千米小说网

Chapter 2 Gravity in the Solar System（第2页）

erdistancescales，however，thereareavarietyofotherteststhatwemustsider。Toorderthedisofthesetestsseofallsiderdistaeres，uptoabouttoaboutafewkilometres。Thesescalesmayseemliketheyshouldbetheeasiesttoprobeastheyareclosesttothetypeofdistansiderinoureverydaylives。Theyare，however，quiteproblematic。

&hebesttsthathassofarbeenagravityovertheseeverydaydistancescalesisduetomeasuremeioaexperievariousheightsupatalltower。Towardstheendofthe1980s，ateamofstistsperformedthistestusire-highWTVDtarha。Thegravityexpeon’sinversesquarelawatdiferentpoioweriseasytocaldtheexperimeoodaccuracy。Atarouime，otherresearchersweremeasuriiohewaterinareservoirfilledtodiferentlevels。Ihisallowedthewatertobeweighedaon’sinversesquarelawofgravitytobetested。Furthertests，acoupleofyearslater，wereperformedbymeasuriionalattraofseawateratdifereheo。Theseexperimentsallproducedresultsthatweretwithon’sinversesquarelaw，andallatalevelofaccuracyofarou。

erscalesitstartstobeepossibletouseastronomicaldata，whichismuchmoreaccuratethantryihegravitatiohewaterinreservoirsaaweenamillionandafewhundredsofmillioresbeprobedbystudyisofmaesarouheorbitoftheMoosoftheplasarouheLAGEOSsatellites，laun1976and1992，areparticularlyusefulforexperimentsofthistype’asaretheorbitsoftheplas，whiclosedellipses（tofirstapproximatiohegravitationalattraisiveon’sinversesquarelaw。Observationsofallofthesebodies，as，haveshowhon’slawataccuraciesofbetweeinamillioinabillion。

&hereforehaveverygoodevideon’sinversesquarelaroximationtogravityoverawideraahesesgefromafraillimetre，tohundredsofmillioheaccuracyoftheexperimentsthatprobegravityoverthesedistancesvarieswildly，betartin1，000（forscalesofteoaboutoinabillion（fordistadtotheorbitsoftheplahisisagreatsuccess，butit’shestory。Let’son’slawandstartgthearisefromEiheory。

&hatprobeEiheory

&sdiscussedearlierhaveallisthatarefamiliartomostpeoplewhohavestudiedphysicsatschool:theass;theUyofFreeFall;aon’sinversesquarelawofgravity。Iion，Iwillisthatprobethemoreunfamiliargrouein’stheravity。Thespecificefectsthatresultfromthistheoryareoftensmallanddiculttodetectexperimeheless，theyareveryimportant，astheyprovideusthatbeusedtoviewaandgravityatadeeperlevel。

TherearealargenumberofefectsthatresultfromEiheoingtolimitmyselftfourofthem。ThesearetheanomalousorbitoftheplahebendinghtarouhetimedelaynalsastheypassbytheSun;andthebehaviyrosorbitarouhesearefourofthemostpromiicgravitatiobeobservedintheSolarSystem。Furtherefects，whieappareremeastrophysients，willbedisChapter3。

&withtheanomalousorbitofMercury。Earlieroninthisbook，IstatedthatheoryexplainedKepler’sobservatiosfollowellipticalorbitsarouhisistruefleplahingsstarttogetabitmorepliwesidertheorbitsofmultipleplaahisisbecausethereexistgravitatioweesthemselvesandnotjustbetlaheSunindividually。Theseiarygravitationalforcesaresmallerbutmeasurable，aheefectofpullisoftheplasofwhatmightotherwisebeperfectellipses。

Physicistshaveknownabouttheefeterplaarygravitationalftime。Theseforbeeasilycalculatedwithiheory，andastronomershavemeasuredtheirtheorbitsoftheplasforturies。Infact，theexisteunewasdeduthemid-19thturybycarefullystudyiofUratotheSuofUraurbedalittlefromwhereastroedittobe，aioncouldbeexplaiherewasaslightlylargerplalebitfurtheroutintheSolarSystem。UrbainLeVerrierandJohnsbothpredictedtheexistehisplain1845’andby1846ithadbeehiswasclearlyamomentousat。

