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INTERFERENCE D A V I D D . PurdueUniversity TheHistoryofOptical InterferometryandtheScientists whoTamedLight GreatClarendonStreet,Oxford,OX26DP, UnitedKingdom
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PREFACE Thewavelengthoflightistheultimateruler.Itsetsthescale againstwhichthephenomenaoftheuniversearemeasured. Fromtheimagingofdistantblackholestothedetectionof exoplanetswithinourgalaxy,fromthegenerationofentangled particlesinquantumcomputerstothetransmissionofphotons downfiber-opticcables,fromthecreationofvirtualrealityby holographicdisplaystothemicroscopicimagingofnanoscale objectsinsidelivingcells—lighthelpsilluminatethebeautyand complexityofthemall.Yetthewavelengthoflightisexceedingly small—aboutthesizeofabacterialcell—amillionthofameter. How,then,canitmeasureablackholethesizeofoursolarsystem thatis50millionlightyearsaway?Howcanitrenderimaginary scenesinhighdefinition?Howcanitimageobjectsthatare nobiggerthantinyfractionsofitsownlength?Theansweris interference!
Interferenceoccurswhenonewaveoverlapsanotherandthey areaddedtogether,summedup,buildingconstructivelyatplaces wherepeaksandtroughsofthewavesaddtogether,anddestructivelyelsewherewherepeaksfilltroughsandcancelout.Thephenomenonofinterferenceliesatthecoreofphysics.Interference iswhyfloatingsoapbubbleshavesuchbrilliantcolors,andwhy theybecomealmostinvisiblejustbeforetheypop.Atamoreprofoundlevel,interferenceamongquantumfluctuationsintheBig
Bangmaybewhytheuniverseiscomposedalmostexclusivelyof matterratherthanantimatter.Interferenceiswhyatomsbondinto molecules,whythechairyousitinissolid,andwhyneutronstars don’timplode.Interferenceisalsoascientifictool,allowingscientiststobuildgravitationalwavedetectorstomeasurethemergerof blackholesindistantgalaxies,tolaunchsatellitestolookforalien planetsorbitingnearbystars,andtobuildquantumcomputers thatmayonedaycomputeincomputablethings.
Interference:TheHistoryofOpticalInterferometryandtheScientistswho TamedLight tellsthestoriesbehindtheinventionsandadvancementsofopticalinterferometry.Interferometersarethemost sensitivemeasurementdeviceseverdevisedbymankindandare poisedtochangeourviewofourplaceintheuniverse.Thisbook tellshowsuperpositionandinterferencehelpedshapethemodern physicsofoptics,showinghowinterferenceandinterferometers areopeningnewworldsthatmaybewilderandmorealienthan weimagined.Thebookisaimedatthecuriousbuddingscientist, atcollegeundergraduates,aswellasatexperiencedpractitioners. Itisforthosewhowonderwherethediscoveriestheyhearabout, learnabout,orusedailyintheirwork,camefrom.Thereader willbeintroducedtothepersonalhistoriesofthescientistswho, oftenworkingagainstgreatodds,madetheirdiscoveries,and whatdrovethem.
OurtalebeginswithThomasYoung,theBritishpolymath unusualevenamongpolymaths,performingthefirstdouble-slit interferenceexperiment,thenturningasidetodecipherEgyptian hieroglyphics.ThestorycrossestheEnglishChanneltoFrance andtheworkofFrenchphysicists,FrancoiseArago(aswashbucklingfutureprimeministerofFrance)andAugustinFresnel (banishedtothecountrysidetobuildroadsandbridgesfor
Napoleon’sarmies),whotogetherfoughtagainsttheentrenched forcesofLaplaceandPoissontoverifythewavenatureoflight. Aragoconstructedthefirsttrueinterferometricapparatusand inspiredagenerationofFrenchphysicistsduringthegoldenage ofFrenchoptics,includingafraughtfriendshipbetweenLéon FoucaultandHippolyteFizeauwhoracedeachothertouse Arago’sinterferometertomeasurethespeedoflight.
Ournarrativethenturnstothegiantofinterferometry—Albert Michelson—whocreatedhisiconicinterferometertomeasure thespeedoflightthroughtheether,onlytocomeupemptywith anullresult.Disappointedwithhisexperiments(hecouldnot foreseethathisresultswouldhelplaunchEinstein’srelativity revolutionyearslater),Michelsondesignedthefirstmetrological interferometertocalibratethemeterbarinParisinunitsofwavelengthsofredcadmiumlight,forwhichhebecamethefirstUS Nobelprizewinner.Michelsonlaterextendedhisinterferometer toastronomywherehemeasuredthesizeofthestarBetelgeuse inthefirstofalonglineofincreasinglysophisticatedandsensitiveinterferometers.DescendantsofMichelson’sinterferometer todayaremeasuringthebarestwhisperofdistantcollidingblack holes—aseemingparadoxthatcrossesastronomicalscalesto detectaphenomenonabillionlightyearsawaybymeasuring mirrordisplacementsthatareatinyfractionoftheradiusofa proton.
