Michael Eckert Arnold Sommerfeld. Science, Life and Turbulent Times

Michael Eckert translated by Tom Artin Arnold Sommerfeld Science, Life and Turbulent Times 1868–1951 Michael Eckert Deutsches Museum Munich , Germany

Translation of Arnold Sommerfeld: Atomphysiker und Kulturbote 1868–1951, originally published in German by Wallstein Verlag, Göttingen

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In the case of such a source-dependent work, fi rst thanks are due Sommerfeld’s heirs for access to the private papers and for permission to make full use of their content. Sommerfeld’s granddaughter, Monika Baier, deserves particular thanks for ordering her grandfather’s private papers and for providing valuable information on the family background. Th e archivists of the various public institutions in which Sommerfeld correspondence resides, too, are gratefully acknowledged for their readiness to help. Th e bibliography reveals how much support the project has thereby received. Cited passages from letters and texts from other source materials have been adapted to modern notation for the sake of legibility. Numerous letters cited here only in excerpts are printed in full in editions of correspondence. In these cases, references are given indicating the respective volume of the published corre- spondence (e.g., ASWB I and ASWB II as abbreviations for the two volumes of Arnold Sommerfeld – Wissenschaftlicher Briefwechsel, Band I und II: since in those volumes the letters are printed in chronological order, there is no need to specify page or letter numbers). Th us the reader can study the respective letters in full and in the context of the other correspondence reproduced there. Since Sommerfeld’s private letters are not publicly accessible, they are identifi ed only by date, and no source is given. It is conventional for the published work of a scientist to take precedence over unpublished papers at the beginning of a biographical endeavor. Th is approach refl ects his life’s work as a scientist as it has been perceived by his contemporaries. Usually this kind of biographical work begins with the mile-stone birthdays mark- ing decades of a scientist’s life. 1 Th us, festschrifts were assembled to celebrate Sommerfeld’s 60th, 70th, and 80th birthdays. In 1968, on the occasion of his 100th birthday, the physicists of the University of Munich, organized an “Arnold Sommerfeld Centennial Memorial Meeting,” and an “International Symposium on the Physics of the One- and Two-Electron Atoms.” On this occasion, his succes- sor at the Institute for Th eoretical Physics (Fritz Bopp) was commissioned by the Bavarian Academy of Sciences to arrange for Sommerfeld’s most important scien- tifi c papers to be published in the form of a four-volume edition. 2 Th e biographer is gratefully obliged to all those involved in this preliminary work. Th e editors of the scientifi c correspondence (the author and Karl Märker) had the support of the Munich Physics Department (especially Harald Fritzsch and Herbert Wagner), and the Bavarian Academy of Sciences (in the person of Past President Arnulf Schlüter). Special thanks are also due the Dean of the Faculty of Physics (Axel Schenzle) of Munich University for the fi nancial support that has enabled the translation of the German original of this biography into English.

1 See chapter 14. 2 Sauter, Sommerfeld , 1969.

v acknowledgment

Th e biographer owes thanks and acknowledgment also to his colleagues in the history of science who have concerned themselves from a historical perspective with Sommerfeld and his fi elds of research. Atop the list are John L. Heilbron, and Paul Forman, who as participants in the SHQP project actually fi rst set Sommerfeld research in motion. Subsequently, above all Armin Hermann, Ulrich Walter Benz, and Karl von Meyenn have helped cause this spark to jump over to Germany. In recent years, a new project on the history of quantum physics 3 at the Max Planck Institute for History of Science in Berlin brought further impetus to analyze Sommerfeld’s work within the network of modern atomic and quantum theory. For this biography, it was a piece of good fortune to participate in this project. It would lead too far afi eld to list all the names of colleagues and friends who in this and in previous decades have researched the history of quantum physics—and thus also important aspects of the fi eld of Sommerfeld biography. Even if, in retrospect, quantum physics be deemed Sommerfeld’s most signifi - cant area of research, the less spectacular mathematical, physical, and technical work to which he devoted himself in the course of his long scientifi c career deserves inclusion in his biography. Th e gratitude of the historian of science and biographer is therefore directed to all those who have devoted their scholarly attention to these aspects of Sommerfeld’s work, and thus in diverse ways have advanced the work of this biography. Th eir names and contributions are to be found in the bibliography. Special thanks to our colleagues at the Research Institute of the Deutsches Museum, which constitutes a particularly sympathetic setting for the work of history of sci- ence. Last but not least, thanks to the German Research Foundation, without whose fi nancial support this project would not have been possible.

