OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi CONSTRUCTING QUANTUM MECHANICS OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi Constructing Quantum Mechanics Volume One The Scaffold: 1900–1923 Anthony Duncan and Michel Janssen 1 OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi 3 GreatClarendonStreet,Oxford,OX26DP, UnitedKingdom OxfordUniversityPressisadepartmentoftheUniversityofOxford. ItfurtherstheUniversity’sobjectiveofexcellenceinresearch,scholarship, andeducationbypublishingworldwide.Oxfordisaregisteredtrademarkof OxfordUniversityPressintheUKandincertainothercountries ©AnthonyDuncanandMichelJanssen2019 Themoralrightsoftheauthorshavebeenasserted FirstEditionpublishedin2019 Impression:1 Allrightsreserved.Nopartofthispublicationmaybereproduced,storedin aretrievalsystem,ortransmitted,inanyformorbyanymeans,withoutthe priorpermissioninwritingofOxfordUniversityPress,orasexpresslypermitted bylaw,bylicenceorundertermsagreedwiththeappropriatereprographics rightsorganization.Enquiriesconcerningreproductionoutsidethescopeofthe aboveshouldbesenttotheRightsDepartment,OxfordUniversityPress,atthe addressabove Youmustnotcirculatethisworkinanyotherform andyoumustimposethissameconditiononanyacquirer PublishedintheUnitedStatesofAmericabyOxfordUniversityPress 198MadisonAvenue,NewYork,NY10016,UnitedStatesofAmerica BritishLibraryCataloguinginPublicationData Dataavailable LibraryofCongressControlNumber:2019909012 ISBN978–0–19–884547–8 DOI:10.1093/oso/9780198845478.001.0001 Printedandboundby CPIGroup(UK)Ltd,Croydon,CR04YY LinkstothirdpartywebsitesareprovidedbyOxfordingoodfaithand forinformationonly.Oxforddisclaimsanyresponsibilityforthematerials containedinanythirdpartywebsitereferencedinthiswork. OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi Preface How did physicists in the first three decades of the last century come to recognize that theyneededanewframeworktoaccountforagrowinglistofphenomenathattheycould not account for with what we now call classical physics? That is the central question of this book. To answer it, we analyze what we have identified as the main strands in thedevelopmentofthisnewframework.Thisfirstvolumetakesusfromthelate1890s to the early 1920s. The second volume will take us from the early to the late 1920s. Ourstartingpointcanbecomparedtothatofsomeonefirstencounteringanarch.This personwillwonderhowitwasbuiltandcometorealizethatitwasdonewiththehelpof ascaffold.Ifwethinkofquantummechanicsasanarch,wecanlikewisesaythatitwas puttogetherwiththehelpofascaffold,providedinthiscasebyelementsofthetheory itreplaced.Byusingthearch-and-scaffoldmetaphorintheirsubtitles,weacknowledge thebackwards-lookingperspectiveweareadoptinginthesevolumes.1 Decisionsabout which strands in the developmentof quantum mechanics we cover in detail and which ones we treat more cursorily (if at all) were informed by our assessment,in hindsight, of their relative importance for the scaffold on which the arch was erected. However, our analysis of the strands selected for detailed discussion is given entirely in terms of insights and concepts available to the historical actors at the time.To find meaningful historically sensitive answers to our central question, we scrupulously adhered to this methodologicalprinciple. Theauthorsoftwoprevioushistoriesofquantumphysics,MaxJammer(1966)and Jagdish Mehra and Helmut Rechenberg (1982a,b,c,d, 1987, 2000–2001), addressed questions similar to ours and faced similar historiographical challenges. Our book, however,isnotonlymoreexplicitabouttheapproachtakenbutdiffersfromtheseearlier efforts in several other respects.A reader familiar with these older histories will notice themostimportantdifferencesjustleafingthroughthepagesofthisbookandglancing atitstableofcontents. First of all, our choice of topics is more selective than either Jammer’s or Mehra andRechenberg’s.Withoutbeingselective,wewouldhavehadnohopeofcoveringthe developments we cover at the level of detail we cover them in just two volumes.It took Mehra and Rechenberg, depending on how one counts, six or nine volumes to cover roughlythesameperiodandJammeronlymanagedtodosoinonevolumebyomitting thederivationofmostresults. 