Movingfromcosmicoratomicscalestotheintermediate microscale,interferenceofphotonscreatesholograms,invented bytheHungarianphysicistDenisGabor,exiledfromhiscountry bytheeventsofWWII,whileworkingataBritishelectronicscompanywherehewasisolatedfromtheotheremployees becausehewasdeemedanenemyalien.Holographycaptures
three-dimensionalinformationofanilluminatedobjectby recordinglightinterferenceonatwo-dimensionalplanetocreate theamazingnoveltyhologramsweseeinstoresandmuseums. Butholographygoesmuchfurther,usedforopticaldatastorage andfastretrievalofmassiveamountsofdataandfordirectoptical communicationinsidecomputersandinfiber-optictelecommunicationsystems.Holographyisalsousedforlaserranging, muchlikeradar,seeingintoturbidmediaandlivingtissuethat areopaquetoordinaryoptics,trackingobjectsmovingwith speedsdowntonanometerspersecond.Theunderlyingoptical processesofholography—diffractionandinterference—arethe samethatsupportopticalmicroscopy,includingsurprisingnew methodsthatallowmicroscopestoseewithfinerresolutionthan previouslythoughtpossible.
Ourstoryreturnsbrieflytostellarinterferometry,wherethe BritishscientistsR.HanburyBrownandRichardTwiss(known collectivelyasHBT)createdanewtypeofinterferometer,known asanintensityinterferometer,thatdifferedradicallyfromMichelson’s,expandingthenumberofstarsthatcouldbemeasured interferometrically.TheHBTinterferometer,performedonthe classicallightfromstars,hadtheunexpectedsideeffectoflaunchingthefieldofquantumopticsthatmanipulatessinglephotons andharnessestheirquantumpropertiestoperformnoveltypesof communicationandcomputation.Whenaquantumcomputer calculatessomethingmuchmorequicklyandcheaplythana classicalcomputer,itiscalled“quantumadvantage.”Itwouldbe acalculationthattakesaquantumcomputerminutesorhours whenthefastestclassicalcomputer(whichincludesdistributed classicalcomputingoverthecloudthatcanharnessthousandsof computationalnodes)wouldtakemonthsoryearsormore.There
PREFACE arealreadyreportsthatquantumadvantagehasbeenachieved, andmoreclaimswillbecomingsoon,enabledbythesubtle powerofopticalinterferometry.
Thesearesomeofthestoriestoldherein Interference,gathered intoasinglevolumeforthefirsttime,highlightingthehumanelementinthepursuitofscience.Igratefullyacknowledgethehelp ofProf.DanMilisavljevicatPurdueUniversityforpre-readingthe chapteronstellarinterferometryandProf.Z.H.(Jeff)Ouatthe UniversityofHongKongforprovidingsuggestionsforthechapteronquantumoptics,andasalways,Iamgratefultomyeditor atOxford,SonkeAdelung,forhiscontinuedsupportandencouragement.IalsoextendathankfulacknowledgmenttoNicholas Noltewhoreadthequantumchaptersandpointedoutinteresting improvements.Thisbookwouldnothavebeenpossiblewithout theloveandsupportofmywife,Laura,whohasalwaysbeenmy biggestfan.
Lafayette,IN November2022
DAVIDD.NOLTE
1 ThomasYoungPolymath TheLawofInterference Ihavefoundsosimpleandsodemonstrativeaproofofthegenerallawoftheinterferenceoftwoportionsoflight...thatIthink itrighttolaybeforetheRoyalSocietyashortstatementofthe factswhichappeartomesodecisive. 1
ThomasYoung, BakerianLecture (1803)
Thedecisionin1798bytheexecutivecommitteeoftheFrench Revolution,knownastheDirectory,tosendanoverlyambitiousgeneraltothefarendoftheMediterraneanSeatoconquer EgyptlikeAlexandertheGreat,wastheunlikelylinkamonga setofepicdiscoveriesthatchangedthecourseofscience.Oneof thesediscoverieswasthemostfamouslinguisticdeciphermentin history.Anotherwasthemostimportanttheoreminthemathematicsoffunctionalanalysis.Joiningthesewerediscoveries thatexposedthenatureoflight.Allofthesetookplaceoverthe decadesfollowingtheinvasion,buttheywereconnected,like thethreadsofacobweb,tothefatefuldecisionbythegeneralto complywiththeDirectory’swishes.
