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Chapter 1 From Newton to Einstein（第1页）

Chapter1FromoEinstein

Gravityisbyfartheweakestofthefourfualforcesthatexistihersbeiromagicfordthestrongandweaknuclearforces。Yetedistaisgravitythatdomihisisbecausegravityisoiveaeverbesed。Sowhilemeobjectsareelectrieutral，theyeverbegravitatiohegravitatioweehmassalullthoseobjectstogether，andalwaysiheybeassive。

Itisthankstogeniuseslikeohatweuandgravityatall，yetitisgravity，morethananyotherforce，thatuestopreseswiththemostenigmaticofpuzzles。Toua'sstartatthebeginning，ahehistorientofgravity。

&ravity

Itseemssafetoassumethatmankindhasalhenwedropawillfalldownwards。Inthissense，we'vealwaysbeeheexistenceofgravity。Itwastheotionthatappearstohavebeenthefoahinkers。

Aristotle，who'sPhysicsplayedadominantroleinEuropeailaslateasthe17thtury，basedhisexplanationofgravityoobjeovetowardstheirnaturalplatheUhisplacewas，hereasoned，shouldbedetermiioiion。Morespecifically，itshoulddependonhowmutaihefourelemeer，Air，andFire。

AristuedthatobjeposedpredominantlyfromEarthandWatershouldmovetowardsthetreoftheUohim，thetreoftheUheplaeathhisfeet。ObjectsmadefromEarth，andthatarethrownintotheair，mustthereforemovedround。Water，hereasoned，islighterthah，asbeverifiedbydumpioahalf-fullglassofit。AllWatermustthereforeetorestontopoftheEarth。Similarly，AirislighterthanWater，asbubblesriseihenaturalplaceforAiristhereforeaboveWater，auralplaceforFireisabovetheAir。

Thisframeworkprovidesalogicalsortofordertotheworldthatwasobserved，intermsofwhatwerethoughttobethebasistituentsofmatteratthetime。Itevenallowsoatementsaboutthespeedatwhichobjectsshouldfall。Aristuedthatthevelocityofafallishouldbeinproportiontoitsweight，andiioyofthemediumthroughwhichittravels。Thatis，Aristotlethoughtthataweighs2kgshouldfalltwiceasfastasoweighs1kg。

Sadly，Aristotle'stheoryotbecorreowknowthatthereisheUniverseforobjeovetowards。Weonstrate，bydirectexperimentation，thattherateatwhiobjectacceleratesduetogravityisnotinproportiontoitsmass。Infabeshownthatallobjectsfallatthesamerate。Thisdiscoverywasoohemoderandingofgravity，andsoitbearssomefurtherexplanation。

&hatallobjectsaccelerateuyatthesamerateisnotanobviouso，ifIweretodropafeatherfrommylefthand，andalumpht，Ishouldohittheflooratthesametime。Thelumpettherefirst。Sowhatismeaement‘allobjectsaccelerateuyatthesamerate’？Touhisphrase，wehavetothinkaboutalloftheforcesthatatheseobjects。

&goofthefeatheritisabythefravity，butitisalsoabyotherforces。Asitstartstodrop，thereisresistasmotionfromtheairaroundit。Thisslowsthefeathermorethaheheavierlumpofiron。Anyslightgustofairwillalsohavealargeefethefeatherwhileperturbiionofthelumpofmetalbyonlyatinyamount。So，themeaningofthephrase‘allobjectsaccelerateuyatthesamerate’isnotastatementaboutthemotiosieeher，itisastatementaboutwhatshouldhappentoaweretofalluheinfluenceofgravityalois，ifallotheriionsaresuppressed，thesshouldfallatthesamerate。

