諾貝爾獎(jiǎng)凸顯神經(jīng)網(wǎng)絡(luò)的物理學(xué)根源 Nobel Prize highlights neural networksphysics roots

NobelPrizehighlightsneuralnetworks’physicsroots罡

Theroadtothemodernmachine-learningmarvelswaspavedwithideasfromstatisticalmechanicsandcollectivephenomena.

JohannaL.Miller

PhysicsToday77(12),12–16(2024);

/10.1063/pt.qjmx.snxw

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NobelPrizehighlightsneuralnetworks’

physicsroots

Theroadtothemodern

MATTRASPANTI/PRINCETONUNIVERSITY

JOHNNYGUATTO/UNIVERSITYOFTORONTO

machine-learningmarvelswaspavedwithideasfromstatisticalmechanicsandcollectivephenomena.

“G

arbagein,garbageout.”According

totheoldadagefromcomputersci-

JohnHop?eld

Geo?reyHinton

onthoseideastodevelopthealgorithmsusedbyneural-networkmodelstoday.

Glassymemory

Itwasfarfromobvious,atfirst,thatneuralnetworkswouldevergrowtobesopowerful.Asrecentlyas2011,theflashiestmilestonesinAIwerebeingachievedbyanotherapproachentirely.IBMWatson,thecomputerthatbeatKenJenningsandBradRutteratJeop-ardy!,wasnotaneuralnetwork:Itwasexplicitlyprogrammedwithrulesforlanguageprocessing,informationre-trieval,andlogicalreasoning.AndmanyresearchersthoughtthatwasthewaytogotocreatepracticalAImachines.

Incontrast,theearlyworkonneuralnetworkswascuriosity-drivenresearch,inspiredmorebyrealbrainsthanbycomputersandtheirapplications.Butthenatureoftheinterdisciplinaryconnectionwassubtle.“ThequestionsHopfieldad-dressedarenotunrelatedtothingsneuro-scientistswereworriedabout,”saysPrinceton’sWilliamBialek.“Butthisisn’tabout‘a(chǎn)pplicationofphysicstoX’;rather,it’saboutintroducingawholepointofviewthatjustdidn’texistbefore.”

Bythe1980s,neuroscientistshadknownfordecadesthatthebrainiscom-posedofneurons,whichareconnectedtooneanotherviasynapsesandalter-natebetweenperiodsofhighandlowelectricalactivity(colloquially,“firing”and“notfiring”),andtheywerestudy-ingsystemsofafewneuronstounder-standhowoneneuron’sfiringaffected

thoseitwasconnectedto.“Somethoughtofneuronsintermsoflogicgates,likeinelectronics,”saysStanfordUniversity’sJayMcClelland.

Inalandmark1982paper,Hopfieldtookadifferentapproach.1Inphysics,heargued,manyimportantpropertiesoflarge-scalesystemsareindependentofsmall-scaledetails.Allmaterialscon-ductsoundwaves,forexample,irrespec-tiveofexactlyhowtheiratomsormole-culesinteract.Microscopicforcesmightaffectthespeedofsoundorotheracous-ticproperties,butstudyingtheforcesamongthreeorfouratomsrevealslittleabouthowtheconceptofsoundwavesemergesinthefirstplace.

Sohewrotedownamodelofanet-workofneurons,withaneyemoreto-wardcomputationalandmathematicalsimplicitythanneurobiologicalrealism.Themodel,nowknownasaHopfieldnetwork,issketchedinfigure1.(Thefig-ureshowsafive-neuronnetworkforeaseofillustration;Hopfieldwassimulatingnetworksof30to100neurons.)Eachneuroncanbeinstate1,forfiring,orstate0,fornotfiring.Andeachneuronwasconnectedtoalltheothersviacouplingconstantsthatcouldhaveanypositiveornegativevalue,dependingonwhethereachsynapsefavorsordisfavorstheneu-ronstobothbefiringatthesametime.