3 http://quantum-history.mpiwg-berlin.mpg.de/main/ (28 January 2013) vi Contents

Prologue...... xi

 Königsberg Roots ......  . Childhood......  . School Years ......  . University Study......  . A Competition ......  . Th e Dissertation......  . A Mechanical Basis of Electrodynamics ...... 

 Setting the Course......  . Missed Opportunities ......  . Military Service ......  . Mineralogical Interlude ......  . Stick It Out, or Resign? ......  . Approach to Felix Klein ......  . Physics or Mathematics?...... 

 Klein’s Assistant ......  . Physical Mathematics ......  . Th en I’ll Grow into the Lectureship ......  . Reading Room and Model Collection ......  . Habilitation ......  . Lecturer ......  . Th e Engagement...... 

 Clausthal ......  . An Off er from America ......  . Th e Appointment at the Clausthal School of Mining ......  . Th e School of Mining ......  . Th e Wedding ......  . Gyroscope Matters and Electrodynamic Problems......  . Encyclopedia Travels......  . Gyroscope + Encyclopedia = Aachen-Recommendation ...... 

vii contents

 Aachen ......  . Backgrounds of an Appointment ......  . Rapprochement with Engineering......  . Technological Expert ......  . Family Life ......  . Duties and Inclinations ......  . Th e “Super-Mechanics” of Electrons......  . “In Truth I Am No Engineering Professor; I Am a Physicist” . . . . . 

 Munich ......  . Academic Traditions......  . Quarrel over Electron Th eory ......  . Th e Origins of the Sommerfeld School ......  . Th e Mathematical Attack ......  . Th e “h-Discovery” ......  . Th e First Solvay Congress ......  . X-Rays and Crystals ...... 

 Physics in War and Peace ......  . “For Me, the Political Future Lies in Utter Darkness” ......  . Return to Th eory ......  . Letters from the Front ......  . Elaboration of Bohr’s Atomic Model ......  . Success ......  . Military Physics ......  . An Emotional Roller Coaster ...... 

 The Quantum Pope ......  . Atomic Structure and Spectral Lines......  . Th e German Physical Society: Internal Strife ......  . Visiting Bohr ......  . A New Quantum Number ......  . Teacher and Students ......  . Th e Bible of Atomic Physics......  . Th e Lusitania Medal......  . Karl Schurz Professor at Madison ......  . California Impressions......  . Practical Spectroscopy ......  viii contents

 Wave Mechanics......  . Th e Crisis of the Models......  . “We Believe in Heisenberg, but We Calculate with Schrödinger” . . .  . Electron Th eory of Metals ......  . Th e Planck Succession......  . “Not Sommerfeld, but Schüpfer” ......  . Th e Volta Congress ......  . A Wave Mechanics Supplement ...... 

 Cultural Ambassador......  . German Science on the International Stage ......  . Impressions of India ......  . German Science at Chinese Outposts ......  . Birthday in Japan ......  . Visiting Professor in Pasadena ......  . Th e Second American Tour ......  . Critique of Positivism ......  . Quarrel with Stark ......  . On the Road Again ......  . Consolidation of the New Th eories ...... 

 Descent ......  . Consequences of the New Civil Service Law ......  . A Deceptive Normalcy......  . “Retirement” with Postponement ......  . “German Physics” ......  . Science on the Sidelines ......  . Th e Seventieth Birthday ......  . Th e Decision in the Succession Dispute ...... 