Galileoiswidelycreditedwithhavingshowhofthisproposition。In1638，hesupposedlydroppedballswithdiferehetopoftheleaningtowerofPisa。Theballswerefoundtofallatthesamerate，iheiroreredperhapseveically，thesameresultwasdemoheApolloastronautDavidScott。Scottdroppedafeatherandahammerwhilestandingonthesurfaoon。Thereisnoairooslowthemotioher，andsobothobjededathisfeetatthesametime（seeFigure1）。TodaywerefertothispheheUyofFreeFall。Itisakeyihon'saein'stheravity，aswewillseelater。

1。（a）GalileballsoftheleaningtowerofPisa;and（b）apaintingofDavidScahammeraheronthesurfaoon。

&heravity

SirIsaa’stheravityandmotionublishedinhisPrincipiaMathemati1687，anditgedtheworldforever。Thiswasthefirstgeifictheoryofhowgravityworked。Ule，ondidn’ttryandseekairavity。Iifieditsefedintheprocessdeducedphysicallawsthataccuratelydescribedthemotionofnotosoalsothemotioself，aswellasallotherbodiesintheSolarSystem。

&owastrulyaweednewbrahematidforthefirsttimedemohelawsofphysicsthatapplytous，oh，arethesameasthosethatapplytotheheaveheplexityofmotion，whichAristotlehadtriedtoexplain，wasboileddowntoahandfulofsimplelaws。helorious，aungedforover200years。Inonebook，ioheworldsofsdustry，androvidedtheframehiyofusstillthinktoday。

ThebasigredieheoryaretheexistenceofabsolutespadtimeasthestageonwhichallmotionodtheexisteniversalgravitatiosiweeneverypairofmassivebodiesintheUhatisall。

&onearstomostofusinourday-todayexperience，simplytheeternalandungarenawithinwhichallmatterexists。Ipositiooinspadworkoutthedistaoa，Y，bysimplyusiapeheldoutinastraightliohereisnoambiguityinthisprocess。ObjedYmaymoveihespaceitselfisfixedandfed。

Likewise，oionoftimefollowedtheeverydayintuitionthatmostofusgrowupwith。Instantsoftimeunfoldiheory，oher。Objegetheirpositiaime，buttimeitselfisuniversalandthesameforeveryoheoryalltrueeasuretimeinthesameway，justasalltruemeasuringtapesmeasurethesamedistawogives。

Ntoon，allbodiesmoveatatrate，uedoerhisarturefromAristotle’sphysics）。Ifaforceshouldaobjetheefectofthatforceistocausetheobjecttoaccelerate。Moreforsmoreac，and，ifahasmoremass，thenitoreachthesameac。Withinthisframewravityissimplyaeringonallmassivebodies，pulliher。hathisgravitatiobeproportionaltothemassofeachoftheobjectsitactsbetween，andinverselyproportionaltothesquareofthedistahatis，thefravitybetweentwomassivebodiesmustbegiveionoftheform:

whereMahemassesofthetwoobjequestion，andristhedistahissimpleequatiohon'slawsofmotion，issuittogetagoodapproximatioiomajorityofastrophysicalbodiesaswellasallbodiesthatexistoh。

Thatallbodiesfalluon'sgravityatthesamerate，andhenceobeyGalileo'sobservation，beseehation'smeiforcecausesaheavybodytoacceleratemoreslowlythanalightone。siderthistogetherwiththefactthat，agtohegravitatioihmass。Ion'stheorythesetwothiexactlytherequiredratetoceleachotherout。Abodybeingabyon'sgravity，aon'slawsofmotion，mustthereforeaccelerateatafixedrate，ismass。ThatthishappensisheUyofFreeFallisbuiltion'stheoryfromthestart。

&greatsuewton'stheorywasthatitcouldbeusedtoderivethelawsofmotios。TheselawshadbeendeducedempiricallybyJohannesKeplerearlierihtury，usingg-edgeastronomicaldata。Kepler'slawsstatedthat:

&ofaplaaheSunisatohefociofthisellipse。

·IfalineisdrawlaheSuheplastheSun，thatlinewillsweepoutanequalareaofspaytwointervalsofequaltime。