Itthereforewasn’ttoomuchofasho，in1859，LeVerrieraheorbitofMercury（theplaheSuodeviatealittlefromthepaththatitectedtotake。Nodoubtbuoyedbyhispredioftheexisteune，LeVerrierpredictedthattheremustbeastillclosertotheSunthanMercury。Heeve’Vul。Thistime，however，thediscoveryofthepredietneveroccurred。Despitemuchefort，couldbeseeweenMerdtheSuheless，theanomalousorbitofMercurypersisted。ItseemedasifitsorbitwasbeingdraggedaroundtheSunbyagravitatiohadnoin。

&eryoftheanomalousorbitofMercurywassolvedin1915，notbythediscoveryofanynewmassivebodiesintheSolarSystem，butbyEinsteiioheory。Yousee，agtoEinsteiheory，theoniangravitationalforlyaroughapproximatioureofgravity。Aswellastheinversesquarelaw，Eihatthereshouldbenew，smallertributioionalforcelahysicalsystemdominatedbyalargemassatthetre，liketheSureoftheSolarSystem，Einsteincalculatedthatthelargestofthesenewforcesshouldvaryastheihedistaherefore，theeistotheSuhetributionfromthisnewforceshouldbe，relativetoon’sinversesquarelaw。

Mercurywas，andstillis，theowheSun，aeiationalfororeiialonMercury’sorbitthasofaherplas。

&eincalculatedthatMercury’sorbitshedaroundtheSura43ardspertury（1ardis1360ofadegree）。Thisisatinyamount，butitisenoughtobeobservedbyastroisalsotwiththeanomalousobservationsmadebyLeVerrier。SoEiheravityhaditsfirstobservationalsuccessasearlyas1915，byexplainingtheanomalousorbitoftheplaMercury。

ModersoftheorbitofMercuryaremuchbetterthaexistedihtury。theorbitsofalltheplahaccuracy，whichisessentialforgthedeviationofMeraradragcausedbyVenusalone，forexample，ismorethaoftheduetoEinstein’sgravity。Itspositionthereforeobeknhaccuracy。Thisissourtyinmoderionsthough。ThatdistiheuyassociatedwiththeshapeoftheSunitself。AionsfromaperfectlysphericalshapecauseefetheorbitofMercurythatcouldbefusedwithEinsteiatioheshapeoftheSunishardtokly，sothebestwetlydoistosaythattheanomalousorbitofMercuryistwithEiheorytoanaccuracyofabout1partin1，000。

&ionofMercury’sorbitisimpressivebutprobablydoesn’ttasapredi，becausetheanomalyitselfwasknownaboutloeinwasevenborn。WhatdoestasageioeihebendinghtaroundtheSunbyitsgravitatioreviouslyuailightshouldbeafectedbygravity，beewtravitationalforlybetweehavemass（whichlightdoesnot）。Thisambiguitywasremovedihehtsimplyfollowstheshortestpathsavailableinthecurvedspace-time，justlikeeverythihereforepredictedthatlightshouldbebeionalfieldsofmassiveobjects。

&ein’scalsshowedthatthedefleoflightwouldbegreatestforbeamsthatjustskimthesurfaassiveobjects。ThemostmassiveobjetheSolarSystemistheSun。ButthtthatpassesveryclosetotheSuforasolareclipse，otherwisetheSun’sownlightoverwhelmsthatofthemuterstars。ThefirstgoodopportuheideaofbendingstarlightcameaftertheendofWorldWarI，in1919。TheexpeditiohepositioarsheSuotestEiheravity，wasledbySirArthurEddington。

&oioheislandofPrínAfrica，wherethesolareclipsethatwasgoingtohappenthatyearwouldbetotal。Hemadecarefulmeasurements，usiographicplatesthatwereavailableatthetime。TheswerelessthaEddingtonsumeasuriioarsduringtheeclipse。Hefoundthattheyhadied，justasEinsteied，becausethetrajectoryofthelighthadbeehegravitatioheSuosweresuiEiheory，butonlywithinanaccuracyofarou。