NapoleonBonaparte,thegeneralinquestion,hadhiseyes onEgyptandtheMiddleEastwithaplantoseverBritain’sties toIndia,buthisambitionsweremorethanmilitary.Whenhe Interference.DavidD.Nolte,OxfordUniversityPress.©DavidD.Nolte(2023). DOI:10.1093/oso/9780192869760.003.0001
invadedEgypt,inadditiontobringingcannonsandhorsesand legionsofsoldiers—hebroughtalegionof savants.Thesewere mathematiciansandhistoriansandbotanistsandphilologists. Thesewereengineersandchemistsandarchitectsandarcheologists.Theycamewithtrunkspackedwitharcanebooks,delicate scientifictools,andthemostadvancedmeasuringinstrumentsof theday.Thesavants,astheysteppedaboardtheirfleetofships sailingfromToulonnearMarseilles,didnotevenknowwhere theywerebound,sosecrethadthetargetofthemissionbeen kept.2 Theysimplywererespondingtothecharismaticpullof Napoleonandthepromisethatitwouldbetheopportunityofa lifetime.Thepromisewaspropheticallytrueforthosewhowere fortunatetoreturnalive.
TheInstituteofEgypt TheFrenchinvasionlandedatAlexandriaandmovedtoward Cairo,wheretheyweremetbytheEgyptianarmyseveralmiles outsidethecity.Thefamouspyramidswerevisibleonthehorizon astheFrenchroutedtheEgyptians—thebattlelatercametobe knownastheBattleofthePyramids.Napoleon’sarmyentered Cairowhereofficersandscientiststookpossessionoflavish palacesabandonedbytheMamelukerulers.Thenewlyformed InstituteofEgyptsetupinthepalaceofHassan-Kashifonthe outskirtsofCairo,stilloccupiedbyabandonedservantsandan ampleharem.
ThepresidentoftheInstitutewasthemathematicianGaspard Monge(1746–1818),withJosephFourier(1768–1830)asthesecretaryandNapoleonhimselfasvice-president.Thefirstmeeting
oftheInstitutewasheldonAugust24,1798with48members. MongeandFourieraswellasNapoleon(whofanciedhimself goodatmath)weremembersofthemathematicssection.The physicsandnaturalhistorysectionincludedClaudeBerthollet (achemistwhoestablishedtheprinciplesofchemicalequilibria),René-NicolasDesgenettes(adoctorandclosefriend ofNapoleon),DéodatdeDolomieu(therocktypeknownas dolomiteisnamedafterhim),andGeoffroySaint-Hilaire(acolleagueandproponentofLamark).Theliteratureandartssection includedBaronDominiqueDenon(thefirstdirectoroftheLouvre Museum).AlthoughthemissionoftheInstitutewasostensiblyto developthefledglingfieldofEgyptology,Napoleonvalueditas anintelligenceagencyinanenemyland.
Intime,thisgrandadventurethathadseducedthesavantswith thedesiretounlockthemysteriesofEgyptturnedfromadream intoanightmare.TheBritishfleetunderHoratioNelsonengaged theFrenchduringtheBattleoftheNileinAboukirBaythatraged forthreedaysfromAugust1toAugust3,1798.TheFrenchships weredestroyed,captured,orscattered.DirectcontactandresupplyfromFrancewerecutoff,andtheprospectsforreturning homedimmed.Despitethedefeat,Napoleondidnotabandonhis campaign,andhistroopsadvancedintoSyria,passingthrough GazatobesiegetheTurkishgarrisonintheportcityofJaffain present-dayIsrael.
Amongthemilitaryengineerswhoaccompaniedthecampaign wasÉtienneLouisMalus(1775–1812),amemberoftheInstitute ofEgyptinthemathematicsbranch.Maluswasthesonofa financialadvisortoLouisXVI.AttheoutbreakoftheFrenchRevolutionhehadtoleaveschooltojointhearmytoavoidscrutiny, prison,orworsebecauseofhisassociationwiththeoldregime.
THOMASYOUNGPOLYMATH Hiscommanders,surprisinglycognizant,recognizedhisstrong mathematicaltalentandtransferredhimin1794totheEcole PolytechniqueinPariswherehewasamongthefirststudents toattendthenewschool.WhileattheEcole,hecametothe attentionofitsdirectorMonge,whotookapersonalinterestin him.Aftergraduation,Malusreturnedtothearmyasanengineer, andwhenNapoleonaskedMongetoassembleacorpsofsavants toaccompanytheEgyptiancampaign,Maluswasoneofthefirst hepicked.
