Mathematics meets physics Studien zur Entwicklung von Mathematik und Physik in ihren Wechselwirkungen
Die Entwicklung von Mathematik und Physik ist durch zahlreiche Verknüpfungen und wechselseitige Beeinflussungen gekennzeichnet. Die in dieser Reihe zusammengefassten Einzelbände behandeln vorran- gig Probleme, die sich aus diesen Wechselwirkungen ergeben. Dabei kann es sich sowohl um historische Darstellungen als auch um die Analyse aktueller Wissenschaftsprozesse handeln; die Untersuchungsge- genstände beziehen sich dabei auf die ganze Disziplin oder auf spezielle Teilgebiete daraus. Karl-Heinz Schlote, Martina Schneider (eds.)
Mathematics meets physics
A contribution to their interaction in the 19th and the first half of the 20th century Die Untersuchung der Wechselbeziehungen zwischen Mathematik und Physik in Deutschland am Beispiel der mitteldeutschen Hochschulen ist ein Projekt im Rahmen des Vorhabens Geschichte der Naturwissenschaften und Mathematik der Sächsischen Akademie der Wissenschaften zu Leipzig.
Das Vorhaben Geschichte der Naturwissenschaften und der Mathematik der Sächsischen Akademie der Wissenschaften wurde im Rahmen des Akademien- programms von der Bundesrepublik Deutschland und dem Freistaat Sachsen gefördert.
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Bibliografische Information der Deutschen Nationalbibliothek Die Deutsche Nationalbibliothek verzeichnet diese Publikation in der Deut- schen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über http://dnb.d-nb.de abrufbar.
ISBN 978-3-8171-1844-1
Die Bilder auf der Umschlagseite zeigen den Physiker O. Wiener (links) und den Mathematiker B. L. van der Waerden (rechts) (Quelle: Universitätsarchiv Leipzig).
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1. Auflage 2011 © Wissenschaftlicher Verlag Harri Deutsch GmbH, Frankfurt am Main, 2011 Druck: fgb – freiburger graphische betriebe
In vielen Bereichen der Naturwissenschaften wird von mathematischer Durchdringung gesprochen, doch gibt es wohl kaum Gebiete, in denen die wechselseitige Beeinflussung stärker ist als zwischen Mathematik und Physik. Ihr Wechselverhältnis war wiederholt Gegenstand erkennt- nistheoretischer und historischer Untersuchungen. Eine wichtige, nur selten im Zentrum der Betrachtungen stehende Frage ist dabei die nach der konkreten Ausgestaltung dieser Wechselbeziehungen, etwa an einer Universität, oder die nach prägenden Merkmalen in der Entwicklung dieser Beziehungen in einem historischen Zeitabschnitt. Diesem Problemkreis widmete sich ein Projekt der Sächsischen Akade- mie der Wissenschaften zu Leipzig, das die Untersuchung der Wechsel- beziehungen zwischen Mathematik und Physik an den mitteldeutschen Universitäten Leipzig, Halle-Wittenberg und Jena in der Zeit vom frühen 19. Jahrhundert bis zum Ende des Zweiten Weltkriegs zum Gegenstand hatte. Das Anliegen dieses Projektes war es, diese Wechselbeziehungen in ihren lokalen Realisierungen an den drei genannten Universitäten zu untersuchen und Schlussfolgerungen hinsichtlich der Entwicklung und Charakterisierung der Wechselbeziehungen abzuleiten. Die in dem Projekt vorgelegten Ergebnisse dokumentieren die große Variabilität in der Ausgestaltung dieser