That’sexactlythesameformasaspinglass,afamouslythornysystemfromcondensed-matterphysics.(SeePhysicsToday,December2021,page17.)Unlikeaferromagnet,inwhichthecouplingsareall

ence,whatyougetfromacomputerisnobetterthanwhatyougiveit.Anditwouldseemtoimplythatbecausecom-puterscan’tthinkforthemselves,theycanneverdoanythingmoresophisti-catedthanwhatthey’vebeenexplicitlyinstructedto.

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Butthatlastpartappearstobenolongertrue.Neuralnetworks—computingarchitectures,inspiredbythehumanbrain,inwhichsignalsarepassedamongnodescalledartificialneurons—have,inrecentyears,beenproducingwaveafterwaveofstunningresults.(See,forexam-ple,page17ofthisissue.)Individualartificialneuronsperformonlythemostelementaryofcomputations.Butwhenbroughttogetherinlargeenoughnum-bers,andwhenfedonenoughtrainingdata,theyacquirecapabilitiesuncannilyreminiscentofhumanintelligence,seem-inglyoutofnowhere.

Physicistsarenostrangerstotheideaofunexpectedphenomenaemergingfromsimplerbuildingblocks.Afewel-ementaryparticlesandtherulesoftheirinteractionscombinetoyieldalmostthewholeofthevisibleworld:super-conductors,plasmas,andeverythinginbetween.Whyshouldn’taphysicsap-proachtoemergentcomplexitybeap-pliedtoneuralnetworkstoo?

Indeed,itwas—andstillis—asshow-casedbythisyear’sNobelPrizeinPhys-ics,whichgoestoPrincetonUniversity’sJohnHopfieldandtheUniversityofTo-ronto’sGeoffreyHinton.Beginningintheearly1980s,Hopfieldlaidthecon-ceptualfoundationsforphysics-basedthinkingaboutbrain-inspiredinforma-tionprocessing;Hintonwasatthefore-frontofthedecades-longefforttobuild

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FIGURE1.AHOPFIELDNETWORK,formallyequivalenttoaspinglass,functionsasanassociativememory:Whenpresentedwithapartiallyrecalledstate,itusesanenergy-loweringalgorithmtofillinthegaps.Thememoriesarestoredinthestrengthsofthe

connectionsamongthenodes.WhenJohnHopfieldshowedthatwiththerightcombinationofconnectionweights,thenetworkcouldstoremanymemoriessimultaneously,hesetthestageforphysics-basedthinkingaboutneuralnetworks.(FigurebyFreddiePagani;rabbitphotobyJMLigeroLoarte/WikimediaCommons/CCBY3.0.)

positiveandthesystemhasacleargroundstatewithallitsspinsaligned,aspinglassalmostalwayslacksastatethatsatisfiesallitsspins’energeticpreferencessimul-taneously.Itsenergylandscapeiscom-plex,withmanylocalenergyminima.

Hopfieldarguedthatthelandscapecouldserveasamemory,witheachoftheenergy-minimizingconfigurationsservingasastatetoberemembered.Andhepresentedanelegantwayofset-tingtheconnectionstrengths—inspiredbywhathappensatrealsynapses—sothatthememorywouldstoreanyde-siredcollectionofstates.

ButtheHopfieldnetworkisfunda-mentallydifferentfromanordinarycom-putermemory.Inacomputer,eachitemofdatatobestoredisencodedasastringofonesandzerosinaspecificplace,andit’srecalledbygoingbacktothatplaceandreadingoutthestring.InaHopfieldnetwork,alltheitemsarestoredsimulta-neouslyinthecouplingstrengthsofthewholenetwork.Andtheycanberecalledassociatively,bygivingthenetworkastartingpointthatsharesjustafewfea-tureswithoneoftherememberedstatesandallowingittorelaxtothenearestenergyminimum.Moreoftenthannot,itwillrecallthedesiredmemory.(SeealsothearticlesbyHaimSompolinsky,PhysicsToday,December1988,page70,andJohnHopfield,PhysicsToday,Feb-ruary1994,page40.)

Thoseareboththingsthathappeninrealbrains.“Itwasknownexperimen-tallyinhigheranimalsthatbrainactivitywaswellspreadout,anditinvolved

manyneurons,”saysHopfield.Andas-sociativememoryissomethingyou’vedirectlyexperiencedifyou’veeverre-calledasongyou’veheardbeforeafterhearingonerandomline.