 Bitter Years......  . Th e Scandal Intensifi es......  . Th e End of “German Physics” ......  . Political Misgivings ......  . A Research Assignment for the Navy......  . Lectures on Th eoretical Physics ......  . Relativity Th eory Without Einstein? ...... 

ix contents

 Carrying On ......  . Denazifi cation ......  . A Provisional Fresh Start ......  . “I’ll Bet on the Anglo-Americans” ......  . Recognition for the Teacher ......  . Th e Eightieth Birthday......  . Th e Late Work ......  . Th e Last Years ...... 

 Legacy ......  . Obituaries ......  . Leading Figure for the History of Physics ......  . Th e Fine-Structure Constant......  . Th e “Sommerfeld Puzzle” ......  . From the Pacifi c Problem to Dark Matter ......  . Th e Nobel Prize Denied ...... 

 Epilogue ...... 

Abbreviations ...... 

Bibliography...... 

Index ...... 

x Prologue

Who was Arnold Sommerfeld? Along with Max Planck (1858–1947), Albert Einstein (1879–1955) and Niels Bohr (1885–1962), he belongs among the founders of theo- retical physics, which developed into an independent discipline during his lifetime (1868–1951). Among his best known achievements is the elaboration of the Bohr atomic theory established a century ago. Even among physicists of the twenty-fi rst century, the “Bohr-Sommerfeld-Atom” and the “Sommerfeld fi ne-structure con- stant,” remain current concepts. Older physicists associate Sommerfeld’s name with the fi rst “school” of modern theoretical physics, and with the work known as the “Bible of atomic physics,” Atomic Structure and Spectral Lines . Th is legendary textbook was spread throughout the world in many editions and translations, and initiated generations of physics students into the fi eld of nuclear physics. Additionally, Sommerfeld’s Lectures on Th eoretical Physics, published in six volumes, and reissued long after his death in ever new editions, conveys a sense of the char- ismatic teacher’s personality. At the University of Munich, where he taught and pursued research from 1906 for over three decades, the tradition of the Sommerfeld school continues at the “Arnold Sommerfeld Center for Th eoretical Physics.” Here, the latest fi ndings in string theory and other areas of theoretical physics are discussed. A hundred years ago, the Munich “nursery of theoretical physics” (as Sommerfeld liked to describe his institute) was a haven for the new quantum physics. Sommerfeld’s students included Nobel Prize winners Peter Debye (1884– 1966), Max von Laue (1879–1960), Wolfgang Pauli (1900–1958), Werner Heisenberg (1901–1976), Linus Pauling (1901–1994) and Hans Bethe (1906–2005). With his 81 nominations, Sommerfeld himself holds the sad record of having been proposed for the Nobel Prize more often than any other physicists . . . without ever receiving the coveted distinction. 1 It is not just his contributions to modern physics that suggest the need for a biography of Sommerfeld, however. In mathematics too, and in technology, evi- dence of his infl uence is apparent. For engineers who deal with the theory of hydro- dynamic bearings, the “Sommerfeld number” is a term; in this technological discipline, Sommerfeld is counted as one of the “Men of Tribology.” 2 Moreover, Sommerfeld was recognized beyond the boundaries of his own subject. In the 1920s, he traveled widely in the capacity of “Cultural Ambassador” campaigning for Germany’s reputation abroad.3 In the time of the “Th ird Reich,” he became the target of attacks from a group of Nazi ideologues who sought to replace the “Th eoretical Cartel” around Sommerfeld, denounced as Jewish, with a “German physics.” Following Sommerfeld’s retirement, the most fanatical member of this

1 See chapter 14.6 2 See chapter 5.3. 3 Th is was his son’s designation for him; see Chapter 10.

xi prologue group, a professor of aerodynamics, was appointed his successor. In 1939, the Institute, once renowned for modern physics, became for several years an arena of aggressive Nazi propaganda; later, it served the history of science as an example of the devastating eff ects of an ideologically driven science.4

