P1:FJU FinalPages EncyclopediaofPhysicalScienceandTechnology EN-Index-Fm August30,2001 15:45 ADVISORY BOARD JohnG.Bollinger HerbertFriedman UniversityofWisconsin HulbertCenterforSpaceResearch,Washington,D.C. MichaelBuckland R.A.Mashelkar UniversityofCalifornia,Berkeley CouncilofScientificandIndustrialResearchofIndia JeanCarpentier KarlS.Pister OfficeNationald’EtudesetdeRecherechesAerospatiales,France UniversityofCalifornia,SantaCruz LudwigFaddeev GordonR.Slemon V.A.SteklovMathematicalInstitute,St.Petersburg,Russia CanadianAcademyofEngineering ii P1:FJU FinalPages EncyclopediaofPhysicalScienceandTechnology EN-Index-Fm August30,2001 15:45 EXECUTIVEADVISORYBOARD AllenJ.Bard YuanT.Lee UniversityofTexas AcademicaSinica EliasJ.Corey BenoitMandelbrot HarvardUniversity YaleUniversity PaulJ.Crutzen GeorgeOlah MaxPlanckInstituteforChemistry UniversityofSouthernCalifornia DanielS.Goldin MelvinSchwartz Washington,D.C. ColumbiaUniversity StephenHawking FrederickSeitz CambridgeUniversity RockefellerUniversity Klausv.Klitzing PhillipA.Sharp MaxPlanckInstituteforSolid-StatePhysics MassachusettsInstituteofTechnology AaronKlug EdwardTeller MedicalResearchCouncil,UK HooverInstitution,StanfordUniversity ii P1:FJU FinalPages EncyclopediaofPhysicalScienceandTechnology EN-Index-Fm August30,2001 15:45 Editor-in-Chief Robert A. Meyers, Ramtech, Inc. Tarzana, California, USA iii P1:FYX/FYX P2:FYX/UKS QC:FYX/UKS T1:FYX EncyclopediaofPhysicalScienceandTechnology org-fm May10,2001 19:57 SUBJECTAREAEDITORS RuzenaBajcsy BarrieA.Gregory GeorgeS.Philander UniversityofPennsylvania UniversityofBrighton PrincetonUniversity Robotics InstrumentationandSpecialApplications DynamicOceanography forElectronicsandElectricalEngineering PhysicalandChemicalOceanography AllenJ.Bard UniversityofTexas GordonHammes PeterH.Rose AnalyticalChemistry DukeUniversity OrionEquipment,Inc Biochemistry MaterialsScienceforElectricaland Wai-KaiChen ElectronicEngineering UniversityofIllinois FrederickHawthorne CircuitTheoryandCircuits UniversityofCalifornia,LosAngeles JeromeS.Schultz InorganicChemistry UniversityofPittsburgh JohnT.Christian Biotechnology Consultant,Waban,Massachusetts LeonardV.Interrante MiningEngineering RensselaerPolytechnicInstitute MelvinSchwartz Snow,Ice,andPermafrost MaterialsScienceandEngineering ColumbiaUniversity SoilMechanics Physics BruceH.Krogh RobertColeman CarnegieMellonUniversity PhilipA.Sharp U.S.GeologicalSurvey ControlTechnology MassachusettsInstituteofTechnology EarthScience MolecularBiology JosephKung Geochemistry Geomorphology UniversityofNorthTexas MartinShepperd GeologyandMineralogy Mathematics BournemouthUniversity ComputerSoftware EliasJ.Corey WilliamH.K.Lee HarvardUniversity U.S.GeologicalSurvey StevenShore OrganicChemistry Geodesy IndianaUniversity SolidEarthGeophysics Astronomy PaulJ.Crutzen TerrestrialMagnetism LeslieSmith MaxPlanckInstituteforChemistry YuanT.Lee AtmosphericScience UniversityofBritishColumbia AcademicaSinica HydrologyandLimnology GordonDay PhysicalChemistry SteinSture NationalInstituteofStandards Radio,Radiation,andPhotochemistry andTechnology UniversityofColorado LasersandMasers RobertW.Lenz Mechanical,Industrial,andCivil OpticalMaterials UniversityofMassachusetts Engineering PolymerChemistry JackDongarra RobertTrappl IrvinE.Liener UniversityofTennessee UniversityofVienna ComputerHardware UniversityofMinnesota SystemsTheoryandCybernetics AgriculturalandFoodChemistry GerardM.Faeth NicholasTsoulfanidis LarryB.Milstein UniversityofMichigan UniversityofMissouri Aeronautics UniversityofCalifornia,SanDiego NuclearTechnology Communications WilliamH.Glaze M.VanRossum DonaldR.Paul UniversityofNorthCarolina CatholicUniversityofLeuven EnvironmentalChemistry,Physics, UniversityofTexas Components,ElectronicDevices,and andTechnology AppliedChemistryandChemical Materials Engineering TomifumiGodai R.H.WoodwardWaesche S.S.Penner NationalSpaceDevelopmentAgencyofJapan ScienceApplicationsInternationalCorporation SpaceTechnology