Chapter 2
Origins of Quantum Mechanics
Wheredocorrectideascomefrom?Dotheydropfromthesky?No!Aretheyinnate
inthemind?No!Theycomefromsocialpractice,andfromitalone.
- MaoTse-Tung
Thesuggestionthatallmatteriscomposedofatoms,andthename”atom”itself,
areduetotheGreekthinkerDemocritus,wholivedfivecenturiesbeforeChrist.Not
untilthe19thcentury,however,didevidenceforthishypothesisbegintoaccumulate,
particularlyfromthermodynamics. Theevidencewasindirect,but compelling: as-
sumingthatgasesarecomposedofatoms,onecouldderiveanalyticallytheequation
ofstateforidealgasesPV =nRT,whichhadbeendiscoveredempiricallybyBoyle
andothers. Inaddition,assumingthatsolidsaswellasgasesarecomposedofatoms,
onecoulddeducetheirspecificheats,whichagreedfairlywellwiththeexperimental
valuesathightemperatures.Bytheearly20thcentury,improvementsintechnology
andthediscoveryofradioactivityhadenabledphysiciststostudyinsomedetailthe
internalstructureofatoms,themassandchargeoftheelectron,andtheinteraction
ofatomswithlight.
Certainaspectsofatomicphysicswhichemergedfromtheseearlyinvestigations
werepuzzlingandevenparadoxical,inthesensethattheobservedbehaviorofelec-
trons,atoms,andlightseemedincontradictiontotheknownlawsofmechanicsand
electromagnetism.Theseaspectsfellroughlyintothreecategories:
- TheParticle-likeBehaviorofLightWaves
Black-bodyradiation,thephotoelectriceffect,theComptoneffect. - ThePuzzlingStabilityoftheAtom
Whydoesn’t theelectron fallinto thenucleus? What isthe originof atomic
spectra? - TheWave-likeBehaviorofParticles
Electrondiffraction.