Hopfield’smodelwasavastsimplifica-tionofarealbrain.Realneuronsarein-trinsicallydynamic,notcharacterizedbystaticstates,andrealneuronconnectionsarenotsymmetric.Butinaway,thosedifferenceswerefeatures,notbugs:They

showedthatcollective,associativemem-orywasanemergentlarge-scalephenom-enon,robustagainstsmall-scaledetails.

Learninghowtolearn

“NotonlyisHopfieldaverygoodphysi-cist,buttheHopfieldmodelisexcellentphysicsbyitself,”saysLeovanHemmen,oftheTechnicalUniversityofMunich.Still,its1982formulationleftmanyin-triguingopenquestions.Hopfieldhadfocusedonsimulationstoshowhowthesystemrelaxestoanenergyminimum;wouldthemodeladmitamorerobustanalyticaltreatment?Howmanystatescouldthemodelremember,andwhatwouldhappenifitwasoverloaded?Weretherebetterwaysofsettingthecon-nectionstrengthsthantheoneHopfieldproposed?

Thosequestions,andothers,weretakenonbyaflurryofphysics-trainedresearcherswhowereinspiredbyHopfield’sworkandenteredtheneural-networkfieldoverthe1980s.“Physicistsareversatile,curious,andarrogant—inapositiveway,”saysEytanDomany,oftheWeizmannInstituteofScienceinIsrael.

“They’rewillingtostudythoroughlyandthentackleaproblemthey’veneverseenbefore,ifit’sinteresting.Andeveryoneisexcitedaboutunderstandingthebrain.”

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AnotherpartoftheappealwasinhowHopfieldhadtakenatraditionalphysicsproblemandturneditonitshead.“Inmostenergy-landscapeprob-lems,you’regiventhemicroscopicinter-actions,andyouask,Whatisthegroundstate?Whatarethelocalminima?Whatistheentirelandscape?”saysHaimSompolinsky,oftheHebrewUniversityofJerusalem.“The1982paperdidtheopposite.Westartwiththegroundstatesthatwewant:thememories.Andweask,Whatarethemicroscopicinteractionsthatwillsupportthoseasgroundstates?”

Fromthere,itwasashortconceptualleaptoask,Whatifthecouplingstrengthsthemselvescanevolveontheirownen-ergylandscape?Thatis,insteadofbeingpreprogrammedwithparameterstoen-codespecificmemories,canthesystemimproveitselfbylearning?

Machinelearninginneuralnetworkshadbeentriedbefore.Theperceptron—aneural-network-likedevicethatsortedim-agesintosimplecategories,suchascirclesandsquares—datesbacktothe1950s.Whenprovidedwithaseriesoftrainingimagesandasimplealgorithmforupdat-ingitsconnectionsbetweenneurons,itcouldeventuallylearntocorrectlyclassifyevenimagesithadn’tseenbefore.

Buttheperceptrondidn’talwayswork:Withthewaythenetworkwasstructured,sometimestherewasn’tanywayofsettingtheconnectionstrengths

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FIGURE2.ABOLTZMANNMACHINEextendstheHopfieldnetworkintwoways:Itaugmentsthenetworktoincludehidden

nodes(showninthecenterofthenetworkingray)thataren’tinvolvedinencodingthedata,anditoperatesatanonzeroeffectivetemperature,sothattheentirespaceofconfigurationscanbecharacterizedbyaBoltzmannprobabilitydistribution.Geoffrey

HintonandcolleaguesdevelopedawaytotraintheBoltzmannmachineasagenerativemodel:Whenpresentedwithseveralinputsthatallsharedacommonfeature,itproducedmoreitemsofthesametype.(FigurebyFreddiePagani.)

toperformthedesiredclassification.“Whenthathappened,youcoulditer-ateforever,andthealgorithmwouldneverconverge,”saysvanHemmen.“Thatwasabigshock.”Withoutaguid-ingprincipletochartapathforward,thefieldhadstalled.

