费曼物理学讲义 第1章 英文版.docx

费曼物理学讲义 第1章 英文版.docx

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费曼物理学讲义第1章英文版

1AtomsinMotion

1–1Introduction

Thistwo-yearcourseinphysicsispresentedfromthepointofviewthatyou,thereader,aregoingtobeaphysicist.Thisisnotnecessarilythecaseofcourse,butthatiswhateveryprofessorineverysubjectassumes!

Ifyouaregoingtobeaphysicist,youwillhavealottostudy:

twohundredyearsofthemostrapidlydevelopingfieldofknowledgethatthereis.Somuchknowledge,infact,thatyoumightthinkthatyoucannotlearnallofitinfouryears,andtrulyyoucannot;youwillhavetogotograduateschooltoo!

Surprisinglyenough,inspiteofthetremendousamountofworkthathasbeendoneforallthistimeitispossibletocondensetheenormousmassofresultstoalargeextent—thatis,tofindlawswhichsummarizeallourknowledge.Evenso,thelawsaresohardtograspthatitisunfairtoyoutostartexploringthistremendoussubjectwithoutsomekindofmaporoutlineoftherelationshipofonepartofthesubjectofsciencetoanother.Followingthesepreliminaryremarks,thefirstthreechapterswillthereforeoutlinetherelationofphysicstotherestofthesciences,therelationsofthesciencestoeachother,andthemeaningofscience,tohelpusdevelopa“feel”forthesubject.

Youmightaskwhywecannotteachphysicsbyjustgivingthebasiclawsonpageoneandthenshowinghowtheyworkinallpossiblecircumstances,aswedoinEuclideangeometry,wherewestatetheaxiomsandthenmakeallsortsofdeductions.（So,notsatisfiedtolearnphysicsinfouryears,youwanttolearnitinfourminutes?

）Wecannotdoitinthiswayfortworeasons.First,wedonotyetknowallthebasiclaws:

thereisanexpandingfrontierofignorance.Second,thecorrectstatementofthelawsofphysicsinvolvessomeveryunfamiliarideaswhichrequireadvancedmathematicsfortheirdescription.Therefore,oneneedsaconsiderableamountofpreparatorytrainingeventolearnwhatthewordsmean.No,itisnotpossibletodoitthatway.Wecanonlydoitpiecebypiece.

Eachpiece,orpart,ofthewholeofnatureisalwaysmerelyanapproximationtothecompletetruth,orthecompletetruthsofarasweknowit.Infact,everythingweknowisonlysomekindofapproximation,becauseweknowthatwedonotknowallthelawsasyet.Therefore,thingsmustbelearnedonlytobeunlearnedagainor,morelikely,tobecorrected.

Theprincipleofscience,thedefinition,almost,isthefollowing:

Thetestofallknowledgeisexperiment.Experimentisthesolejudgeofscientific“truth.”Butwhatisthesourceofknowledge?

Wheredothelawsthataretobetestedcomefrom?

Experiment,itself,helpstoproducetheselaws,inthesensethatitgivesushints.Butalsoneededisimaginationtocreatefromthesehintsthegreatgeneralizations—toguessatthewonderful,simple,butverystrangepatternsbeneaththemall,andthentoexperimenttocheckagainwhetherwehavemadetherightguess.Thisimaginingprocessissodifficultthatthereisadivisionoflaborinphysics:

therearetheoreticalphysicistswhoimagine,deduce,andguessatnewlaws,butdonotexperiment;andthenthereareexperimentalphysicistswhoexperiment,imagine,deduce,andguess.

Wesaidthatthelawsofnatureareapproximate:

thatwefirstfindthe“wrong”ones,andthenwefindthe“right”ones.Now,howcananexperimentbe“wrong”?

First,inatrivialway:

ifsomethingiswrongwiththeapparatusthatyoudidnotnotice.Butthesethingsareeasilyfixed,andcheckedbackandforth.Sowithoutsnatchingatsuchminorthings,howcantheresultsofanexperimentbewrong?

Onlybybeinginaccurate.Forexample,themassofanobjectneverseemstochange:

aspinningtophasthesameweightasastillone.Soa“law”wasinvented:

massisconstant,independentofspeed.That“law”isnowfoundtobeincorrect.Massisfoundtoincreasewithvelocity,butappreciableincreasesrequirevelocitiesnearthatoflight.Atruelawis:

ifanobjectmoveswithaspeedoflessthanonehundredmilesasecondthemassisconstanttowithinonepartinamillion.Insomesuchapproximateformthisisacorrectlaw.Soinpracticeonemightthinkthatthenewlawmakesnosignificantdifference.Well,yesandno.Forordinaryspeedswecancertainlyforgetitandusethesimpleconstant-masslawasagoodapproximation.Butforhighspeedswearewrong,andthehigherthespeed,themorewrongweare.

Finally,andmostinteresting,philosophicallywearecompletelywrongwiththeapproximatelaw.Ourentirepictureoftheworldhastobealteredeventhoughthemasschangesonlybyalittlebit.Thisisaverypeculiarthingaboutthephilosophy,ortheideas,behindthelaws.Evenaverysmalleffectsometimesrequiresprofoundchangesinourideas.

Now,whatshouldweteachfirst?