UniversityofCalifornia,SanDiego PropulsionandPropellants EnergyResources PowerSystems v P1:FYX/FYX P2:FYX/UKS QC:FYX/UKS T1:FYX EncyclopediaofPhysicalScienceandTechnology org-fm May10,2001 19:57 Foreword T he editors of the Encyclopedia of Physical Sci- Theinfluenceofthesteamengineonthedevelopmentof enceandTechnologyhadadauntingtask:tomake thermodynamics is the classic example. A more recent an accurate statement of the status of knowledge one would be the stimulus that the problem of noise in across the entire field of physical science and related communications channels gave to the study of informa- technologies. tiontheory. No such effort can do more than describe a rapidly As technology has developed, it has increasingly be- changingsubjectataparticularmomentintime,butthat cometheobjectofstudyitself,sothatnowmuchofsci- doesnotmaketheeffortanylessworthwhile.Changeis ence is focused on what we have made ourselves, rather inherentinscience;science,infact,seekschange.Because than only on the natural world. Thus, the very existence ofitsassociationwithchange,scienceisoverwhelmingly of the computer and computer programming made pos- the driving force behind the development of the modern sible the development of computer science and artificial world. intelligenceasscientificdisciplines. The common point of view is that the findings of ba- Thewholeprocessofinnovationinvolvesscience,tech- sicsciencemoveinalinearwaythroughappliedresearch nology, invention, economics, and social structures in andtechnologydevelopmenttoproduction.Inthismodel, complexways.Itisnotsimplyamatterofmovingideasout allthemovementisfromsciencetoproduct.Technology ofbasicresearchlaboratories,throughdevelopment,and depends on science and not the other way around. Sci- onto factory floors. Innovation not only requires a large ence itself is autonomous, undisturbed by technology or amountoftechnicalinvention,providedbyscientistsand anyothersocialforces,andonlythroughtechnologydoes engineers,butalsoarangeofnontechnicalor“social”in- scienceaffectsociety. ventionprovidedby,amongothers,economists,psychol- Thissuperficialviewisseriouslyinerror.Amoreaccu- ogists,marketingpeople,andfinancialexperts.Eachadds rateviewisthatmanycomplexconnectionsexistamong value to the process, and each depends on the others for science,engineering,technology,economics,theformof ideas. ourgovernmentandthenatureofourpolitics,andlitera- Beyond the processes of innovation and economic ture,andethics. growth, science has a range of direct effects on our Althoughadvancesinscienceclearlymakepossiblead- society. vances in technology, very often the movement is in the Scienceaffectsgovernmentandpolitics.TheU.S.Con- otherdirection:Advancesintechnologymakepossiblead- stitutionwasaproductofeighteenthcenturyrationalism vancesinscience.Thedependenceofradioastronomyand andowesmuchtoconceptsthatderivedfromthescience high-energyphysicsonprogressindetectortechnologyis ofthattime.Toaremarkableextent,theFoundingFathers agoodexample.Moresubtly,technologymaystimulate werefamiliarwithscience:Franklin,Jefferson,Madison, sciencebyposingnewquestionsandproblemsforstudy. and Adams understood science, believed passionately in ix P1:FYX/FYX P2:FYX/UKS QC:FYX/UKS T1:FYX EncyclopediaofPhysicalScienceandTechnology org-fm May10,2001 19:57 x Foreword empiricalinquiryasthesourceoftruth,andfeltthatgov- disease-made possible by the technology of the micro- ernmentshoulddrawonscientificconceptsforinspiration. scopedestroyedthenotionthatdiseasewassentbyGod Theconceptof“checksandbalances”wasborrowedfrom asajustretributionforunrepentantsinners. Newtonianphysics,anditwaswidelybelievedthat,like Scienceaffectsethics.Becausesciencehasalargepart theorderlyphysicaluniversethatsciencewasdiscovering, in creating our reality, it also has a significant effect on socialrelationsweresubjecttoaseriesofnaturallawsas ethics. Once the germ theory of disease was accepted, well. it could no longer be ethical, because it no longer