Findingcommonground

Hintondidn’tcometoneuralnetworksfromabackgroundinphysics.ButhiscollaboratorTerrenceSejnowski—who’dearnedhisPhDunderHopfieldin1978—did.Together,theyextendedtheHop-fieldnetworkintosomethingtheycalledtheBoltzmannmachine,whichvastlyextendedthemodel’scapabilitiesbyex-plicitlydrawingonconceptsfromstatis-ticalphysics.2

InHopfield’s1982simulations,he’deffectivelyconsideredthespin-glassnet-workatzerotemperature:Heallowedthesystemtoevolveitsstateonlyinwaysthatwouldloweritsoverallenergy.Sowhateverthestartingstate,itrolledintoanearbylocalenergyminimumandstayedthere.

“TerryandIimmediatelystartedthinkingaboutthestochasticversion,withnonzerotemperature,”saysHinton.In-steadofadeterministicenergy-loweringrule,theyusedaMonteCarloalgorithmthatallowedthesystemtooccasionallyjumpintoastateofhigherenergy.Givenenoughtime,astochasticsimulationofthenetworkwouldexploretheentireen-ergylandscape,anditwouldsettleintoaBoltzmannprobabilitydistribution,withallthelow-energystates—regardlessof

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whetherthey’relocalenergyminima—representedwithhighprobability.

“Andin1983,wediscoveredareallybeautifulwaytodolearning,”Hintonsays.Whenthenetworkwassuppliedwithtrainingdata,theyiterativelyup-datedtheconnectionstrengthssothatthedatastateshadhighprobabilityintheBoltzmanndistribution.3Moreover,whentheinputdatahadsomethingincommon—liketheimagesofthenu-meral3infigure2—thenotherhigh-probabilitystateswouldsharethesamecommonfeatures.

Thekeyingredientforthatkindofcommonalityfindingwasaugmentingthenetworktoincludemorenodesthanjusttheonesthatencodethedata.Thosehiddennodes,representedingrayinfigure2,allowthesystemtocapturehigher-levelcorrelationsamongthedata.

Inprinciple,theBoltzmannmachinecouldbeusedformachinerecognitionofhandwritingorfordistinguishingnormalfromemergencyconditionsinafacilitysuchasapowerplant.Unfortu-nately,theBoltzmannmachine’slearn-ingalgorithmisprohibitivelyslowformostpracticalapplications.Itremainedatopicofacademicresearch,butitdidn’tfindmuchreal-worlduse—untilitmadeasurprisingreappearanceyearslater.

Howthenetworkswork

Aroundthesametime,HintonwasworkingwithcognitivescientistDavidRumelhartonanotherlearningalgo-rithm,whichwouldbecomethesecretsauceofalmostalloftoday’sneural

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networks:backpropagation.4Thealgo-rithmwasdevelopedforadifferentkindofnetworkarchitecture,calledafeed-forwardnetwork,showninfigure3.IncontrasttotheHopfieldnetworkandBoltzmannmachine,withtheirbidirec-tionalconnectionsamongnodes,signalsinafeedforwardnetworkflowinonedirectiononly:fromalayerofinputneu-rons,throughsomenumberofhiddenlayers,totheoutput.Asimilararchitec-turehadbeenusedinthemultilayerperceptron.

Supposeyouwanttotrainafeed-forwardnetworktoclassifyimages.Yougiveitapictureofarabbit,andyouwantittoproducetheoutputmessage“Thisisarabbit.”Butsomethingiswrong,andinsteadyougettheoutput“Thisisaturtle.”Howdoyougetthingsbackontrack?Thenetworkmighthavedozensorhundreds—ortoday,trillions—ofinter-nodeconnectionsthatcontributetotheoutput,eachwithitsownnumericalweight.There’sadizzyingnumberofwaystoadjustthemalltotrytogettheoutputyouwant.

Backpropagationsolvesthatproblemthroughgradientdescent:First,youde-fineanerrorfunctionthatquantifieshowfartheoutputyougotisfromtheoutputyouwant.Then,calculatethepartialde-rivativesoftheerrorfunctionwithre-specttoeachoftheinternodalweights—asimplematterofrepeatedlyapplyingcalculus’schainrule.Finally,usethosederivativestoadjusttheweightsinawaythatdecreasestheerror.

