22 CHAPTER2. ORIGINSOFQUANTUMMECHANICS
Quantummechanicsemergedasanattempttoexplainthesephenomenaand,as
inthebible,thestorybeginswithlight.
2.1 Black-Body Radiation
IsaacNewtonbelievedthatlightiscomposedofparticles,andhehadgoodreason
tothinkso. Allwavemotionexibitsinterferenceanddiffractioneffects,whichare
thesignatureof anyphenomenoninvolvingwaves. Newtonlookedfortheseeffects
bypassing lightthroughsmallholes,but nodiffractioneffectswereobserved. He
concludedthatlightisastreamofparticles.
OneofNewton’scontemporaries,ChristianHuygens,wasanadvocateofthewave
theoryoflight. Huygenspointedoutthattherefractionoflightcouldbeexplained
iflightmovedatdifferentvelocitiesindifferentmedia,andthatNewton’sinability
tofinddiffractiveeffectscouldbeduesimplytotheinsensitivityofhisexperiments.
Interferenceeffectsaremostapparentwhenwavelengthsarecomparableto,orlarger
than,thesizeoftheholes.Ifthewavelengthoflightwereverysmallcomparedtothe
sizeoftheholesusedbyNewton,interferenceeffectswouldbeveryhardtoobserve.
Huygensturnedoutto be right. More sensitive opticalexperiments byYoung
(1801)andFresneldemonstratedtheinterferenceanddiffractionoflight,andmea-
surementsbyFoucault(1850)showedthatthespeedoflightinwaterwasdifferent
fromthespeedoflightinair,asrequiredtoexplainrefraction. ThenMaxwell,in
1860,byunifyingandextendingthelawsofelectricityandmagnetism,demonstrated
thatelectricandmagneticfieldswouldbeabletopropagatethroughspaceaswaves,
travelingwithavelocityv= 1 /
√
μ 0! 0 ,whichturnedouttoequal,withinexperimen-
talerror,theknownvelocityoflight. Experimentalconfirmationoftheexistenceof
electromagneticwavesfollowedshortlyafter,andbythe1880stheviewthatlightis
awavemotionoftheelectromagneticfieldwasuniversallyaccepted.
Itisalittleironicthatfollowingthisgreattriumphofthewavetheoryoflight,
evidencebegantoaccumulatethatlightis,afterall,astreamofparticles(or,atleast,
lighthasparticlepropertieswhichsomehowcoexistwithitswaveproperties). The
firsthintofthisbehaviorcamefromastudyofblack-bodyradiationundertakenby
MaxPlanck,whichmarksthehistoricalbeginningofquantumtheory.
Anyobject,atanyfinitetemperature,emitselectromagneticradiationatallpos-
siblewavelengths. Theemissionmechanismissimple: atomsarecomposedofnega-
tivelychargedelectronsandpositivelychargednuclei,anduponcollisionwithother
atomsthesechargesoscillateinsomeway. AccordingtoMaxwell’stheory,oscillating
chargesemit(andcan alsoabsorb)electromagneticradiation. Soitisnomystery
thatifwehaveametallicboxwhose sidesarekeptat someconstanttemperature
T,theinterioroftheboxwillbefilledwithelectromagneticradiation,whichiscon-
stantlybeingemittedandreabsorbedbytheatomswhichcomposethesidesofthe
box. Therewas,however,somemysteryintheenergydistributionofthisradiation
asafunctionoffrequency.