千米小说网

Chapter 4 Duality（第2页）

Theseplearosefromthestudyoflightemittedbyatoms，pioneeredbyaSwisssaster，JohannesBalmer。We’lllookatthismemoredetailitheimportahelightisiributionofitsspectrum，showninFigure23a。Iisverydifferentfromsunlight（theSunisagoodexampleofahotbody），whichhasthefamiliar‘rairum，showninFigure23b，gofallcoloursredtoviolet（aead—justnotvisibletous）。Bytrast，asemitsadiscretesetofcolours—asetofspees’ofparticularwavelengths—specificallyassociatedwiththeiructureofthepartiinvolved。

Bothofthesephenomenarequiredaradicalrevisionofthinkingaboutlight，becausetheyotbeexplaihiemporarymodelsofwavemotionandatomicstructure。

MaxPlangatHumboldtUyihelate19thtury，firstcameupwithaoexplairumemittedbyhotobjects—so-called‘blackbodies’。Hejecturedthatwhenlightaeract，theydosoonlybyexgdiscrete‘packets’，orquanta，y。Planizedtherevolutionarynatureofhisidea，andwasthereforereluttoioomuchaboutlightitselffromit，thoughitwouldeventuallydramatigeourviewofwhatlightis。Hisheideaoflightasastreamofparticles—discreteobjectsthatcarriedafixedamoucouldbeabsorbedbyatomsoremittedbythem。

23。Spectrumoflightemittedfroma。theSun（a‘blackbody’），andb。aneonlamp。Theformerhasuousbandofcolours，whereaslattershowsthedisesofparticularcoloursthatarea‘fi’oftheoms。

Thisseemedlikearetrogradestep:thewavemodeloflightcouldexplainallofthehithertoobservedeffeditwasilton’sworkthateveory-likebehaviour，previouslythemostobviousevidenceofaparticle-likeentity，emergedfromawavemodeloflightiainonsituations。Sotheideaofaparticleoflightappearednottobenecessary。Surelyitlyacalal‘fix’，thoughtuptogetoutofatightspot，auallybereplacedbyamorereasonablytpictureoflight。However，bihBalmer’sobservations，thisjecturewassettoradigephysics。

IntheyearsimmediatelyafterPlanck’ssuggestioeiheideaofdiscreteexgesybetweenlightaoexposeanotherpieceofphysicsthathadeludedexplaoelectriceffect。Thiseffewhenlightshinesoal。Ses—elefact—areejeetal。Thespeedwithwhichtheelesareejedsohofthelight。Thelightmustbesuffitly‘blue’，thatishaveashhwavelength，ioseearoall。Asitgetsblueraheelesareejectedwithmoreandmy，andthushigherandhigherspeed。

&eihisbynotingthattheelesrequiredaminimumamouoescapetheetal，andbyassignisytoaparticleoflight—thephoton—proportionaltothefrequehetofproportionalitybeingknolanstant，h），sothatwhenthefrequeishighenough（andthusthewavelengthshh）thelight，whenitisabsorbed，provideenyforaoescape。HismodelsuggestedthattheieexgeybetweenlightaistraltoPlanotioheactualdiscretecharacteroflight—afullrevivaloftheparticlemodel。

ThisideaicelywithBalmer’sobservationsofdisespectraoflightemissionfromatoms。Butafullexplanationofthisphenomenonclearlyrequiresabitmhtaboutwhyatomswoulddeliverlightinsuchpackets。ThekeyideaNielsBohr，aDanishphysicistwier。Hesuggestedthatthereasonlightwasemittedasdiscretepacketsywasthattheatomsthemselveslyexistiainfigurations。Heimagiomsasanalogoustotiems:eleorbitarouralheele‘jump’betweentwostableorbits，emittinghtasitdoesso（dependihejumpistoanorbitherehecharacteroftheseorbits—orquantumstates—depeailsoftheatom:howmaronsandthesizeofthehereforetheeakenupwherowoquantumstatesisasighepartiitself。Sowhentheenergyofalightpartiatchesthatofthedifferewoquaheeleanatom，thenabsorptionoremissionispossible。Bohr’sideasexplainedBalmer’sobservatioly，andaddedsomeweighttotheideaoflightbeamsasaofdiscreteparticles。

AllthesedevelopmentshadthepotentialtouhewavemodeloflightthathadbeenslyaffirmedbyMaxwell’stheory。Theywentbeyooiooriesaioheyappearedtobefusimplytheresultofanapproximationaboutsizeaheyreopeioureoflighton。

IaylinCambridge，perf’sdouble-slitexperimeio—soweakthatetherewaslessthaousatahestillsawinterfereouteisverystrahihfromthelightsourcetothedetectviao，ahviatheotherslit，thewohotofromsourcetodetector。Theevideerferengesposedadilemmathatcaughttheattentionoftheleadisoftheday。Bohrcapturedthediffiotingthatwewould‘beobligedtosay，ohatthephotonalwayseofthetwoways，ahatitbehavesasifitpassedthroughboth’。Eveiexhibitwave-likebehaviour。