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FIGURE3.AFEEDFORWARDNETWORK,trainedbybackpropagation,isthebasicstructureoftheneuralnetworksusedtoday.

Bypassingnumericalsignalsfromaninputlayerthroughhiddenlayerstoanoutputlayer,feedforwardnetworksperformfunctionsthatincludeimageclassificationandtextgeneration.(FigurebyFreddiePagani;rabbitphotobyJMLigeroLoarte/Wikimedia

Commons/CCBY3.0;haikugeneratedbyGPT-4,OpenAI,22October2024.)

theerrorcloseenoughtozero—andyou’llwanttomakesurethatthenetworkgivestherightoutputformanyinputs,notjustone.Butthosebasicstepsareusedtotrainallkindsofnetworks,includingproof-of-conceptimageclassifiersandlargelan-guagemodels,suchasChatGPT.

Gradientdescentisintuitivelyele-gant,anditwasn’tconceptuallynew.“Butseveralelementshadtocometo-gethertogetthebackpropagationideatowork,”saysMcClelland.“Foronething,youcan’ttakethederivativeofsome-thingifit’snotdifferentiable.”Realneu-ronsoperatemoreorlessindiscreteonandoffstates,andtheoriginalHopfieldnetwork,Boltzmannmachine,andper-ceptronwerealldiscretemodels.Forbackpropagationtowork,itwasneces-sarytoshifttoamodelinwhichthenodestatescantakeacontinuumofvalues.Butthosecontinuous-valuednetworkshadalreadybeenintroduced,includingina1984paperbyHopfield.5

Asecondinnovationhadtowaitforlonger.Backpropagationworkedwellfornetworkswithjustacoupleoflayers.Butwhenthelayercountapproachedfiveormore—atriflingnumberbyto-day’sstandards—someofthepartialde-rivativesweresosmallthatthetrainingtookanimpracticallylongtime.

Intheearly2000s,Hintonfoundasolution,anditinvolvedhisoldBoltz-mannmachine—orrather,aso-calledrestrictedversionofit,inwhichtheonlyconnectionsarethosebetweenonehid-denneuronandonevisible(non-hidden)neuron.6RestrictedBoltzmannmachines(RBMs)areeasytocomputationally

model,becauseeachgroupofneurons—visibleandhidden—couldbeupdatedallatonce,andtheconnectionweightscouldallbeadjustedtogetherinasinglestep.Hinton’sideawastoisolatepairsofsuccessivelayersinafeedforwardnetwork,trainthemasiftheywereRBMstogettheweightsapproximatelyright,andthenfine-tunethewholenetworkusingbackpropagation.

“Itwaskindofahackything,butitworked,andpeoplegotveryexcited,”saysGrahamTaylor,oftheUniversityofGuelphinCanada,whoearnedhisPhDunderHintonin2009.“Itwasnowpos-sibletotrainnetworkswithfive,six,sevenlayers.Peoplecalledthem‘deep’networks,andtheystartedusingtheterm‘deeplearning.’”

TheRBMhackwasn’tusedforlong.Computingpowerwasadvancingsoquickly—particularlywiththerealizationthatgraphicsprocessingunits(GPUs)wereideallysuitedtothecomputationsneededforneuralnetworks—thatwithinafewyears,itwaspossibletodoback-propagationonevenlargernetworksfromacoldstart,withnoRBMsrequired. “IfRBMlearninghadn’thappened,wouldGPUshavecomealonganyway?”asksTaylor.“That’sarguable.Buttheex-citementaroundRBMschangedtheland-scape:Itledtotherecruitmentandtrain-ingofnewstudentsandtonewwaysofthinking.Ithinkattheveryleast,itwouldn’thavehappenedthesameway.”

What’snewisold

Today’snetworksusehundredsorthou-sandsoflayers,buttheirformislittle

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changedfromwhatHintondescribed.“Ilearnedaboutneuralnetworksfrombooksfromthe1980s,”saysBernhardMehlig,oftheUniversityofGothenburginSweden.“WhenIstartedteachingit,Irealizedthatnotmuchisnew.It’sessen-tiallytheoldstuff.”Mehlignotesthatinatextbookhewrote,publishedin2021,part1of3isaboutHopfield,andpart2isaboutHinton.