Methodology

Th e biography of a physicist of such broad infl uence cannot content itself with a presentation of his scientifi c life’s work. Needing to assess the signifi cance of the his- torical events occurring during the life of the individual it depicts, it thus assumes its place in the domain of science history. Many biographies of great thinkers and learned scholars of earlier periods, however, evince little of the fi ndings of history profession- als, so that historians have long stood in an uneasy relation to biographies in general, and those of scientists in particular. Th e genre—so the allegations have run—suff ers from weak theoretical underpinnings, and tends to be outmoded. Biography has been regarded as a relic of historicism, a “fossil of a long surpassed historiography of great events and personalities, when the historian still believed it possible through intuition and imagination to grasp and represent the inner logic of the historical evolution and actions of his biographical subjects.” Th is is how, from the perspective of recent social history, this professional unease has aptly been described. Th at this characterization prefaced a biography of Fritz Haber (1868–1934) shows to what extent the genre of scientifi c biography today has distanced itself from historicist hero worship. For the modern discipline of history, biography constitutes simultaneously both “challenge and opportunity” to demonstrate “the interweaving of an individual life with its historical context.” Th e socio-historically stamped biography traces “the historical scope of action of the individual,” and is “as it were microscopically” focused on historical details to which history otherwise pays scant attention. “For this, Fritz Haber’s transparently lived life presents itself almost ideally.”5 Th is applies equally to the biography of Sommerfeld, the same age almost to the day as Haber, and whose path crossed Haber’s on several occasions. But even if a modern science biography is committed to both social history and the history of science, the unique traits characterizing a life which, either through family back- ground or in some other way lend a subject his quite individual personality, require of the biographer—more than of a “microscopically” oriented historian—consider- able empathy to convey the personality of his subject to the reader. In an analyti- cally oriented historical study, such an intimate view would rather be frowned upon since historical science values objectivity and critical distance. For biographers, however, empathy is nonetheless a prerequisite, on which theorists of this genre

4 See chapters 11, 12. 5 Szöllösi-Janze, Fritz Haber, 1998, p. 12; Szöllösi-Janze: Lebens-Geschichte , 2000; Daston/ Sibum: Scientifi c Personae , 2003. xii prologue place great value. Th is proximity must also fi nd narrative expression: the biographer needs to employ formulations that are as authentic as possible, and not obscure them through idiosyncratic terminology.6 In the fi eld of history of science, it has been and continues to be especially diffi cult for biography to establish itself as an independent genre. On the one hand, biographers of renowned scientists are liable to the stigma of outmoded hero-worship if they display empathy; on the other hand, biographies often serve as vehicles to illustrate scientifi c processes in a social context. Th e scientifi c biography is most convincing, however, precisely when it presents not only the scientifi c issues of the period, but also the ambitions, pas- sions, and moral choices of a life in science. Th is at any rate was the judgment of one science historian on the incorporation of biography into her fi eld.7 Th e scientifi c biography has yet to emancipate itself in this respect, and to refl ect on the strengths of its own genre, claimed Th omas Söderqvist, who as a biographer and historian of science has made contributions in both spheres. Biography is a genre that deals with existential choices. Söderqvist coined the term “existential biography.” Th e science biographer needs to convey the existential choices that life as a scientist entails. 8 Th is far exceeds the demand that history of science inspired by the social sciences be integrated into its social context. Th e genre of scientifi c biography adheres to its own set of rules and possibilities, and is not simply a tool in the service of history of science. 9 Th e biography of a modern theoretical physicist like Sommerfeld, however, is a tight-rope balancing act in yet another way. Anyone who has not studied physics will hardly know where to begin with the details of Sommerfeld’s work. It is the peculiar challenge for the biographer of mathematicians and physicists to be true to the scientifi c content and yet not pitch the demands on the reader so high that he can follow the biography only in the context of a technical course of study. If high priority is given to scientifi c content, the biography of a physicist or mathematician most often falls into chapters containing general accounts of their lives on the one hand, and chapters of physical or mathematical detail studies, larded with formulas and technical jargon, on the other. If broad accessibility is given priority, scientifi c content tends to fall by the wayside. Th e narrative essence of biography, however, requires a balanced presentation not divided up into chapters of divergent reading styles. Among scientifi c biographers, these problems have long been debated. 10 In mathematical biographies, a compromise between technical precision and broader accessibility seems impossible when very abstract topics are in question. In physics, though, even in highly complex areas, there is often an apparent relation to objects in the world of experience, so that at least a rough presentation of the essential