1 Thearch-and-scaffoldmetaphorforthedevelopmentofscientifictheoriesispresentedandthehistorio- graphicalissuesitraisesarediscussedinJanssen(2019).ThemetaphorwasinspiredbyAlexanderGraham Cairns-Smith(1985,pp.58–60),whousedittoillustratehissuggestionthatthecomplexnucleotidesofRNA andDNAwerefirstassembledonminuteclaycrystals. OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi vi Preface This brings us to a second important difference. We include derivations of all key resultsatalevelthatareaderwithacommandofphysicsandmathematicscomparable to that of an undergraduate in physics should be able to follow without having to take outpencilandpaper.Wefrequentlyusemodernnotationandstreamlinederivationsbut conceptually we always remain faithful to the historical sources. As we will explain in more detail below,this feature makes our book suitable for classroom use.But first we want to emphasize another way in which our book goes beyond the books by Jammer andMehraandRechenberg. Sincethepublicationofthesebooks,moreresearchontheearlyhistoryofquantum physicshasbeenpublishedandadditionalprimarysourcematerialhasbecomeavailable. In the latter category, we mention the collected papers of Niels Bohr (1972–2008) and Albert Einstein (1987–2018) as well as editions of the scientific correspondence of Arnold Sommerfeld (2000, 2004) and Hendrik Antoon Lorentz (2008, 2018). We have taken full advantage of these primary sources and the annotation provided by the editors. We should also note that it made our work considerably easier that so much material is available on-line these days. This is true not only for many of the primary andsecondarysourcesweusedbutalsoforbiographicalinformationforallbutthemost obscure characters in our story. Library support nonetheless remained crucial for our workandwethankthededicatedstaffofthelibrariesoftheUniversityofMinnesota,the UniversityofPittsburgh,and,especially,theMaxPlanckInstituteforHistoryofScience inBerlin. In terms of secondary sources, Thomas S. Kuhn’s (1978) book on black-body radiationhasledtoareevaluationofPlanck’sroleintheearlyhistoryofquantumtheory, as illustrated, for instance, by the first part of Olivier Darrigol’s (1992) book on the roleofclassicalanalogiesinthedevelopmentofquantumtheory(seenote8inSection 2.1). This literature has been taken into account in our discussion of both Planck’s work and Einstein’s reaction to it. Darrigol’s book also deepened our understanding of Bohr’s correspondence principle, which informed our discussion of this important principle. More recent books on topics covered in this volume include Helge Kragh’s (2012) history of the Bohr model of the atom from its inception in 1913 to its demise in1925andMichaelEckert’s(2013b)biographyofArnoldSommerfeld,whoplayeda leadingroleintheelaborationofBohr’smodelanditsconfrontationwithempiricaldata. We also drew on Eckert’s (2013a) introduction to an edition of Sommerfeld’s (1915a, 1915b)firstpapersextendingtheBohrmodel.2 Booksbyyoungerscholars(e.g.,Staley 2008,Seth2010,Navarro2012,Badino2015)havealsobeenusefultouseventhough their approach is sometimes markedly different from ours.Many older sources remain invaluable.InadditiontoJammer(1966)andMehraandRechenberg(1982a),wesingle outtheclassicpaperby John L.Heilbronand Kuhn (1969) onthegenesisoftheBohr atom and Martin J. Klein’s (1970a) magisterial partial biography of Paul Ehrenfest, covering its subject’s most important contributions to the developments discussed in this volume. We should note, however, that much more has been written on the early 2 ForEnglishtranslationsandcommentary,seeSommerfeld(2014a,2014b)andEckert(2014). OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi Preface vii workcoveredinPartOneofthisvolumethanonthedevelopmentscoveredinPartTwo, where,insomeplaces,weworkedalmostexclusivelyfromprimarysources. Researchforthisbookwasdoneinthecontextofalargeinternationalprojectinthe