Wechselbeziehungen, die Vielzahl der dabei eine Rolle spielenden Einflussfaktoren sowie deren unterschiedliche Wirkung in Abhängigkeit von der jeweiligen historischen Situation. Auf der internationalen wissenschaftshistorischen Fachtagung «Ma- thematics meets physics – General and local aspects», die vom 22. – 25. März 2010 in Leipzig stattfand, wurden die Ergebnisse dieser lokalen Detailstudien in einen breiteren Kontext eingebettet und mit einem Fach- publikum diskutiert. International anerkannte Wissenschaftshistoriker und Fachwissenschaftler präsentierten ihre Untersuchungsergebnisse zu den Wechselbeziehungen zwischen Mathematik und Physik, wobei sie in ihrer Schwerpunktsetzung die Rolle innerdisziplinärer Entwicklun- gen, einzelner Wissenschaftlerpersönlichkeiten bzw. wissenschaftlicher ii Vorwort
Schulen oder institutioneller Veränderungen in den Mittelpunkt ihrer Analysen rückten und somit die in dem Akademieprojekt gewonnenen Erkenntnisse in vielerlei Hinsicht ergänzten. Außerdem versuchten einzelne Referenten von einem allgemeineren, philosophischen Stand- punkt aus, das Wesen und die Entwicklungslinien der Wechselbezie- hungen zwischen Mathematik und Physik durch einige Merkmale zu charakterisieren. Im Ergebnis lieferte die Konferenz einen guten Einblick einerseits in die aktuellen Forschungen zu den Beziehungen zwischen Mathematik und Physik mit all ihrer Diversität und andererseits in die auch in der abschließenden Podiumsdiskussion formulierte, wohl etwas überraschende Einsicht, dass die in früheren Darstellungen skizzierte kontinuierliche Entwicklung der Wechselbeziehungen einer deutlichen Revision und Spezifizierung bedarf. Mit diesem Tagungsband werden die vorgetragenen Ergebnisse nun einer breiten wissenschaftlichen Öffentlichkeit vorgelegt. Dabei will der Band nicht nur Einblicke in die gegenwärtige Forschung gewähren, sondern zugleich neue Untersuchungen anregen. Er enthält 14 der insgesamt 18 präsentierten Vorträge in einer überarbeiteten Fassung. Vier der Tagungsteilnehmer haben aus unterschiedlichen Gründen ihr Referat leider nicht zur Publikation eingereicht. In einigen Fällen werden sie ihre Ergebnisse in ein größeres eigenes Werk einfließen lassen. Um dem Leser einen vollständigen Überblick über die vorgetragenen Themen zu geben, ist am Ende des Buches das Tagungsprogramm angefügt. Die Konferenzsprachen waren Deutsch und Englisch. Wir haben als Herausgeber diesen zweisprachigen Charakter der Tagung bewusst für diesen Band übernommen und es den Autoren überlassen, die Ausarbeitung ihres Vortrags in Deutsch oder in Englisch zu präsentieren. In den meisten Fällen gab es sowohl gute Gründe für die Wahl der deutschen Sprache, als auch für die Wahl des Englischen. In die Gestaltung der Artikel haben wir nur sehr vorsichtig und nur formale, keine inhaltlichen Aspekte betreffend eingegriffen. Ne- ben der Anpassung an ein einheitliches Layout wurde jedem Artikel eine inhaltliche Übersicht vorangestellt, die wir aus der vom Autor vorgenommenen Gliederung seines Beitrags erzeugten. Aufgrund der individuellen Gewohnheiten ergaben sich dabei deutliche Unterschiede zwischen den einzelnen Artikeln, die wir als persönliche Note des Autors interpretiert haben und nicht