ThesiegeofJaffawasoverquickly,buttheaftermathwasendless.Maluswrote,“Thetumultandthecarnage,thebrokendoors, thehomesshakenbythenoiseofthefightingandofarms,the screamingofthewomen…furioussoldiersrespondingtothe criesofthewoundedbycriesofrageandrepeatedblows,finally ofmensatisfiedofbloodandgold,fallingofwearinessonthe heapsofcadavers….”3 NapoleonwasmercilesstotheTurkish prisoners.FourthousandTurkssurrenderedandwerebayonetted todeathonthebeachbecauseNapoleondidnotwanttowaste bullets.AnothereviloutcomeofthesackingofJaffawasthe pestilenceitunleashedontheFrenchvictors,andMalusrecognizedtheevent:“Thefrenziedpillagedeliveredamiasma.Itwas containedintheclothestheyhadgreedilytaken.Themortaleffect wasrapid.Theillnessappearedonthebattlefield,inbuboesand carbuncles.Thehorrifyingcry‘It’stheplague!’spreadthroughthe armyandstruckterrorinthemostcourageousandinvincible.”4
Asamilitaryengineeronthecampaign,Maluswasorderedto setupahospitaltoholdtherapidlyincreasingnumberofplague victims.Hetookoveraconventtooutfititasahospital,butit wasunderstaffedandoverwhelmedbynumbers.WhenNapoleon
marchedupthecoasttoattackAcre,thosewhogottheplague wereshippedbacktoJaffawheresoldiersweredyingattherate of30perday,continuingforsixweeks.Withintendaysofsetting upthehospital,Maluswasstrickenwiththediseaseandfellintoa nightmarishsequenceoffeversandconfusion.Hewasneardeath whenhewrotelettersinabriefmomentofclaritysayinggoodbye tohisfriendsandfamily.Byluck,atransportshiparrivedtorelieve thepressofpatients,andittransportedmanybacktoCairo,Malus amongthem.Assoonashebreathedthefreshseaairhebeganto recoverandwasfinallyabletoeatbythetimehearrivedinCairo. Unfortunately,allpatientswerequarantinedinaprisonwherethe onlymedicalstaffconsistedofthegravediggerswhogavenocare tothepatientsandstrippedthemoftheirpossessionsbeforethey wereevendead.Miraculously,Malussurvivedthisordeal,butthe FrenchcouldnotsurviveinEgypt.
Towardtheendof1801,astheFrenchresourcesfinallygave out,theyhadnothinglefttodefendthemselvesfromtheBritish. Thoseofthesavantswhoremained(onlytwothirdsoftheoriginalgroupwereleft)hopedtobeallowedsafepassagehome, buttheFrenchcommanderGeneralMenouwouldnotletthem go.WhentheFrenchfinallysurrendered,Menoucaredlittle forthetremendouscacheofnotesanddrawingsandartifacts thatthesavantshadamassedduringtheirtimeinEgypt,andhe useditasabargainingchipinthetermsforsurrender,promisingtogiveitalltotheBritishinreturnforotherconcessions. Thesavantswereappalled.WhentheBritishmovedtoconfiscatethistreasuretrove,GeoffroySainte-Hilairedeliveredan impassionedspeechthatmadehimfamousinalltheFrench societies.
THOMASYOUNGPOLYMATH Youaretakingfromusourcollections,ourdrawings,ourplans, ourcopiesofhieroglyphics,butwhowillgiveyouthekeytoallof this?Theyareonlypreliminarydraftsthatourpersonalimpressions,ourobservations,ourmemoriesmustcomplete.Without us,thesematerialsareadeadlanguage,fromwhichyouwillhear nothing,neitheryounoryoursavants.Wehavespentthreeyears conquering,onebyone,theseriches,threeyearsgatheringthem fromallofthecornersofEgypt,fromPhilaetoRosetta:toeach ofthemisattachedaperilsurmounted,amonumentseenand engravedinourmemories.5
HethenstatedwithconvictionthatiftheBritishmovedtotake thematerials,theFrenchwoulddestroyitallbeforetheyhadthe chance.Heaskedwhattheworldwouldthinkifatreasureas greatastheAlexandrianLibrarywereburnedonceagain?The Britishrelented,lettingthesavantskeeptheirmaterials—except foroneartifactthattheBritishcovetedmorethanalltheirother manuscriptsandrelics.
Passagehomeforthesurvivorswastakenonamotleyarrayof BritishandGreekvessels,somemeretrawlers,sufferingdangers asgreatasthosefacedinEgypt.WhentheylimpedintoMarseilles,thesavantsweresequesteredintoquarantineforamonth. Onlyin1802didtheyfinallywalkfreeontoFrenchsoil.Later, asNapoleonrosetotakeabsolutepower,herewardedhismost faithfulservants,makingBertholletandMongecounts.Berthollet setupasocietyinArcueil,wherearecoveredMalusbecamea member,givinghimthefreedomtopursueideasthathehad conceivedinEgyptonthepropertiesoflight.Laplacealsojoined thesmallgroup,andhetookMalusunderhiswing.
Sainte-Hilaire’swordswereprophetic;thevastcollectionof diversematerialsthesavantscarriedbacktoFrancebecamethe sourcefor Descriptiondel’Egypte,acomprehensivedescriptionof
theancientworldthatthesavantshadrediscoveredduringtheir expedition.Thetaskofpreparingtheworkforpublicationwas takenonbyFourier.Hehadrecognizedtheuniqueopportunityas earlyas1798toshedlightonthemysteriouslandonthefarsideof theirownMediterraneanSeathathadcapturedtheimagination ofscholarsforcenturies.ThefirstvolumewaseditedandpublishedbyFourierin1809underthedirectionofNapoleon(always lookingforanopportunityforself-aggrandizement).Manymore volumesappearedlaterundertheauspicesoftherestoredKing LouisXVIIIafter1814,withthefinalvolumepublishedin1829. Theefforttook30years,consistingof24volumes,compilingthe workofthe150savantswhosailedwithNapoleonfromToulon in1798withoutknowingtheirdestination.The Description drew fromthevastmaterialsthattheBritishhadobliginglyreturned, buttheyretainedthatonemostcoveteditem—theRosettaStone.