history and foundations of quantum physics that ran from 2006 to 2013.This project was a joint initiative of Jürgen Renn’s department at the Max Planck Institute for the HistoryofScienceandthetheorydepartmentoftheFritzHaberInstitute,bothlocated inBerlin.Theprojectledtotheestablishmentofaseriesofconferencesonthehistoryof quantumphysics.AfterasmallworkshopinBerlinintheSummerof2006(HQ0),four such conferences have been held so far:HQ1 (Berlin,July 2–6,2007),HQ2 (Utrecht, July 14–17,2008),HQ3 (Berlin,June 28–July 2,2010),and HQ4 (San Sebastián,July 15–18,2015).WewereinvolvedwiththisBerlinquantumprojectfromthebeginningand presentedourworkbothattheoriginalworkshopandatallfoursubsequentconferences. These presentations resulted in four papers, all on topics that will be covered in the secondvolumeofourbook(DuncanandJanssen2007,2008,2009,2013). A pair of papers on a topic covered in this volume grew out of the presentation we gaveataconferencetocommemoratethecentenaryoftheBohratom,heldJune11–14, 2013 in Copenhagen and attended by many members of the Berlin quantum project. These two papers are on the explanation of the Stark effect, the splitting of spectral lines in electric fields, both in the old and in the new quantum theory (Duncan and Janssen 2014,2015).One of us co-authored a paper on Ehrenfest’s adiabatic principle with another member of this large international collaboration, the Barcelona historian of physics Enric Pérez (Duncan and Pérez 2016). These papers formed the basis for parts of Sections 5.2 and 6.3 in this volume. We are grateful to Elsevier, publisher of StudiesinHistoryandPhilosophyofModernPhysics,and the Royal Danish Academy of SciencesandLetters,publisheroftheproceedingsoftheBohrcentenaryconference,for permissiontousethismaterial. As indicated above, we wrote this book with the idea that it could be used for courses in the history of modern physics aimed at undergraduate majors in physics or beginning graduate students in the history and philosophy of science. We both have experience teaching such courses. The past couple of years, one of us (MJ) has been using drafts of chapters of this book to teach the history of quantum physics to an audience of predominantly physics juniors and seniors at the University of Minnesota. Wearegratefultothestudentsintheseclassesfortheirfeedback. Several sections in Part One of this volume were written with the express purpose in mind of having students read them in tandem with some of the classic papers by Planck (1900d,1901a),Einstein (1905a,1907,1909b,1916b) (all available in English translation),and Bohr (1913b,1913c,1913d).Part Two likewise prepares the student forreadingmanyoftheoriginalpapersoftheoldquantumtheory(thoughmanyofthese have not been translated) as well as the four editions of the “bible of the old quantum theory,”AtombauundSpektrallinien(Sommerfeld1919,1921,1922a,1924).AnEnglish translation(AtomicStructureandSpectralLines)ofthethirdeditionappearedshortlyafter theGermanversionwaspublished(Sommerfeld1923). Toencourageandsupportfacultyinphysicsdepartmentsinterestedinofferingaclass on the history of quantum mechanics based on the latest scholarship in the field, we OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi viii Preface partneredwiththeCenterforHistoryofPhysicsandtheEmilioSegrèVisualArchives oftheNielsBohrLibrary&Archives,bothattheAmericanInstituteofPhysicsinCollege Park, MD, to set up a website that will make supplementary materials for a course based on our book available to students and instructors. This will include electronic versions of the primary sources mentioned above,materials from the course offered at theUniversityofMinnesota,andselectedimagesfromtheEmilioSegrèVisualArchives. Oxford University Press has set up a companion website featuring a series of “web resources” with more detailed treatments of various topics covered in the book. The URLforthiscompanionwebsiteis:www.oup.com/companion/constructingquantum. The present volume has two appendices with crucial background material on clas- sical mechanics and spectroscopy. The additional information provided in our “web resources” is not critical for understanding the main text of this volume. To give two examples:oneofthesewebresourcesisonclassicaldispersiontheoryandincludesthe derivationofakeyresultPlanckneededtoderiveaformulaforthespectraldistribution of black-body radiation (see Eq.