Shouldweteachthecorrectbutunfamiliarlawwithitsstrangeanddifficultconceptualideas,forexamplethetheoryofrelativity,four-dimensionalspace-time,andsoon?

Orshouldwefirstteachthesimple“constant-mass”law,whichisonlyapproximate,butdoesnotinvolvesuchdifficultideas?

Thefirstismoreexciting,morewonderful,andmorefun,butthesecondiseasiertogetatfirst,andisafirststeptoarealunderstandingofthefirstidea.Thispointarisesagainandagaininteachingphysics.Atdifferenttimesweshallhavetoresolveitindifferentways,butateachstageitisworthlearningwhatisnowknown,howaccurateitis,howitfitsintoeverythingelse,andhowitmaybechangedwhenwelearnmore.

Letusnowproceedwithouroutline,orgeneralmap,ofourunderstandingofsciencetoday（inparticular,physics,butalsoofothersciencesontheperiphery）,sothatwhenwelaterconcentrateonsomeparticularpointwewillhavesomeideaofthebackground,whythatparticularpointisinteresting,andhowitfitsintothebigstructure.So,whatisourover-allpictureoftheworld?

1–2Matterismadeofatoms

If,insomecataclysm,allofscientificknowledgeweretobedestroyed,andonlyonesentencepassedontothenextgenerationsofcreatures,whatstatementwouldcontainthemostinformationinthefewestwords?

Ibelieveitistheatomichypothesis（ortheatomicfact,orwhateveryouwishtocallit）thatallthingsaremadeofatoms—littleparticlesthatmovearoundinperpetualmotion,attractingeachotherwhentheyarealittledistanceapart,butrepellinguponbeingsqueezedintooneanother.Inthatonesentence,youwillsee,thereisanenormousamountofinformationabouttheworld,ifjustalittleimaginationandthinkingareapplied.

Figure1–1

Toillustratethepoweroftheatomicidea,supposethatwehaveadropofwateraquarterofaninchontheside.Ifwelookatitverycloselyweseenothingbutwater—smooth,continuouswater.Evenifwemagnifyitwiththebestopticalmicroscopeavailable—roughlytwothousandtimes—thenthewaterdropwillberoughlyfortyfeetacross,aboutasbigasalargeroom,andifwelookedratherclosely,wewouldstillseerelativelysmoothwater—buthereandtheresmallfootball-shapedthingsswimmingbackandforth.Veryinteresting.Theseareparamecia.Youmaystopatthispointandgetsocuriousabouttheparameciawiththeirwigglingciliaandtwistingbodiesthatyougonofurther,exceptperhapstomagnifytheparameciastillmoreandseeinside.This,ofcourse,isasubjectforbiology,butforthepresentwepassonandlookstillmorecloselyatthewatermaterialitself,magnifyingittwothousandtimesagain.Nowthedropofwaterextendsaboutfifteenmilesacross,andifwelookverycloselyatitweseeakindofteeming,somethingwhichnolongerhasasmoothappearance—itlookssomethinglikeacrowdatafootballgameasseenfromaverygreatdistance.Inordertoseewhatthisteemingisabout,wewillmagnifyitanothertwohundredandfiftytimesandwewillseesomethingsimilartowhatisshowninFig. 1–1.Thisisapictureofwatermagnifiedabilliontimes,butidealizedinseveralways.Inthefirstplace,theparticlesaredrawninasimplemannerwithsharpedges,whichisinaccurate.Secondly,forsimplicity,theyaresketchedalmostschematicallyinatwo-dimensionalarrangement,butofcoursetheyaremovingaroundinthreedimensions.Noticethattherearetwokindsof“blobs”orcirclestorepresenttheatomsofoxygen（black）andhydrogen（white）,andthateachoxygenhastwohydrogenstiedtoit.（Eachlittlegroupofanoxygenwithitstwohydrogensiscalledamolecule.）Thepictureisidealizedfurtherinthattherealparticlesinnaturearecontinuallyjigglingandbouncing,turningandtwistingaroundoneanother.Youwillhavetoimaginethisasadynamicratherthanastaticpicture.Anotherthingthatcannotbeillustratedinadrawingisthefactthattheparticlesare“stucktogether”—thattheyattracteachother,thisonepulledbythatone,etc.Thewholegroupis“gluedtogether,”sotospeak.Ontheotherhand,theparticlesdonotsqueezethrougheachother.Ifyoutrytosqueezetwoofthemtooclosetogether,theyrepel.

Theatomsare1 or 2×10−8 cminradius.Now10−8 cmiscalledanangstrom（justasanothername）,sowesaytheyare1or2 angstroms（Å）inradius.Anotherwaytoremembertheirsizeisthis:

ifanappleismagnifiedtothesizeoftheearth,thentheatomsintheappleareapproximatelythesizeoftheoriginalapple.

Nowimaginethisgreatdropofwaterwithallofthesejigglingparticlesstucktogetherandtaggingalongwitheachother.Thewaterkeepsitsvolume;itdoesnotfallapart,becauseoftheattractionofthemoleculesforeachother.Ifthedropisonaslope,whereitcanmovefromoneplacetoanother,thewaterwillflow,butitdoesnotjustdisappear—thingsdonotjustflyapart—becauseof