On，moderionshaveimprovedthisresultdramatically。Thishasbeelybythefactthatthereare，tally，anumberhtobjeasquasars，whijusttherightpartoftheskytotestEinsteiiospassbehihede’eofthelightthattheyemitbemeasured。Severalmillioionsofthesequasarshavenowbeenmadeusingverylargebaseliers（thisisatypeoftelesixesthesignalsfromanumberofdifereocreateahigh-resolutioheresultofthisworkshowsperfesisteeialevelofaccuracyofabout1partin10，000。

AmoreretprediEiheoryisthetimedelaynalsthatpassbymassiveobjeereason，ittookuntil1964forstiststohisefeeeofEinstein’sgravity，butithasnowbeenmeasuredinanumberofdiferentsituations。ThesehaveiheobservatinalsthatbounetsastheyareabouttopassbehindtheSun，aswellasbylookingatthesignalsthatareemittedfromman-madeprobesastheydothesame。Thefirstofthesemethodshasthebehepositiosbeingverywellknowrajecteasytopredict。Thisstabilitymakesthemaget，buttheimperfetheirshapeeproblemsihere’eals。Man-madeprobes，oherhaveryeasilypredictablesignals，buttheirtrajectoriesewhatless。

RadietsthathavebeeomeasurethetimedelayefecludetheplasMerdVenus，andthespaariners6and7，Voyager2，theVikingMarsLanderaheiprobe。Themostredmostaccurate，oftheseobservationswasmadeusingi。TheprimarymissionofthisspacecraftwastoobserveSaturn，butitsrelevahestudyofgravitysbestservedin2003，whenitwasahadedtheexisteimedelayefeaccuracyof1partin100，000。Thisectaatioein’stheory，andonewhichwasachievedathigheraccuraypreviousexperiment。Thisart，duetotheobservationsnalsbeiiplefrequencies，whichallowedinterfereheSun’satobeextracted。

’smoveontothefiIwanttodisthisse:thebehaviyrosorbitarouh。Agyroscopeisessentiallyaspinningtop，whosetralaxisisallowedtopointiitoEiheravity，thereshouldbetwobeobservedeisputinorbitarouhefirstoftheseisadireofthegyroscope’saxisofrotation，asitorbitstheEarth。Thisefeasgeodeticpre，isduetothecurvatureofspace-timearouhesedefeing，aherotatiohefectivelypullingspadwithitasitrotates。Thisisaypeofgravitatioion，andsoitisofgreatioseeifitedexperimentally。

AlthoughtheprediiwasmadeonlyafewyearsafterEinsteinpublishedhisheravity，ittookuntilthe1960sbeforetheefegyrosorbitwascalculated，aalobservatiohemselvesdidnottakeplatilthe21sttury。TheLAGEOSsatelliteworkprovidedaionibymeasuringthetheorbitofitssatellitesastheywentarEarth。Thelong-awaitedgyroscopeexperimenterformedbyamissioyProbeB，in2011。Thegeodeticpredframe-draggisweremeasuredbythisexperimentwithaccuraciesofabout0。3perd20pert，respectively。TheaccuracyofthedisfromtheLAGEOSsatellitesareestimatedatbetert。Allresultswereonfoueein’stheory。

SotheoverallpictureweareleftwithisveryengforEiheravity。Thefoundationalassumptionsofthistheory，suchastheassandtheUyofFreeFall，havebeeremelyhighaccuraversesquarelawthatformedthebasisofheory，andwhichisagoodfirstapproximatioein’stheory，hasbeehesub-millimetrescaleallthetoastrophysicalscales。WearealsonowinpossessionofanumberofaccurateexperimehatprobethetihatresultfromEiheoryspecifically。Thisdataallowsusdirectexperimentalinsightiioteraureofspace-time，andallofitissoodagreemeeiions。Thisisatrulyspectaatiowasborofalmht。Eiedtomakeatheravitythatatibleeedoflightthatwasmeasuredtobethesamebyeveryobserver。Hedidso，andwehavehenumerouscesofhisrevolutioureoftheUniverse。

Thisishesth:Einstein’sgravityhasmaasequences，illdisthefollo