(2.7) in Section 2.3);another is on Woldemar Voigt’s classical theory of the Zeeman effect, which played an important role in attempts to accountfortheeffectintheoldquantumtheory(seeSection7.2.2). WearegratefultoGregoryGood,DirectoroftheCenterforHistoryofPhysics,for agreeing to partner with us in developing and maintaining this website in support of our book, and to Sonke Adlung, our editor at Oxford University Press, for agreeing to have the American Institute of Physics host a website connected to a book in their catalog.The materials from the course on history of modern physics at the University of Minnesota were developed as part of the project “Digital Essays in the History of Quantum Mechanics” led by Robert Rynasiewicz of Johns Hopkins University and supportedbytheNationalScienceFoundationunderNSFGrantNo.SES-1027018. It remains for us to thank a number of individuals and organizations that made our work possible.Our main debt of gratitude is to the Berlin quantum project.We wrote partsofthisvolumeasguestsinJürgenRenn’sdepartmentattheMaxPlanckInstitute for History of Science and we thank the institute for its support. One of us (MJ) has beenaregularvisitorattheinstitutesinceitsinceptionintheearly1990sandfocusedon this book project in the Spring of 2016 during the second half of a sabbatical from the UniversityofMinnesota.ThisworkinBerlinwassupportedbyaResearchAwardfrom theAlexandervonHumboldtFoundation.Therecipient(MJ)wouldliketoexpresshis gratitude to the foundation for its support.For arranging our stays in Berlin,we thank ShadiyeLeather-Barrow. Most of the book was written in Pittsburgh where we have been getting together regularly to work on the history of quantum physics. We started the actual writing of this book in the Fall of 2015 when one of us (MJ) spent the first half of the aforementioned sabbatical at the Center for Philosophy of Science at the University of Pittsburgh.We thank the staff at the Center,especially its director John Norton,for its hospitality and support.The company of Edith Cohen,Donna Naples,Ted Newman, MaxNiedermaier,andMerrileeSalmonhasmadeourget-togethersinPittsburghallthe moreenjoyable. OUPCORRECTEDPROOF – FINAL,20/7/2019,SPi Preface ix We greatly benefited from discussions with Massimiliano Badino, Michael Eckert, AllanFranklin,ClaytonGearhart,TedJacobson,ChristianJoas,AnneKox,JoeMartin, CharlesMidwinter,JohnNorton,EnricPérez,SergioPernice,JürgenRenn,SergeRudaz, RobertRynasiewicz,RobertSchulmann,AndreySharapov,RogerStuewer,Jeroenvan Dongen, and Brian Woodcock. We are especially grateful to Olivier Darrigol for his detailedcommentsonacompletedraftofthisvolume.Althoughtheyaretoonumerous tomentionindividually,wealsolearnedmuchfromotherhistorians,philosophers,and physicistsinvolvedintheBerlinquantumprojectandattendingtheHQconferences.We thankChristianJoasforprovidinguswiththeLATEXtemplatethatweusedforthebook andforsolvingvariousLATEXproblemsweranintoalongtheway. WethankSonkeAdlung,AniaWronski,CherylBrant,andAlannahSantraatOxford University Press and its partner production company SPi Global for shepherding our bookthroughtheproductionprocess.WethankJulianThomasforhismeticulouscopy- editingofourmanuscript. Finally,weexpressourgratitudetotheEmilioSegrèVisualArchives,theDeutsches Museum in Munich, the Niels Bohr Archive in Copenhagen, the Royal Society in London, and the Stichting “Pieter Zeeman-Fonds” in Amsterdam for granting us permission to include various photographs in their collection in this volume. These photographs,mostly portraits of physicists playing an important role in our story,can befoundintheplatesinsertedbetweenPartOneandPartTwo.