made Sciencealsopervadesmoderngovernmentandpolitics. sense,toberatethesickfortheirsins.Gulliver’svoyage AlargepartoftheFederalgovernmentisconcernedeither tothelandoftheHouynhymsmadethepoint: withstimulatingresearchordevelopment,asaretheNa- By inventing a society in which individuals’ illnesses tionalAeronauticsandSpaceAdministration(NASA)and wereactsoffreewillwhiletheircrimeswerearesultof theNationalScienceFoundation,orwithseekingtoregu- outside forces, he made it ethical to punish the sick but latetechnologyinsomeway.Thereasonthatscienceand notthecriminal. technology have spawned so much government activity Knowledge—most of it created by science—creates is that they create new problems as they solve old ones. obligations to act that did not exist before. An engineer, This is true in the simple sense of the “side effects” of forinstance,whodesignsapieceofequipmentinaway newtechnologiesthatmustbemanagedandthusgiverise thatisdangerous,whenknowledgetosafelydesignitex- tosuchagenciesastheEnvironmentalProtectionAgency ists,hasviolatedbothanethicalandalegalprecept.Itis (EPA).Moreimportantly,however,theavailabilityofnew nodefensethattheengineerdidnotpersonallypossessthe technologiesmakespossiblechoicesthatdidnotexistbe- knowledge;thesimpleexistenceoftheknowledgecreates fore,andmanyofthesechoicescanonlybemadethrough theethicalrequirement. thepoliticalsystem. Inanothersense,sciencehasapositiveeffectonethics Biotechnologyisagoodexample.TheFederalgovern- by setting an example that may be followed outside sci- menthassupportedthebasicscienceunderlyingbiotech- ence.Sciencemustsettruthasthecardinalvalue,foroth- nologyformanyyears.Thatscienceisnowmakingpossi- erwiseitcannotprogress.Thus,whileindividualscientists blechoicesthatwereonceunimagined,andintheprocess maylapse,scienceasaninstitutionmustcontinuallyreaf- a large number of brand new political problems are be- firmthevalueoftruth.Tothatextentscienceservesasa ingcreated.Forexample,whatsafeguardsarenecessary moralexampleforotherareasofhumanendeavor. beforegeneticallyengineeredorganismsaretestedinthe Science affects art and literature. Art, poetry, litera- field? Should the Food and Drug Administration restrict ture, and religion stand on one side and science on the the development of a hormone that will stimulate cows othersideofC.P.Snow’sfamousgulfbetweenthe“two to produce more milk if the effect will be to put a large cultures.” The gulf is largely artificial, however; the two numberofdairyfarmersoutofbusiness?Howmuchrisk sides have more in common than we often realize. Both shouldbetakentodevelopmedicinesthatmaycuredis- scienceandthehumanitiesdependonimaginationandthe easesthatarenowuntreatable? useofmetaphor.Despitewidespreadbelieftothecontrary, These questions all have major technical content, but sciencedoesnotproceedbyarationalprocessofbuilding at bottom they involve values that can only be resolved theoriesfromundisputedfacts.Scientificandtechnolog- throughthepoliticalprocess. ical advances depend on imagination, on some intuitive, Scienceaffectsideas.Scienceisanimportantsourceof creative vision of how reality might be constructed. As ourmostbasicideasaboutreality,aboutthewaytheworld Peter Medawar puts it: [Medawar, P. (1969). Encounter isputtogetherandourplaceinit.Such“worldviews”are 32(l),15-23]:Alladvanceofscientificunderstanding,at criticallyimportant,forwestructureallourinstitutionsto everylevel,beginswithaspeculativeadventure,animagi- conformwiththem. nativepreconceptionofwhatmightbetrue—apreconcep- Inthemedievalworldview,theheavenswereunchang- tionthatalways,andnecessarily,goesalittleway(some- ing,existingforeverastheywereonthedayofcreation. times a long way) beyond anything that we have logical ThenTychoBraheobservedthe“newstar”—thenovaof orfactualauthoritytobelievein. 