versuchten zu beseitigen. Die Angaben zur Vorwort iii verwendeten Literatur und die Zitierweise wurde von uns ebenfalls nicht vereinheitlicht. Dennoch folgen sie im Wesentlichen einem einheitlichen Schema, indem die Verweise in den Fußnoten bei der Literatur durch Angabe des Autors und des Erscheinungsjahres bzw. bei Archivalien entsprechend der offiziellen Abkürzungen der Archive vorgenommen werden. Sind von einem Wissenschaftler mehrere Arbeiten aus einem Jahr aufgeführt worden, so wird an die Jahreszahl der Buchstabe a, b oder c entsprechend der Auflistung im Literaturverzeichnis angefügt. Die Auflösung der Kürzel wird im Literaturverzeichnis vorgenommen, das am Ende des jeweiligen Artikels steht. Während das Literatur- und Quellenverzeichnis bei dem jeweiligen Artikel belassen wurde, sind die Personennamen in einem gemeinsamen Personenverzeichnis am Ende des Buches zusammengestellt. Soweit bekannt bzw. ermittelbar wurden die Lebensdaten der Personen angefügt. Schließlich haben wir die Reihenfolge der Artikel aus inhaltlichen Gründen gegenüber der Vortragsfolge im Programm leicht abgeändert. Die Durchführung der Tagung in dem geplanten Umfang wurde erst durch die finanzielle Unterstützung seitens der Deutschen Forschungs- gemeinschaft möglich. Für diese Hilfe danken wir sehr herzlich. Ebenso danken wir der International Commission on the History of Mathematics, die die Tagung als förderungswürdig anerkannte und ihr eine größere, internationale Aufmerksamkeit verschaffte. Bei der Vorbereitung der Tagung stand uns die Kommission für Wissenschaftsgeschichte der Sächsischen Akademie der Wissenschaften zu Leipzig, insbesondere ihr Vorsitzender Herr Professor M. Folkerts, mit Rat und Tat zur Seite, wofür wir uns herzlich bedanken. Doch was wäre eine Tagung ohne die fleißigen Helfer im Hintergrund. Ein besonderer Dank gilt diesbezüglich mehreren Mitarbeitern in der Verwaltung der Akademie, von denen stellvertretend Frau E. Kotthoff und Herr A. Dill besonders genannt seien. Bei der Drucklegung des Buches konnten wir wie gewohnt auf die gute Zusammenarbeit mit dem Verlag, speziell Herrn K. Horn, und dessen Kooperationspartner Herrn Dr. S. Naake bauen. Trotz des gegenüber vorangegangenen Publikationen deutlich größeren Aufwandes hat Herr Dr. Naake uns bei der Gestaltung des Buches sehr kompetent beraten sowie unsere Vorstellungen mit viel Geduld und großer Sorgfalt umgesetzt, beiden einen herzlichen Dank. Weiterhin möchten wir Frau iv Vorwort
Dr. H. Kühn für ihre tatkräftige Unterstützung und die zahlreichen Hinweise bei der Vorbereitung des Buchmanuskripts und während der Fahnenkorrektur danken. Die Tagung ist ein wichtiges Element des eingangs genannten For- schungsprojektes der Sächsischen Akademie im Rahmen des Akade- mievorhabens: «Geschichte der Naturwissenschaften und der Mathe- matik». Dem Bundesministerium für Bildung und Forschung sowie dem Sächsischen Staatsministerium für Wissenschaft und Kunst danken wir für die finanzielle Absicherung dieses Akademieunternehmens und somit auch des Druckes dieses Tagungsbandes. Der Band bildet die Abschlussveröffentlichung des Projektes und ist zugleich die letzte Publikation in dem in einem Monat auslaufenden Akademievorhaben.
