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&hiedup，theygecolour.Takealumpofmetal.Asitgetshotterafirstglowsred，thehenwhite.Whydoesthishappen?Thisquestionstumpedmastistsofthetime，ingMaxwellhimself.TheproblemwasthatMaxwell’stheht，liedtothisproblem，ihecetbluerahetemperatureihoutalimit，eventuallymovingouteofhumanvisioraviolet—beyioruButthisdoesnothappeninpractice.
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—discreteobjectsthatcarriedafixedamoucouldbeabsorbedbyatomsoremittedbythe
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，youwouldbeeffectivelygtheonsinthebea
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.ThisisrelateddirectlytothedensityofphotobeaSimilarly，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，whiedforhi
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.
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