1572—and the inescapable fact of its existence forced a The difference between literature and science is that reconstructionofreality.Kepler,Galileo,andNewtonfol- inscienceimaginationiscontrolled,restricted,andtested lowedanddestroyedtheearth-andhuman-centereduni- byreason.Withinthestricturesofthediscipline,theartist verseofmedievalChristianity. or poet may give free rein to imagination. Although we Darwinestablishedthecontinuityofhumanandanimal, may critically compare a novel to life, in general, litera- thusunderminingbothourviewofourinnatesuperiority tureorartmaybejudgedwithoutreferencetoempirical andagoodbitofreligiousauthority.Thegermtheoryof truth.Scientists,however,mustsubjecttheirimaginative P1:FYX/FYX P2:FYX/UKS QC:FYX/UKS T1:FYX EncyclopediaofPhysicalScienceandTechnology org-fm May10,2001 19:57 Foreword xi constructiontoempiricaltest.Itisnotestablishedastruth comingawaytocreateknowledgethatisasrevolutionary untiltheyhavepersuadedtheirpeersthatthistestingpro- asexperimentationwas300yearsago. cesshasbeenadequate,andthetruththeycreateisalways Artificialintelligenceisonlyjustbeginning;itsgoalis tentativeandsubjecttorenewedchallenge. toduplicatetheprocessofthinkingwellenoughsothatthe The genius of science is that it takes imagination and distinctionbetweenhumansandmachinesisdiminished. reason,whichtheRomanticsandthemoderncountercul- Ifthiscanbeaccomplished,theconsequencesmaybeas turebothholdtobeantithetical,andcombinesthemina profoundasthoseofDarwin’stheoryofevolution,andthe synergisticway.Bothscienceandart,theopposingsidesof workingoutofthesocialimplicationscouldbeasdifficult. the“twocultures,”dependfundamentallyonthecreative With astonishing speed, modern biology is giving us use of imagination. Thus, it is not surprising that many the ability to genuinely understand, and then to change, mathematiciansandphysicistsarealsoaccomplishedmu- biologicalorganisms.Theimplicationsformedicineand sicians,orthatmusicmajorshaveoftenbeencreativecom- agriculture will be great and the results should be over- puterprogrammers. whelminglybeneficial. Science,technology,andcultureinthefuture.Wecan Theimplicationsforourviewofourselveswillalsobe onlyspeculateabouthowscienceandtechnologywillaf- great, but no one can foresee them. Knowledge of how fectsocietyinthefuture.Thetechnologiesmadepossible to change existing forms of life almost at will confers a byanunderstandingofmechanics,thermodynamics,en- fundamentallynewpoweronhumanbeings.Wewillhave ergy,andelectricityhavegivenusthetransportationrevo- tostretchourwisdomtobeabletodealintelligentlywith lutionsofthiscenturyandmadelargeamountsofenergy thatpower. availableforaccomplishingalmostanysortofphysicalla- One thing we can say with confidence: Alone among bor.Thesetechnologiesarenowmatureandwillcontinue all sectors of society and culture, science and technol- toevolveonlyslowly.Intheirplace,however,wehavethe ogy progress in a systematic way. Other sectors change, informationrevolutionandsoonwillhavethebiotechnol- but only science and technology progress in such a way ogy revolution. It is beyond us to say where these may thattoday’sscienceandtechnologycanbesaidtobeun- lead,buttheimplicationswillprobablybeasdramaticas ambiguously superior to that of an earlier age. Because thechangesofthepastcentury. scienceprogressesinsuchadramaticandclearway,itis Computers affected first the things we already do, by thedominantforceinmodernsociety. making easy what was once difficult. In science and en- gineering, computers are now well beyond that. We can ErichBloch now solve problems that were only recently impossible. NationalScienceFoundation Modeling, simulation, and computation are rapidly be- Washington,D.C. P1:FJUFinalPages EncyclopediaofPhysicalScienceandTechnology Vol.11-Fm-Bm July31,2001 15:18 Preface W earemostgratifiedtofindthatthefirstandsec- concurred with the idea of expanding our coverage into ond editions of the Encyclopedia of Physical molecular biology, biochemistry, and biotechnology in Science and Technology (1987 and 1992) are recognitionofthefactthatthesefieldsarebasedonphys- nowbeingusedinsome3,000librarieslocatedincenters ical sciences. Military technology