Experimental Tests of General Relativity 0809.3730

arXiv:0809.3730v2[gr-qc]18Jan2009ExperimentalTestsofGeneralRelativity:RecentProgressandFutureDirectionsSlavaG.TuryshevSternbergAstronomicalInstitute,13UniversitetskijProspect,119992Moscow,RussiaandJetPropulsionLaboratory,CaliforniaInstituteofTechnology,4800OakGroveDrive,Pasadena,CA91109-0899,USA(Dated:January18,2009)Einstein’sgeneraltheoryofrelativityisthestandardtheoryofgravity,especiallywheretheneedsofastronomy,astrophysics,cosmologyandfundamentalphysicsareconcerned.Assuch,thistheoryisusedformanypracticallyimportantpurposesinvolvingspacecraftnavigation,geodesy,timetransfer,etc.HereIreviewthefoundationsofgeneralrelativity,discussrecentprogressinthetestsofrelativisticgravity,andpresentmotivationsforthenewgenerationofhigh-accuracytestsofnewphysicsbeyondgeneralrelativity.Space-basedexperimentsinfundamentalphysicsarecapabletodaytouniquelyaddressimportantquestionsrelatedtothefundamentallawsofnature.Idiscusstheadvancesinourunderstandingoffundamentalphysicsthatareanticipatedinthenearfutureandevaluatethediscoverypotentialofanumberoftherecentlyproposedspace-basedgravitationalexperiments.Keywords:Testsofgeneraltheoryofrelativity;StandardModelextensions;cosmology;modifiedgravity;stringtheory;scalar-tensortheories;EquivalencePrinciple;gravitationalexperimentsinspaceI.INTRODUCTIONNovember25,2015willmarktheCentennialofgeneraltheoryofrelativity,whichwasdevelopedbyAlbertEinsteinbetween1905and1915[1,2].Eversinceitsoriginalpublication[3,4,5],thetheorycontinuestobeanactiveareaofboththeoreticalandexperimentalresearch[6,7].Thetheoryfirstdemonstrateditsempiricalsuccessin1915byexplainingtheanomalousperihelionprecessionofMercury’sorbit.ThisanomalywasknownlongbeforeEinstein;itamountsto43arcsecpercentury(”/cy)andcannotbeexplainedwithinNewton’sgravity,therebypresentingachallengeforphysicistsandastronomers.In1855,UrbainLeVerrier,whoin1846predictedtheexistenceofNeptune,aplanetonanextremeorbit,wrotethattheanomalousresidueoftheMercurialprecessionwouldbeaccountedforifyetanotherplanet,theas-yetundiscoveredplanetVulcan,revolvesinsidetheMercurialorbit.BecauseoftheproximityofthesunVulcanwouldnotbeeasilyobserved,butLeVerrierthoughthe,nevertheless,haddetectedit.However,noconfirmationcameinthedecadesthatfollowed.Ittookanother60yearstosolvethispuzzle.In1915,beforepublishingthehistoricalpapercontainingthefieldequationsofgeneralrelativity(e.g.[4]),EinsteincomputedtheexpectedperihelionprecessionofMercury’sorbit.Whenheobtainedthefamous43”/cyneededtoaccountfortheanomaly,herealizedthataneweraingravitationalphysicshadjustbegun!Shortlythereafter,SirArthurEddington’s1919observationsofstarlines-of-sightduringasolareclipse[8]confirmedthedoublingofthedeflectionanglespredictedbygeneralrelativityascomparedtoNewtonianandequivalenceprinciple(EP)arguments.1ObservationsweremadesimultaneouslyinthecityofSobralinBrazilandontheislandofPrincipeoffthewestcoastofAfrica;theseobservationsfocusedondeterminingthechangeinpositionofstarsastheypassedneartheSunonthecelestialsphere.TheresultswerepresentedonNovember6,1919ataspecialjointmeetingoftheRoyalAstronomicalSocietyandtheRoyalSocietyofLondon[11].ThedatafromSobral,withmeasurementsofsevenstarsingoodvisibility,yieldeddeflectionsof1.98±0.16arcsec.ThedatafromPrincipewerelessconvincing.Onlyfivestarswerereliablymeasured,andtheconditionsthereledtoamuchlargererror.Nevertheless,theobtainedvaluewas1.61±0.4arcsec.Bothwerewithin2σofEinstein’svalueof1.74andweremorethantwostandarddeviationsawayfrombothzeroandtheNewtonianvalueof0.87.Theseobservationsbecamethefirstdedicatedexperimenttotestgeneraltheoryofrelativity.2InEurope,whichwasstillrecoveringfromtheWorldWarI,thisresultwas1Eddingtonhadbeenawareofseveralalternativepredictionsforhisexperiment.In1801JohannGeorgvonSoldner[9]hadpointedoutthatNewtoniangravitypredictsthatthetrajectoryofstarlightbendsinthevicinityofamassiveobject,butthepredictedeffectisonlyhalfthevaluepredictedbygeneralrelativityascalculatedbyEinstein[10].Otherinvestigatorsclaimedthatgravitywilldidaffectlightpropagation.Eddington’sexperimentsettledtheseclaimsbypronouncinggeneralrelativityawinner.2Theearlyaccuracy,however,waspoor.Dysonetal.[8]quotedanoptimisticallylowuncertaintyintheirmeasurement,whichwasarguedbysometohavebeenplaguedwithsystematicerrorandpossiblyconfirmationbias,althoughmodernre-analysisofthedatasetsuggeststhatEddington’sanalysiswasaccurate[12].However,considerableuncertaintyremainedinthesemeasurementsforalmost502consideredspectacularnewsandoccupiedthefrontpagesofmostmajornewspapersmakinggeneralrelativityaninstantsuccess.Fromthesebeginnings,thegeneraltheoryofrelativityhasbeenverifiedateverhigheraccuracy;presently,itsuccessfullyaccountsforalldatagatheredtodate.Thetruerenaissanceinthetestsofgeneralrelativitybeganin1970swithmajoradvancesinseveraldisciplinesnotablyinmicrowavespacecrafttracking,highprecisionastrometricobservations,andlunarlaserranging(LLR)(seeFig.1).Thus,analysisof14months’sworthofdataobtainedfromradiorangingtotheVikingspacecraftverified,toanestimatedaccuracyof0.1%,thepredictionofthegeneraltheoryofrelativitythatthe