Neuralnetworksnowinfluenceavastnumberofhumanendeavors:They’reinvolvedindataanalysis,websearches,andcreatinggraphics.Aretheyintelli-gent?It’seasytodismissthequestionoutofhand.“Therehavealwaysbeenlotsofthingsthatmachinescandobetterthanhumans,”saystheUniversityofMaryland’sSankarDasSarma.“Thathasnothingtodowithbecominghuman.ChatGPTisfabulouslygoodatsomethings,butatmanyothers,it’snotevenasgoodasatwo-year-oldbaby.”

Anillustrativecomparisonisthevastdatagapbetweentoday’sneuralnet-worksandhumans.7Aliterate20-year-oldmayhavereadandheardafewhun-dredmillionwordsinlifesofar.Largelanguagemodels,incontrast,aretrainedonhundredsofbillionsofwords,anum-berthatgrowswitheachnewrelease.WhenyouaccountforthefactthatChatGPThastheadvantageofathousandtimesasmuchlifeexperienceasyoudo,itsabilitiesmayseemlesslikeintelli-gence.Butperhapsitdoesn’tmatterifAIfumbleswithsometasksifit’sgoodattherightcombinationofothers.

HintonandHopfieldhavebothspo-kenaboutthedangersofuncheckedAI.

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Amongtheirargumentsistheideathatoncemachinesbecomecapableofbreak-ingupgoalsintosubgoals,they’llquicklydeducethattheycanmakealmostanytaskeasierforthemselvesbyconsolidat-ingtheirownpower.Andbecauseneu-ralnetworksareoftentaskedwithwrit-ingcodeforothercomputers,stoppingthedamageisnotassimpleaspullingtheplugonasinglemachine.

“Therearealsoimminentrisksthatwe’refacingrightnow,”saysMehlig.“Therearecomputer-writtentextsandfakeimagesthatarebeingusedtotrickpeopleandinfluenceelections.Ithinkthatbytalkingaboutcomputerstakingovertheworld,peopletaketheimmi-nentdangerslessseriously.”

Whatcanphysicistsdo?

Muchoftheuneasestemsfromthefactthatsolittleisknownaboutwhatneu-ralnetworksarereallydoing:Howdobillionsofmatrixmultiplicationsadduptotheabilitytofindproteinstruc-turesorwritepoetry?“Peopleatthebigcompaniesaremoreinterestedinpro-ducingrevenue,notunderstanding,”saysDasSarma.“Understandingtakeslonger.Thejoboftheoristsistounder-standphenomena,andthisisahugephysicalphenomenon,waitingtobeunderstoodbyus.Physicistsshouldbeinterestedinthis.”

“It’shardnottobeexcitedbywhat’sgoingon,andit’shardnottonoticethatwedon’tunderstand,”saysBialek.“Ifyouwanttosaythatthingsareemergent,what’stheorderparameter,andwhatisitthat’semerged?Physicshasawayof

makingthatquestionmoreprecise.Willthatapproachyieldinsight?We’llsee.”

Fornow,thebiggestquestionsarestilloverwhelming.“Ifthereweresome-thingobviousthatcametomind,therewouldbeahordeofpeopletryingtosolveit,”saysHopfield.“Butthereisn’tahordeofpeopleworkingonthis,be-causenobodyknowswheretostart.”

Butafewsmaller-scalequestionsaremoretractable.Forexample,whydoesbackpropagationsoreliablyreducethenetworkerrortonearzero,ratherthangettingstuckinhigh-lyinglocalminimaliketheHopfieldnetworkdoes?“TherewasabeautifulpieceofworkonthisafewyearsagobySuryaGanguliatStan-ford,”saysSaraSolla,ofNorthwesternUniversity.“Hefoundthatmosthigh-lyingminimaarereallysaddlepoints:It’saminimuminmanydimensions,butthere’salwaysoneinwhichit’snot.Soifyoukeepkicking,youeventuallyfindyourwayout.”

Whenphysics-trained