such as weapons and of learning and research and development organizations defense systems was eliminated in concert with present world-wide. These include universities, institutes, tech- trends moving toward emphasis on peaceful uses of sci- nologybasedindustries,publiclibraries,andgovernment enceandtechnology.AaronKlug(molecularbiologyand agencies.Thus,wefeelthatouroriginalgoalofproviding biotechnology)andPhillipSharp(molecularandcellbi- in-depthuniversityandprofessionallevelcoverageofev- ology) then joined the board to oversee their fields as eryfacetofphysicalsciencesandtechnologywas,indeed, wellastheoverallEncyclopedia.TheAdvisoryBoardwas worthwhile. completedwiththeadditionofJohnBollinger(engineer- Theeditor-in-chief(EiC)andtheExecutiveBoardde- ing),MichaelBuckland(librarysciences),JeanCarpentier termined in 1998 that there was now a need for a Third (aerospacesciences),LudwigFaddeev(physics),Herbert Edition.Itwasapparentthattherehadbeenablossoming Friedman(spacesciences),R.A.Mashelkar(chemicalen- ofscientificandengineeringprogressinalmosteveryfield gineering),KarlPister(engineering)andGordonSlemon andalthoughtheWorldWideWebisamightyriverofin- (engineering). formation and data, there was still a great need for our A 40 page topical outline of physical sciences and articles, which comprehensively explain, integrate, and technology was prepared by the EiC and then reviewed providescientificandmathematicalbackgroundandper- bytheboardandmodifiedaccordingtotheircomments. spective.Itwasalsodeterminedthatitwouldbedesirable Thisformedthebasisforassuringcompletecoverageof toaddalevelofperspectivetoourEncyclopediateam,by the physical sciences and for dividing the science and bringinginagroupofeminentSectionEditorstoevaluate engineering disciplines into 50 sections for selection theexistingarticlesandselectnewonesreflectingfields of section editors. Six of the advisory board members thathaverecentlycomeintoprominence. decided to serve also as section editors (Allen Bard for TheThirdEditionExecutiveBoardmembers,Stephen analytical chemistry, Elias Corey for organic chemistry, Hawking (astronomy, astrophysics, and mathematics), Paul Crutzen for atmospheric sciences, Yuan Lee for Daniel Goldin (space sciences), Elias Corey (chem- physical chemistry, Phillip Sharp for molecular biology, istry), Paul Crutzen (atmospheric science), Yuan Lee andMelvinSchwartzforphysics).Thirty-twoadditional (chemistry), George Olah (chemistry), Melvin Schwartz section editors were then nominated by the EiC and (physics),EdwardTeller(nucleartechnology),Frederick the board for the remaining sections. A listing of the Seitz (environment), Benoit Mandelbrot (mathematics), sectioneditorstogetherwiththeirsectiondescriptionsis AllenBard(chemistry)andKlausvonKlitzing(physics) presentedonp.v. xiii P1:FJUFinalPages EncyclopediaofPhysicalScienceandTechnology Vol.11-Fm-Bm July31,2001 15:18 xiv Preface Thesectioneditorsthenprovidedlistsofnominatedar- tics, and lasers; 7% mathematics; 8% astronomy, astro- ticles and authors, as well as peer reviewers, to the EiC physics,andspacetechnology;6%energyandpower;6% based on the section scopes given in the topical outline. materials;7%engineering,aerospace,andtransportation. These lists were edited to eliminate overlap. The Board The relative distribution between basic and applied sub- wasaskedtohelpadjudicatethelistsasnecessary.Then, jects is: 60% basic sciences, 7% mathematics, and 33% acompletelistingoftopicsandnominatedauthorswasas- engineeringandtechnology.Itshouldbepointedoutthat sembled.Thiseffortresultedinthedeletionofabout200 asubjectsuchasenergyandpowerwithjusta5%share oftheSecondEditionarticles,theadditionofnearly300 ofthetopicdistributionisabout850pagesintotal,which completely new articles, and updating or rewrite of ap- correspondstoabook-lengthtreatment. proximately480retainedarticletopics,foratotalofover We are saddened by the passing of six of the Board 780articles,whichcomprisetheThirdEdition.Examples