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Chapter 3 Extrasolar tests of gravity（第1页）

Chapter3Extrasolartestsofgravity

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，aionsofthemhaveallowedustravityinwaysthataresimplyimpossibleinourownSolarSystem。Theextremeheseobjectsamplifiestheefesteihat，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—Taylorpulsarwaskofabiwasstillnotpossibletoseetheotherobjethesystem。Thismeantit’tbearegularstar，butithadtobesomethingwithasimilarmasstoastar。Itwasdecidedthatthemostplausibleexplanationulsarartofabiheotherbodyiembeiarthatulsating（or，atleast，wasn’tsendinganypulsesofradiationiion）。Suchasystemisofpartiterestforthestudyofgravitybecausetheobjevolvedaresodenseaiheratsuchextremelyhighvelocities。ThismakesthesmallefectsthatareprediEiheorymuihatoronstarswasemittingasignalthatwasasaccurateasanatomicclockwastheitheatioedfromthissignalaboutthedetailsofthegravitatioion。

DatahasbeeheHulse—Taylorbinarypulsarsisdis1974。ThishasbeendoneprimarilywiththeArecibotelesPuertoRico，whietre-wideradioantenna（andwhichwillbefamiliartoanyonewhohasseenthefilmGoldeisthislargeofdatathatmakesthispartiarypulsarsystemspecial。Later，IwilldiseotherbinarypulsarsystemsthatarenowkhesehavebeenobservedfastheHulse—Taylorpulsar。Thelargedatabasethathasbeenstructedforthissystemallowsforsomeveryprecisetestsofgravitytobeperformed。

&’snowsiderthespecifiationaboutgravityisehepulsar’ssigthishappensisthroughtimedelaysaefectsthatthesignalexperietravelsthroughthegravitatiohepaar。YouwillrecallthatbothoftheseefectshavebeeheSolarSystem。Nowtheybemeasuredintwodistaars，whichorbiteachotheratadistaissimilartotheradiusofourownSuhatwillbefamiliar，andthatisvisibleiaylorbihepreoftheorbit。JustastheorbitofMercuryprecessesarouoothearsiaylorbinarysystempredeachother。ToparewithsimilarefeourSolarSystem，theorbitoftheHulse—TaylorpulsarprecessesasmuadayasMercurydoesiury。

Afiwhichitispossibletomeasurewiththebinarypulsar，andwhichisimpossibletomeasureintheSolarSystem，istheperiodofanorbitduetotheemissionofgravitatioheexistenceofgravitatioyetbeeisadefiioeihereshouldexistripplesihatergyoutofasystem。Thisisaphehasiheory，andisthereforeofpartiterestfEinstein’sgravity。IwillexplaingravitationalwavesiailinChapter4，ingattemptstomeasurethemdirectly。Fornow，IjustwantyoutokeepinmindthattheywerepredictedbyEihattheyshouldayfromthebinarypulsarsystem，astheyareemittedfrbodies。

SotherearethreerelativisticefectsthatbemeasuredinthearrivaltimesofthepulsesfromtheHulse—Taylorsystem。Thesearethetimedelayefethegravitatiohepaar;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—Taylorsystem。Afterdetermiwounkhears，thisleavesfourisofgravityiem。Yetagaiheorypassedthesetestswith’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）。

ItisexpectedthattheobservationsofgravitationalphenomeheSKAwillbeaboutahuimesmoreaccuratethanthosemadebybodieswithintheSolarSystem。Thiswillbeahugeleapforresent，binarypulsarobservatioartiakeobservatioheSolarSystemasthebesttestinggrravity。TheSKAwilldosiderablybetter。Beyohough，theSKAofersafewmoretantalizingpossibilitiesfgravity。OheseiheSKAtotesthowgravityworksedistaheUniverse。Iwillreturntothisiheristhepossibilityofdisgapulsariaroundablackhole。AblackholeisexperavitatioexistsaheUheresultestargisucharatethatnothingstopit。Theexistenarysystemsprisedofapulsarandablackholeareexpectedtoberare，butiftheydoexist，thenthere’sagoodcetheSKAwillfindthem。Suchasystemwouldprovidetheopportuyiremeofalleisaveryexgprospect。

Another，moreimmediate，reashopefulaboutthefutureravitationalphysicsistheretdiscoveryofaypeofpulsarsystem。In2014，ateamofastronomersaheyhaddiscoveredapulsarinorbitwitharfs。TheyriplesystemPSRJ0337+1715。Theorbitsthatarepossibleiarsystemaremuchmorevariedthanthepossibilitiesinabilooksasthoughthesystemisstruahierarchicalway，suchthatthepulsarisiwithoedwarfs，whilethesedwhitedwarforbitsthembothatalargerdistaerearstobeagtheorbitsoftheinnerpair，providiypeoflaboratorywithinwhiggravitationalphysibestudied。

It’sexpectedthatelescopessuchastheSKAwillaidthestudyofbothdoubleandtriplepulsarsystems。Itmayevehatitallowsastrooobservetheframe-draggi（desChapter2），inwhichspaceitselfisdraggedaroundwiththerotatingstars。Suchameasurementwouldnotoerestforthestudyofgravitationalphysics，butalsoforastrophysicistswhowanttoknowhowmatteriarsbehaves。Ifsuchobservationsade，theywouldallowustostudyamaterialwithadensityofaroundatrillionkilogramspertimetre。