TheLinguistandtheLoyalist AyearaftertheBattleofthePyramids,whentheFrenchwerestill diggingin,agangofsoldiersuncoveredanearlyone-tonshattered slabofblackbasaltintheNiledeltanearthetownofRashid,called RosettebytheFrenchandRosettabytheBritish.Itsonepolished surfacewasinscribedwiththreetypesofwriting:onewasGreek, usedtowritetheCopticlanguageofEgypt;anotherwasdemotic, theeverydayscriptoflaterEgyptianantiquity;andthelastwasthe long-forgottenscriptofEgyptianhieroglyphics.Theinscription wasacommemorationofPtolemyVEpiphanes,datedto196BC, understoodthroughtheCopticversionofthetext,whichscholars alreadycoulddecipher.Thetrilingualinscriptionswereassumed
tobeequivalenttranslations,butwhilethedemoticportionwas almostentirelyintact,theCopticwasmissingseveralparts,and thehieroglyphicsectionwasbadlydamagedandnotevenhalf complete.Nonetheless,JosephFourierandÉtienne-LouisMalus inspectedthestoneandsuspectedthattheRosettaStonecouldbe thekeytodeciphermentofEgyptianhieroglyphics.
AtthetimeoftheFrenchinvasion,Egyptianhieroglyphicswere mistakenlybelievedtobeaprimalvisuallanguagethatcommunicatedwisdomdirectlythroughsymbolsimbuedwithmystical meaning.Evenintheseventeenthcentury,mathematicianand philosopherWillhelmGottfriedLeibniz(1646–1716)wassearchingforauniversal“characteristic”—alanguageofpurelogicbased onarrangementsofmathematicalsymbols,andheviewedhieroglyphicsasanarchetype.Forthesereasons,thedeciphermentof Egyptianhieroglyphicspresentedoneofthegreatopportunities oftheEnlightenment.Bythesinglestrokeofapickaxe,thekey toeventualdeciphermenthadbeenuncovered.Withinmonths ofthediscovery,despitethewagingwar,Frenchscientistsmade copiesoftheinscriptionsandsentthemtotheircolleaguesacross Europe.AdecadepassedwithlittleprogressuntiltheBritish physicianandphysicistThomasYoung(1773–1829)turnedhis penetratingmindtotheproblemofthedecipherment.
Atthebeginningof1814,Youngreceivedanenvelopeinthe postthatcontainedacopyofadamagedpapyrusmanuscriptcontainingdemoticscript.Atthattime,hewasa40-year-oldLondon doctorwhosemedicalpracticesufferedforwantofpatientsas hisattentiontendedtowanderovermanynon-medicalsubjects suchaslinguisticsandorthography.Hehadbecomeknownto haveagreatfacilitywithwritingsystems,abletorecognizeand
toreproduceoriginaltextswithmeticulouscare.Thiswasoneof hischieftalents—theabilitytoseesimplepatternsandtoextract thesalientdetailsfromavastvisualjumbleofinformation.As heworkedonthepapyrus,henaturallybegantothinkaboutthe RosettaStone,whichhadfounditswaytotheBritishMuseum inLondon,stubbornlythwartingtheeffortsofprofessional linguists.ToaugmenthisunderstandingofEgyptianwriting, YoungturnedtothepublishedvolumesofFourier’s Description del’Égypte. 6 Thebookcontainedatreasuretroveofexquisitely renderedcoloredplatesofinscriptionsandpapyri.Bythemiddle oftheyear,YoungwasimmersedinEgyptianstudies,assimilating allhecouldfind,comparingandcontrastingscriptsandfigures andcharacters,notingallthesimilaritiesandallthevariations. Inthemidstofthisactivity,hereceivedanotherletterinthepost fromafriendwhoknewofYoung’sintenseinterests.
Youngmusthavethrilledwithexcitementasheopenedit,findingthereproductionofarecentlyuncoveredfunerarypapyrusof ancientEgypt.Thepapyruscarriedtwoscripts—onetherunning demoticscript,andtheotherthehieraticscript,whichlooked vaguelysimilartohieroglyphics,butwrittenwithinkonpapyrus insteadofcarvedinstone.Ashescannedthenewtext,Young musthavebeenstruckbyanapparentsimilaritybetweenseveral ofthedemoticcharactersandtheirhieraticcounterparts.Turning tothereproductionoftheRosettaStone,heisolatedseveralof thesamedemoticcharacters,andthencomparedthemagainst thehieroglyphic,findingthesamestrikingsimilarities.Inaflash ofinsight,herealizedthatdemoticmustbe,atleastinpart,a modifiedversionofhieroglyphicwriting,somewhatlikecursive writingisrelatedtoprintedwritingtoday.