Mainz/Leipzig, November 2010
Martina Schneider Karl-Heinz Schlote Inhaltsverzeichnis
Karl-Heinz Schlote, Martina Schneider: Introduction ...... 1
I Allgemeine Entwicklungen | General developments 15 Jesper Lützen: Examples and Reflections on the Interplay between Mathematics and Physics in the 19th and 20th Century ...... 17 Juraj Šebesta: Mathematics as one of the basic Pillars of physical Theory: a historical and epistemological Survey ...... 43
II Lokale Kontexte | Local contexts 61 Karl-Heinz Schlote, Martina Schneider: The Interrelation between Mathematics and Physics at the Universities Jena, Halle-Wittenberg and Leipzig – a Comparison 63 Karin Reich: Der erste Professor für Theoretische Physik an der Universität Hamburg: Wilhelm Lenz ...... 89 Jim Ritter: Geometry as Physics: Oswald Veblen and the Princeton School . 145
III Wissenschaftler | Scientists 181 Erhard Scholz: Mathematische Physik bei Hermann Weyl – zwischen «Hegel- scher Physik» und «symbolischer Konstruktion der Wirklichkeit» 183 vi Inhaltsverzeichnis
Scott Walter: Henri Poincaré, theoretical Physics, and Relativity Theory in Paris ...... 213 Reinhard Siegmund-Schultze: Indeterminismus vor der Quantenmechanik: Richard von Mises’ wahrscheinlichkeitstheoretischer Purismus in der Theorie physika- lischer Prozesse ...... 241 Christoph Lehner: Mathematical Foundations and physical Visions: Pascual Jordan and the Field Theory Program ...... 271
IV Entwicklung von Konzepten und Theorien | Develop- ment of concepts and theories 295 Jan Lacki: From Matrices to Hilbert Spaces: The Interplay of Physics and Mathematics in the Rise of Quantum Mechanics ...... 297 Helge Kragh: Mathematics, Relativity, and Quantum Wave Equations ..... 351 Klaus-Heinrich Peters: Mathematische und phänomenologische Strenge: Distributionen in der Quantenmechanik und -feldtheorie ...... 373 Arianna Borrelli: Angular Momentum between Physics and Mathematics ..... 395 Friedrich Steinle: Die Entstehung der Feldtheorie: ein ungewöhnlicher Fall der Wechselwirkung von Physik und Mathematik? ...... 441
Vortragsprogramm ...... 487 [lies: 486] Liste der Autoren ...... 489 [lies: 488] Personenverzeichnis ...... 493 [lies: 491] Introduction
Karl-Heinz Schlote, Martina Schneider
Mathematics and physics have interacted in Western science for a very long time. Both disciplines have profited from this on-going process in many ways. To capture the dynamics of this long-lasting interaction is far from simple since a multitude of aspects has to be taken into account. Analytically one might distinguish between developments on different levels of this interrelationship: individual, local, institutional, disciplinary, global. Of course, the outcome of this interaction, i. e. theories and concepts, is part of the parcel. On an individual level, questions about the training of a scientist (who were his or her teachers? which books were studied? which courses were attended?), about his or her contacts, collaboration or publications with other scientists may be asked. This might already reveal networks operating on a local, regional, national or international level. On a local level one might also consider connections between different departments of a university or more generally with local learned societies that focus on mathematics and/or natural sciences, or with local industry. Of course, we should also investigate how the two disciplines were institutionalized and in particular, how mathematical and theoretical physics were institutionalized. Were there chairs especially designated to these fields? How and when were they created? Can we identify a local research group or even a research school with a research program drawing on mathematics and physics alike? If so, what about the training of junior researchers? Or do we find individuals who had a lasting influence on the development? What about the establishment of institutes for mathematical or theoretical physics? Here we touch upon questions of professionalization, institutionalization and the genesis and development of disciplines. 2 Introduction
With regard to the last aspect certain subdisciplines or fields of knowledge – however difficult they are to distinguish – might attract special attention like mathematical and theoretical physics. However, as shown by the articles in the conference proceedings, interaction between mathematics and physics may also take place, perhaps unexpectedly, in other parts of the disciplines, like in experimental physics where experiments might be designed differently to meet the demands of mathematical-theoretical theory building and in pure mathematics in which certain developments are stimulated, advanced, or even triggered by this interplay. We can detect the co-evolution of knowledge, i. e. parallel, but separate developments in the two fields leading to similar concepts or theories, but also developments that are temporally and disciplinarily distinct. Sometimes both fields of knowledge are re-modelled by this interaction.
If a wider context than that of the two disciplines is considered, then their interaction can be studied from a philosophical, or rather epistemological point of view. This can also be done with regard to the various opinions on this subject held by the working scientists. Furthermore, the relations with the development of technology and with industry might be a fruitful perspective. But, of course, other social or cultural developments might be important for the shaping of knowledge in the field, e. g. the two World Wars or also a general public fascination for certain events or gadgets.
