremarkablethingaboutthissystemisthatweseeitalmostexactlyedgeon.Thatis，ourlitothesystemliesalmostexatheorbitalplaronstars.Thisamplifiesthetimedelaynals，asatsomepoiheradiowavesfromthepulsarhavetopassveryclosebytoitspaheymaketheirwaytoEarthtobeobservedbyourastrohepulsaralsohasparticularlystrongandnarroulses，whichmakesitaverygoodfortuahissystemisnotknownwithmuchaccuracy，whichlimitstheprewithwhibeusedasatest-bedfravity.Todate，thissystemhasthereforenotprovidedasmuationaboutgravityastheHulse—Taylorpulsar.
AnotherbinarypulsarsystemofspeterestisPSRJ1738+0333.Thissystemisthoughttoapulsarinorbitarouhebiggerbrotherofthear，describedearlierihespegaboutthissystemisthatthetwoobjevolvedareverydifereher.Thisallowsforaofgravity.Itjustsohappensthatiheorytheemissionofgravitationalwavesfromabiparticularlyseherorwoobjectsaresimilarordiferent.MostofthepossiblealteroEiheorydo，however，predisitivitytothistypeofdiference.BylookingathowmuergyislosttogravitationalwavesinasystemsuchasPSRJ1738+0333，itisthereforepossibletoprovideanadditioofEiheory.IfEinsteihenweshouldexpectPSRJ1738+0333tolyatananomalouslyhighrate.Sofar，nosualyhasbeeed，whichprovidesstillfurtherverifiofEiheory.
However，probablythemostexgsystemthatwasknownaboutbefore2006isPSRJ0737-3039AB.Thissystemwasdis2003，andhasanumberofalmostunbelievableproperties.gsttheseisthefactthatboththearsiemwereobservedtobeemittingpulsesofradiatiooitbeihedoublepulsar.thecasethatthesedobjethesystemlyapassivepanionlyagravitatiohroughwhichtheradiosignalsofthepulsartravelled.Inthissystembothobjectswereemittingpulsesofradiation，whichallowedbothoftheirorbitstobetrawaysthatwerenotpreviouslypossible.Asifthisamazienough，however，italsoturhatthetwopulsarsweremoviremelyhighvelocities（eveandardsofbiars），andthatthesystemwasalmostexactlyedge-on.Thisbinatioiesgreatlyeheefectsofrelativisticgravityiem，tosuextentthatby2008ohepulsarshadprecessedsofarthatitsradiopulsesweofview.
Nolooobserveformaoseethesubtleefestein’sgravity;withthedoublepulsarsystemwecouldseetheminjustafewshortyears.ItishatPSRJ0737-3039ABprovideseveerevideheexistenceofgravitatiohaaylorpulsar.Whilebothpulsarswerevisible，thissystemprovidedsixwaystravitatiohearsparedwiththethreethatareavailablefortheHulse—TaylorsysteAfterdetermiwounkhears，thisleavesfourisofgravityieYetagaiheorypassedthesetestswith’yingcolours.
&ure
&onishingdiscoverieshavealreadybeenmadebyravitatiosideoftheSolarSystem，itishighlylikelythatwehaveeveolookforwardtoiure.ThereasonforthisoptimismispartlyduetothestruofaiohelargestofwhiownastheSKA（SquareKilometreArray）.TheSKAedesigoreceiveradiowavesfromdistawillbebuiltonaseverbeforeseeh.
TheSKAwillsistofthousandsofradioantennasanddishes，spreadoverdistahousaresinSouthAfridAustraliaaswellasseveralothersub-Saharanstates.
&algareaofthetelescope（thesumofalldishesandantennasbined）willbeonemilliores.Itwillbefiftytimesmoresehaelescopeeverbuilt，andwillrequireaputerhacapacitylargerthanallterraibiedcostoftheSKA，atthetime，isaround€2billion，whigsuppliedbyaionalcollaboratioralia，NewZealand，ada，dia，SouthAfrica，Italy，Swedeherlands，theUK，andGermany.Byanystandard，theSKAisamoaluaking.
AprojectthesizeoftheSKAtakesalooplanandbuild，butwhenitstartstakingdatain2020iterimentstobeperformedthatwereneverpreviouslypossible.Forthepurposesofstudyinggravity，oimportantofthesewillbethemeasurementenumbersofpulsars.Firstofall，theSKAisverylikelytenumberofneulsarsystems，eachofwhibeusedtostudygravityidescribed.Sedperhapseveing，theSKAwilllikelydetedredsofrapidlyrotatingmidpulsars（i.e.pulsarsthatrotatemillionsoftimesased）.Theideaisthatbycarefullymeasuringthearrivaltimesofthesignalsfromeachofthesepulsars，theSKAwillbeabletodirectlymeasuretheefegwavelengthgravitatioheypassthroughourpartoftheUhisway，theSKApromisestobeagiantgravitywavedeteethingIwillexplaiailinChapter4）.
ItisexpectedthattheobservationsofgravitationalphenomeheSKAwillbeaboutahuimesmoreaccuratethanthosemadebybodieswithintheSolarSysteThiswillbeahugeleapforresent，binarypulsarobservatioartiakeobservatioheSolarSystemasthebesttestinggrravity.TheSKAwilldosiderablybetter.Beyohough，theSKAofersafewmoretantalizingpossibilitiesfgravity.OheseiheSKAtotesthowgravityworksedistaheUniverse.Iwillreturntothisiheristhepossibilityofdisgapulsariaroundablackhole.AblackholeisexperavitatioexistsaheUheresultestargisucharatethatnothingstopit.Theexistenarysystemsprisedofapulsarandablackholeareexpectedtoberare，butiftheydoexist，thenthere’sagoodcetheSKAwillfindtheSuchasystemwouldprovidetheopportuyiremeofalleisaveryexgprospect.
Another，moreimmediate，reashopefulaboutthefutureravitationalphysicsistheretdiscoveryofaypeofpulsarsysteIn2014，ateamofastronomersaheyhaddiscoveredapulsarinorbitwitharfs.TheyriplesystemPSRJ0337+1715.Theorbitsthatarepossibleiarsystemaremuchmorevariedthanthepossibilitiesinabilooksasthoughthesystemisstruahierarchicalway，suchthatthepulsarisiwithoedwarfs，whilethesedwhitedwarforbitsthembothatalargerdistaerearstobeagtheorbitsoftheinnerpair，providiypeoflaboratorywithinwhiggravitationalphysibestudied.
It’sexpectedthatelescopessuchastheSKAwillaidthestudyofbothdoubleandtriplepulsarsystems.Itmayevehatitallowsastrooobservetheframe-draggi（desChapter2），inwhichspaceitselfisdraggedaroundwiththerotatingstars.Suchameasurementwouldnotoerestforthestudyofgravitationalphysics，butalsoforastrophysicistswhowanttoknowhowmatteriarsbehaves.Ifsuchobservationsade，theywouldallowustostudyamaterialwithadensityofaroundatrillionkilogramspertimetre.
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