Waviher

Asyoumightimagiookatrulyradicalthoughttofigureawayoutofthis。ThatthoughtoccurredtoPaulDirac，aphysicistwatCambridgeinthe192estedthatthefualprhtwasthatitartidawaveatthesametime。Now，thismightappeartoyouassimplysicaltrickthatresolvesnothing。Butthereismuchmorebehinditthanaslogan。WhatDiracdidwastodevelopaquantummeiaxwell’stheoryicfields。Usingthishewasabletoshowthat，ifyoumeasuredthese‘quantumfields’usi-uplikeYoung’sdouble-slitier，youwouldseeinterferescharacteristicofwave-likebehaviour。Whereas，ifyousimplymeasuredtheiyofthelight，youwouldbeeffectivelygtheonsinthebeam。

Thisturobeaveryprofouthequantumfieldupasthefuityonwhiiverseisbuilt—iorwave，butbothatoearticleduality。Itisabeautifulreofallthephelightexhibits，andprovidesaframeworkinwhiderstandallopticaleffects，boththosefromtheclassicalworldofon，Maxwell，andHamiltonahequantumworldofPlaein，andBohr。Butthecostisaperplexingandnon-itheheartofumfield—ofwhichlightisbutoneexample。

Theradicalideathatlightwasbotharticlestimulatedsomemajornewideas。Forinstance，Lgestedthatifthiskindofdualismexistedfht，surelyitshouldforallstoo。So，materialbodiesthatwenormallyhadsideredonlyassofparticlesshouldalsohave‘wave-like’properties，takiepbeyondwhatHamiltonhadsidered。Heevehewavelengthshouldbe（hedeBrogliewavelengthλdB）—proportionaltotheiheparti（i。e。itsmassahetofproportionalityagainbeingPlanstant）:

λdB=hmv

Thissuggeststhattolookforsucheffectsyoushoulduseeitherverylightparticlesorveryesthataremovingvery，veryslowly。Sucheffebeure24showsaternmadeusingadouble-slit-likeiusingmolesteadoflight。Theimplisofthisaremind-boggling。Ifyouthinkofamoleculeasjustaverylightpartiyouotexplaiern，becauseyousiderthatitmusthavepassedthroughoortheother。However，theideathataparticlewithmasscouldbesodelocalizedastohavepassedeffectivelythroughbothslitstoiselfisastoundingandbeggarsbelief。

24。AternmadeusingmoleculespassiimethroughatinyversionofYoung’sapparatus—twoverysmallslitsseparatedbybillionthsofametre。

Animportantquantityfhtisitsiy，proportionaltothesquareoftheamplitudeofthefield。Thisisrelateddirectlytothedensityofphotobeam。Similarly，thesquareofthewavefunisrelatedtothedensityofparticlesatapartitiaparticulartime。Butitisimpossibletosayforthataparticleoccupiesaspecifiataspestant。Thisiermiobeaveryfualpropertyoftheworld，anditisrelateddeeplytothefactthatquantumfieldslieattheheartofthings。

Nothihing

Anotherceofthisfactisthat‘nothing’isaething。Thatis，thepleteabseer（e。g。eles）ht（i。e。photons）stillhasmeasurableproperties。Thisvoidiscalledthe‘eleagumvadisthestateoftheuniversefromwhichallextraergyhasbeeisaseethingmassofactivity，gofflugfieldsbutgnophotoisevenmisthatthequantumvacuumhasobservableces。Howthis‘nothioa>

&hatlightbethoughtofasawavemotioromagicfield。Imagihisfieldasripplesonthesurfaceofthesea。Thesewouldbuffetaboutanyboat，butwouldupanddownorpushitalongasawell-defi。e，theboatdoeshelessrocksbadforth。Nowimagihingfedparticlelikea‘feels’therandomtheeleagicvagbuffetedbythese。Iftheeleisboundinanatom，thisbuffetingisrevealedasashiftoftheehequaheeleayocethefrequehattheatommayabsorbdependsonthedifferewosuchstates，thesegesbeseenihechtthattheatommayabsorb。Thegesaretiny—ashiftoflessthahofthewavelengthofthelight—butmeasurementteiquesforfrequenciesaresoprecisethatsugesdeedbedetermipersontodothis，WillisLamb，winNewYorkinthe1950s，rizefthisfrequencyshift，whiedforhim。

Thedualidentityoflighthasnumerousfaiumworld，thediyofrayandwavedemaion。Thatcameaboutbyuaureofthewavemotionthatlightembodied，andthesatureofobjectswithwhiteracted。Particle-likebehaviour—motionalongawell-defiory—issuffittodescribethesituationwhesaremuchbiggerthahoflight，andhavenes。Quantummeicsputsaonthisduality。Lightactsasapartioreorlesswell-definedeeractswithmatter。Yetitretainsitsabilitytoexhibithehesametime。Theresolutio:thequaparticles—photoionsofthisfield，whichpropagatesagtoquantumversionsofMaxwell’sequatiht>

Quantumfields，ofwhichlightisperhapsthesimplestexample，arenardedasbeingthefuitiesoftheuniverse，underpinningalltypesofmaterialandhings。Theoionisthatthestuffoftheworldisiorwavebutboth。Thisisthenatureofreality。