THOMASYOUNGPOLYMATH Toconfirmhishunch,Youngturnedtothegroupsofcharactersinthehieroglyphicportionsofthestonetextcontainedin a cartouche.Acartoucheisagroupofcharactersseparatedfrom therestofthetext,enclosedbyanoblongovalthatresembled powdercartridges,knownascartouches,carriedbytheFrench soldiersoftheEgyptianExpedition.TheCopticversionofthe RosettatextreferredtoPtolemyinseveralplaces,andYoung locatedthesinglesurvivingcartoucheinthehieroglyphicsection thatwaslikelytocarrythatname.Thecorrespondencewasnot one-to-one,butYoungwasabletoisolateseveralsymbolsin commonbetweenthedemoticandthehieroglyphic.Bylooking forfurthercorrespondencesamongthetexts(seeFigure. 1.1 fora comparison),Youngcorrectlyidentifiedthewritingdirectionas right-to-left.Becausedemoticwaslargelyaphoneticscript,with charactersrepresentingsounds,Younghaddiscoveredthatmany Egyptianhieroglyphswerephonographicinnature,shattering theromanticnotionofthemagicsymbols.
Hieroglyphs
Figure1.1 Hieroglyphs comparedtohieratic, demotic,andCoptic scripts.TheCoptic scriptwasaformof Greek.
ThesediscoveriesbyYoung—thatmanydemoticcharacters werea“cursive”formofhieroglyphic,andthatmanyhieroglyphic characterswerephonetic—allowedhimtocreatetranslationsof severalpropernamesinthehieroglyphictext,andtoidentify thephoneticvaluesofseveralofthehieroglyphiccharacters. ThiswasthegreatestinroadyetmadeintodecipheringEgyptian hieroglyphics.Youngbecamesemi-famousforthisfeat,hisname recognizedinwidecircles.Hewasaskedtowritethe1818entry onEgyptforthe EncyclopediaBritannica,anarticlethatcontained manyofhisinsightsanddiscoveriesinthefield.
HissuccessinthefieldofEgyptologywasthelatestofastring ofsuccessesthatmarkedhissingularlyperipateticcareer.Hewas, afterall,merelyacitydoctorwhotooksummersofffromhispracticetopursueahostofunrelatedactivities,suchasdirectingthe BureauofLongitudeasanexpertinnavigation,andconsulting fortheBritishAdmiraltyasanexpertontheconstructionofships. Yetallofthesesignificantactivitiespaledinrelationtotheworkhe hadaccomplishedattheverybeginningofhiscareer.Longbefore Youngbecameapracticingdoctorwithtoomanysidelines,hewas oneofEngland’sleadingphysicists,hisinterestinthesubjectarisingfromhismedicalstudiesoftheeyeandearwhichhadledhim tostudythephysicsoflightandsound.
Young’supbringinghadbeenstrictbutnotpoor,bornto QuakerparentsinthewestofEnglandneartheBristolChannel thatsnakesitswayouttotheCelticSeabetweenCornwalland Wales.Althoughhisschoolingwastypicaloftheperiod,hewas fundamentallyanautodidact,teachinghimselftoreadbytheage oftwoandcompletingathoroughreadingoftheBiblebytheage ofsix.Asaschoolboy,heimmersedhimselfinstudiesofnatural historyandnaturalphilosophyaswellaslanguages,reading
Newton’s Principia and Opticks,Lavoisier’s ElementsofChemistry, JosephBlack’slecturesonchemistry,andBoerhaave’s Methoclus studiimedici. 7 HeindependentlystudiedHebrew,Chaldean,Syriac, Samaritan,Arabic,Persian,Turkish,andEthiopicinadditionto theusuallanguageslikeLatinandGreek.
Young’sfirstpublicationwasin1793whenhewas20yearsold inhisfirstyearofmedicalschoolattheUniversityinLondon.Part oftheinstructioninvolvedtheroutinedissectionoftheeyeof anox.Youngturnedtheroutineexerciseintoaground-breaking medicalexperimentinwhichheidentifiedthemechanismby whichtheeyefocusesatdifferentdistances(aprocesscalled accommodation)byisolatingthekeyroleofthecrystallinelens andthemusclesthatchangeitscurvature.AspartofYoung’s studies,hepainfullypokedthinkeysintohisowneyestomeasure theircurvaturesanddiameters.Healsodiscoveredtheoriginof astigmatism,causedbytheasymmetricalshapeofsomecorneas. Thepaperhepublishedinthe PhilosophicalTransactions oftheRoyal Societywassowellreceivedthathewaselectedamemberofthe RoyalSocietythenextyear.
