Table Of ContentOUPCORRECTEDPROOF – FINAL,20/7/2019,SPi
CONSTRUCTING QUANTUM MECHANICS
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Constructing Quantum Mechanics
Volume One
The Scaffold: 1900–1923
Anthony Duncan and Michel Janssen
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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.
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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).
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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
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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.
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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.