memberswhoparticipatedinpreviouseditionsofthisEn- of the new articles, which cover science or technology cyclopedia.Thiseditionisthereforededicatedtothemem- areasarisingtoprominenceafterthesecondedition,are: oryofS.Chandrasekhar,LinusPauling,VladimirPrelog, molecular electronics; nanostructured materials; image- AbdusSalam,GlennSeaborg,andGian-CarloRotawith guided surgery; fiber–optic chemical sensors; metabolic gratitudefortheircontributionstothescientificcommu- engineering;self-organizingsystems;tissueengineering; nityandtothisendeavor. humanoid robots; gravitational wave physics; pharma- Finally,IwishtothankthefollowingAcademicPress cokinetics;thermoeconomics,andsuperstringtheory. personnel for their outstanding support of this project: Over1000authorspreparedthemanuscriptsatanav- Robert Matsumura, managing editor, Carolan Gladden eragelengthof17-18pages.Themanuscriptswerepeer andAmyCovington,authorrelations;FrankCynar,spon- reviewed,indexed,andpublished.Theresultistheeigh- soringeditor;NickPanissidi,manuscriptprocessing;Paul teenvolumework,ofover14,000pages,comprisingthe GottehrerandMichaelEarly,production;andChrisMor- ThirdEdition. ris,MajorReferenceWorksdirector. Thesubjectdistributionis:17%chemistry;5%molec- ularbiologyandbiotechnology;11%physics;10%earth RobertA.Meyers,Editor-in-Chief sciences;3%environmentandatmosphericsciences;12% Ramtech,Inc. computers and telecommunications; 8% electronics, op- Tarzana,California,USA FROM THE PREFACE TO THE FIRST EDITION In the summer of 1983, a group of world-renowned sci- provided only by calculus. In addition, they tended ei- entists were queried regarding the need for an encyclo- thertofragmentagivenscientificdisciplineintonarrow pedia of the physical sciences, engineering, and mathe- specificsortopresentsuchbroadlydrawnarticlesastobe matics written for use by the scientific and engineering oflittleusetopracticingscientists. community.Theprojectedreadershipwouldbeendowed In consultation with the senior executive advisory with a basic scientific education but would require ac- board,AcademicPressdecidedtopublishanencyclope- cess to authoritative information not in the reader’s spe- diathatcontainedarticlesofsufficientlengthtoadequately cific discipline. The initial advisory group, consisting of cover a scientific or engineering discipline and that pro- Subrahmanyan Chandrasekhar, Linus Pauling, Vladimir videdaccuracyandaspecialdegreeofaccessibilityforits Prelog,AbdusSalam,GlennSeaborg,KaiSiegbahn,and intendedaudience. EdwardTeller,encouragedthisnotionandofferedtoserve Thisaudienceconsistsofundergraduates,graduatestu- asourseniorexecutiveadvisoryboard. dents,researchpersonnel,andacademicstaffincolleges A survey of the available literature showed that there and universities, practicing scientists and engineers in were general encyclopedias, which covered either all industry and research institutes, and media, legal, and facets of knowledge or all of science including the bio- managementpersonnelconcernedwithscienceandengi- logical sciences, but there were no encyclopedias speci- neeringemployedbygovernmentandprivateinstitutions. ficallyinthephysicalsciences,writtentothelevelofthe Certainadvancedhighschoolstudentswithatleastayear scientificcommunityandthusabletoprovidethedetailed ofchemistryorphysicsandcalculusmayalsobenefitfrom information and mathematical treatment needed by the theencyclopedia. intended readership. Existing compendia generally lim- ited their mathematical treatment to algebraic relation- RobertA.Meyers shipsratherthanthein-depthtreatmentthatcanoftenbe TRW,Inc. PERMISSION ACKNOWLEDGMENTS MaterialhasbeenreproducedinthisreferenceworkwithkindpermissionofNaturePublishingGroup http://www.nature.com/nature Fullcitationsaregivenattherelevantplacesinthetext MaterialhasbeenreproducedinthisreferenceworkwithkindpermissionoftheAmericanAssociationfortheAdvancementofScience http://www.sciencemag.org Fullcitationsaregivenattherelevantplacesinthetext