Despitehistalentsandsuccess,hewasaQuaker,whichwasnot areligionsanctionedbythestate,preventinghimfromearning amedicaldegreefromanEnglishuniversity.Unwillingjustyet togiveuphisfamily’sreligion,heenrolledinthemedicalschool atEdinburghinScotland,transferringoneyearlatertothemedicalschoolattheUniversityinGöttingeninGermanywherehe receivedhisMDin1796.WhenhereturnedfromGermanyin1797, YoungwasstillrequiredtoobtainadegreeatanEnglishuniversity beforebeingallowedtopracticemedicineinEngland.Bythistime hisdedicationtohisQuakerupbringinghadconvenientlyfaded, soheconvertedtotheChurchofEnglandandenteredEmmanuel
CollegeatCambridgewherehemadecontactwithmanytop namesinlearnedsociety,includingEdmundBurkeandWilliam Herschel.
Young’sstudiesatCambridgereignitedhisearlierinterestsin physiologicalperception,especiallyregardinghowtheeyesees andtheearhears.Eitherthroughhisgeniusformakingconnectionsamongseeminglydisparatetopics,orthroughhisinability tokeephisattentionononesubject,hisphysiologicalstudiesled himintophysicalacousticsandthesoundsoforganpipes.He investigatedtherelationshipbetweenthelengthofthepipeand thewavelengthofthestandingpressurewave,payingparticular attentiontotheroleplayedbyinternalreflectionsinestablishing theresonanceinthetube.Ashiswide-rangingmindlookedat thepatternofrowsoforganpipes,arrayedinincreasinglengths highonthewallsofachurch,hemayhaverememberedseeing asimilarpattern,onanunrelatedsubject,fromhisearlydays readingNewton’s Optiks onthesubjectofcoloredfilms.When herereadthe Optiks hemusthavebeenstruckbythestrong similaritiesbetweentheperiodicpatternsofcolorsoflightinthin films(Newton’srings)andtheperiodicpatternsoforganpipes resonatingwithsound.
Thiswasnotthefirsttimesomeonewithinsighthadseenthis similarity.LeonhardEuler(1707–1783)hadmadethisanalogy nearly50yearsbefore,8 thinkingofthecolorsinthinsheets ofglasslikevibratingstringscomposedofoscillatingparticles ofether.Ittookonlysimplearithmetictorelatethethicknessof thefilmtothewavelengthofthelight,andEulerhadproposed thatthedifferentcolorsoflightarosefromdifferentwavelengths ofthevibratingether.9 However,Euler’sopinionwasaminority viewatthetime,10 asNewton’semissionisttheoryofparticles
oflightheldswayovernearlyallnaturalphilosophers.InNewton’stheory,thecolorsoffilmswereattributedtoperiodic“fits” inthematerial—theemissiontheoryadmittednovibratory explanation.
Youngsawitdifferently.Ashecametounderstandthepartial reflectionofsoundwavesfromtheopenendofanorganpipe thatsetupastandingwaveinside,itmadehimlookatthethin glasssheetandthinkofpartialreflectionsoflightattheinterface betweentheairandthetransparentmedium.Inthisview,the thinsheetwasnotresonatinginthewayEulerhadthought,as avibratingstringwherethephysicalmaterialwasoscillating, ratherthepartialreflectionoflightfromeachinterfacecaused astandingwaveoflighttooccurinsidethethinfilm,justlike thestandingwavesofsoundintheorganpipes.Youngmust havebeencaptivatedbythiscloseanalogy,inparticularwiththis insightintotheimportantroleofpartialreflection,andhewrote upadetailed“Outline”thatdocumentedhisideas,includingcalculationsontherelationshipbetweencolorandwavelength.This wasadefiningmomentforYoung,whenhecommittedhimself tothewavenatureoflightdespitetheoverwhelmingmajority opinionagainstit.
AftergraduatingfromCambridgein1800,Youngmovedto Londonwiththehopesofestablishingamedicalpractice,having inheritedthehouseofarichuncle.Unfortunately,Youngwas sociallyawkward,andhismedicalpracticeneversucceededto thesamedegreethathisotherendeavorshad.Hewasatheart agentlemanscholar,andtheinheritancefromhisuncle,ifnot immense,wasatleastlargeenoughtoallowYoungtofollow hismanyinterestscomfortably.OnceinLondon,heattended meetingsoftheRoyalSocietyandsoonwasacclaimedasone
oftheleadingfiguresinthephysiologyofperception.Heread apaper“SoundandLight”totheRoyalSocietyin1800andwas selectedasthatyear’swinneroftheSociety’sBakerianLecture,11 deliveringhisspeech“OntheMechanismoftheEye.”
Buoyedbythereceptionofhisresearchesintothephysiological aspectsoflight,Youngembarkedonamorefocusedtreatment ofthepartialreflectionoflightandthegenerationofstanding waves.Heenvisionedlightasperiodicpatternsthatmovedback andforthbetweeninterfaces,addingtogetherinacomplicated waythatcausedbrightanddimregionsinsidethethinfilm, liketheformationofmoirépatternsseenbyoverlayingtwofine cloths.Hewasstillgropinginthedarkontheactualnatureof thelightwaves,notfullyunderstandinghowthepartialwaves wouldsuperpose,butthiswasaglimpseofafirstcrudeprinciple ofinterference.