6 Frank, Other , 1985. 7 Jo-Nye, Scientifi c Biography , 2006. 8 Söderqvist, Existential projects , 1996. 9 Söderqvist, History and Poetics , 2007. 10 Hankins, Defence , 1979; Carson/Schweber, Studies , 1994.

xiii prologue concepts at issue seems possible. Such was, for example, demonstrated masterfully in a biography of Werner Heisenberg.11 What was possible in the case of Sommerfeld’s prize student ought to apply equally to his teacher. It serves at least as an incentive to achieve in a similar manner a successfully integrated presentation of life and work that satisfi es the demands of modern scientifi c biography. In addition to the methodological requirements of a scientifi c biography, the ques- tion of the premises of the history of science also presents itself. Th is concerns fi rst of all the indispensable sources for any biography. In the history of physics, Sommerfeld has long attracted attention, so that eff orts extend far back to gather source material relevant to his work from the perspective of the history of physics. It began in the 1960s, with the work of the project staff of Sources for History of Quantum Physics (SHQP) .12 Together with other sources for the history of quantum physics, many of Sommerfeld’s letters and manuscripts were archived as microfi lm in the Archive for the History of Quantum Physics (AHQP), and made available to historians of science. Focus was primarily on the development of atomic and quantum physics, so that Sommerfeld’s work was fi rst assessed from this perspective.13 But the AHQP archive also off ered the fi rst opportunity for a biographical approach.14 In the wake of the SHQP project, fi nally Sommerfeld’s students such as Alfred Landé (1888–1976), Wolfgang Pauli, and Werner Heisenberg became subjects of publications in the his- tory of physics, so that the importance of the Sommerfeld school came to the fore.15 In the 1980s, the sources on Sommerfeld were augmented by material from the partial estate of the former Sommerfeld Institute for Th eoretical Physics at the University of Munich, and from the holdings of the Sommerfeld family. In the next two decades, an exhibition at the Deutsches Museum, a monograph on the Sommerfeld school, and a two-volume edition of Sommerfeld’s scientifi c correspon- dence, together with an Internet correspondence database, off ered a wealth of insights into the Sommerfeld papers.16 Th is opened up as well the prospect of an overall view of Sommerfeld’s life and work. Specifi cally, the extensive correspondence in the pos- session of Sommerfeld’s heirs now allows light to be shed on the more private aspects of his personality. Th is is the most important precondition to allowing Sommerfeld himself to speak regarding many aspects of his life, conveying narrative authenticity, and remaining true to the genre in the sense of an “existential biography.”

11 Cassidy, Uncertainty , 1992. 12 Kuhn et. al., Sources , 1967. 13 Hermann, Diskussion , 1967; Hermann, Frühgeschichte , 1969; Nisio, Formation , 1973; Kragh, Structure , 1985. 14 Forman/Hermann, Sommerfeld , 1975; Benz, Sommerfeld , 1975. 15 Forman, Alfred Landé , 1970; Forman, Environment , 1967; Forman, Doublet Riddle , 1968; Heilbron, Kossel-Sommerfeld Th eory , 1967; Cassidy, Core Model , 1979; Meyenn, Paulis Weg , 1980, 1981. 16 Eckert et al., Geheimrat , 1984; Eckert, Atomphysiker , 1993; Eckert/Märker, Arnold Sommerfeld. Wissenschaftlicher Briefwechsel , 2000, 2004 (abbreviated hereafter as ASWB I and ASWB II); http://sommerfeld.userweb.mwn.de/AS_WWW.html (28 January 2013). xiv