YoungwasselectedagainfortheBakerianLectureof1801,this timegivinghislecture“OntheTheoryofLightandColours.” Younghadexpandedhisearlierphysiologicalstudiesintocolor perceptionintheeyeandproposedthattheeyesawtherainbow ofperceivedcolorsusingonlythreecolorreceptors:red,green, andviolet.Forinstance,yellowcouldbeperceivedasthemixture ofredandgreenlight,andblueasthemixtureofgreenandviolet. Hisgraspofthetricolortheoryofcolorperceptionwashelped inpartbyhisacquaintancewithJohnDalton,whowasblind tothecolorredandwhodescribedaperceptionofcolorsthat differedfromwhatmostotherssaw.Youngperformedsimple demonstrationsthatleftlittledoubtaboutthegeneralvalidityof coloradditivity.Hethentookastepfurthertoexplainthatthe colorreceptorsintheeyevibratedinresonancewhendrivenby thedifferentfrequenciesoflight.Usingestimatesofthethickness
offilmsandcoloredfringes,Youngestimatedthewavelengthof light,which,whencombinedwiththespeedoflight,yieldeda temporalfrequencythatcausedthereceptorsintheeyetovibrate.
Young’ssecondBakerianLectureinNovemberof1801waspivotalfortworeasons.First,heopenlysupportedthewavenature oflightandproposedafirstcrudeprincipleofinterference.This aspectofhislecturewasconsideredintriguingbuttoospeculativetobeaccepted.Italsooccasionedthefirstpointedattackson Youngbyotherswhofirmlyheldtotheemissiontheoryoflight.
Second,itestablishedYoungastheleadingscientistinBritainin thebranchofnaturalphilosophyconcernedwithphysiological perceptionandmoregenerallywiththephysicsofsoundand light.Thisputhiminanexcellentpositiontobeconsideredfora positionaslecturerattheRoyalInstitution,newlyfoundedbythe ex-AmericanBenjaminThompson,aRoyalLoyalistwhospied againsttheAmericansintheRevolution,escapedtoLondon,and laterbecameaBavariancountknownasCountRumford.
BenjaminThompsonwasbornonapoorfarminasmalltown nearBoston.Despitenoformalschooling,hewasintelligentand anautodidact,likeYoung,andneverpassedupanopportunityfor advancement.IntheyearsprecedingtheDeclarationofIndependence,ThompsonjoinedaBritishLoyalistbrigadeandhelpedspy onAmerican“agitators”tohisBritishsuperiors.However,after theAmericanvictoryatBunkerHillhewasforcedtoevacuate withHowe’stroopsfromBoston,fleeingtoEngland,wherehe wasappointedtosuccessivelyhigherpositionsofauthorityover colonialaffairsuntiltheendofthewar.12 Alwaysrestlessand lookingforopportunities,hewasrecruitedasamilitaryadvisor toBavariafor16years,restructuringitsmilitary,improvingpublic works,andacquiringapeeragewiththetitleofCount.Because
hewasamanwithoutacountry,withaflairfortheironic,he tookthenameofRumford,theoldernameofConcord,New Hampshire,wherehehadlivedbeforethewar.
AsCountRumfordwasoverseeingtheboringofcannonin Munich,henoticedhowhotthecannonbarrelsbecame,caused bythefrictionfromthebore,insomecasesbecomingredhot. Theincessantmotionoftheborewasconvertedintoheat,contradictingtheprevailingtheorythatheatwasasubstance,and Rumfordrealizedthatheatmustbeaformofmotion.Hewrote uphistreatise“AnExperimentalEnquiryConcerningtheSource oftheHeatwhichisExcitedbyFriction”13 publishedinthe PhilosophicalTransactions in1798.Itwasoneofthefirstseriousattacks againstthecaloric(particulate)theoryofheat.Thispublication ishailedasaseminalstepinthedirectionoftheconservationof energy,whichwoulddevelopoverthenext50years.
ReturningtoEnglandshortlybeforeYoungmovedtoLondon, RumfordhelpedfoundtheRoyalInstitutionin1799witharoyal chartertoadvancepracticalapplicationsofscienceandtoserveas aconduittodisseminatethenewknowledgetothewiderpublic. PubliclecturesbecamethecornerstoneoftheRoyalInstitution, andhehiredHumphreyDavyasthefirstlecturer,whosubsequentlyhiredMichaelFaradaytobehisassistant.Inaddition topopularlectures,theInstitutionalsoprovidedmorespecializedseminars.In1801,JosephBanks,thepresidentoftheRoyal Society,wasimpressedbythebreadthofYoung’sknowledge, especiallyhismasteryofallbranchesofnaturalphilosophy,and BankssuggestedYoungasacandidatetoRumfordasalecturerin naturalphilosophy.YoungmadeafavorableimpressiononRumfordwhentheymet,andonAugust3,1801Youngwasselected as“ProfessorofNaturalPhilosophy,EditoroftheJournals,and