Finally, we might look for global dynamics of the interaction between mathematics and physics. This perspective still seems to be rather difficult when applied to the 19th and the first half of the 20th century since a general, global pattern is not clear, apart from the truism that there exists a growing tendency to mathematize not only physics, but also other branches of knowledge. The dynamics of the interrelationship is so diverse, driven by individuals and/or local research groups who followed often very different ideas, had different aims and used different methods. Instead of trying to unify these different approaches and dynamics, the conference proceedings aim at showing the wealth of forms in which the interrelationship between mathematics and physics manifested itself on the different levels during the 19th and the first half of the 20th century. Introduction 3
The interaction of mathematics and physics is often treated under the heading “the mathematization of physics”. This mathematization does not usually consist of the simple, tool-like application of mathematics (or, to be more precise, of a mathematical theory or concept) to a field of physical knowledge, but rather in this meeting of mathematics and physics both fields undergo changes and modifications – varying in scale – in order to facilitate an input of mathematics into physics. With the mathematization extending to more and more branches of physics and with mathematics increasingly becoming the language of physics, these modifications might become less and less apparent. However, there were and still are some areas of physics, in which mathematics is not – maybe not yet, maybe never – of importance for the understanding of the field. As numerous studies in the history of science have shown, the mathematization of physics takes different forms: There are publications dealing with axiomatization or formalization of existing physical theo- ries. Quantification might play a central role. The elaboration of existing physical theories with regard to mathematical rigour, completeness, exactness and/or consistency is another form of mathematization. Sometimes physical theories are modified on mathematical grounds. Apart from the obvious function to deepen the understanding of physics, mathematization is of particular use for clarifying the structure of a physical theory as well as its formal problems. Sometimes a mathematical concise description of phenomena is achieved. In addition to that, mathematization can contribute to the unification of formerly disjoint fields of physical knowledge, of some of their concepts or even of hitherto different phenomena. In some cases, a new theory is created by mathematization, entailing a new understanding of the phenomena in question. For some scientists, mathematical features, even such vague ones as mathematical simplicity or mathematical beauty, might act as a guide for physical theory-building. If one studies the mathematical-technical side of this process and asks what kind of mathematics is ‘applied’, one is also faced with a great variety. On the one hand, well-established mathematical theories (one might call them ‘old’ theories) are drawn on to capture physical phenomena. Often, these mathematical theories or mathematized physical theories had been applied successfully in other fields of physics 4 Introduction
before, like calculus or mechanics. On the other hand, also new concepts and new mathematical theories are applied or even created. Sometimes mathematical structures are rediscovered by physicists. The mathematical techniques can be found both on a local level, to solve a specific problem as well as on a more global level that affects larger parts of a theory. Various arguments are given by the scientists to legitimize the math- ematization of parts of physics, if this aspect is addressed at all. Scientists might refer to experiments, general experience, consistency with former theories etc. to explain their engagement in mathematization. They might also refer to philosophy to explain and support their practice or even build their own epistemological frameworks. Other important aspects to be investigated are the publication and reception of attempts to mathematize physics. In which journals were the articles published? When did the first monographs and textbooks appear? Do we find joint publications by mathematicians and physicists? How, when and by whom was a certain approach taken up or discarded? Did the topics of mathematization reach the interest of the general public? However, only looking at the mathematization of physics results in a distorted picture of the vast panorama of the interrelationship of mathematics and physics since the impact of physics on mathematical research is missing. One does not have to go as far as to speak of a “physicalization of mathematics” (A. Schirrmacher) to acknowledge the stimulation mathematics gets from the physical sciences. Physics is not only a source of ideas and inspiration for mathematical research. A lot of mathematics was created to solve physical problems. In this process, also physicists have come up with new mathematical entities and concepts worth studying from a mathematical point of view. They, at times, even take up tasks primarily ascribed to mathematics, doing truly mathematical research. Physics may also function to legitimize research in a certain mathematical area. One might even ask in which way the dynamics of mathematical research, e. g. the temporary blossoming of a mathematical field, is related to physical relevance. In general, this side of the interaction of mathematics and physics has not been studied so thoroughly as the mathematization of physics – which is also reflected Introduction 5 in the conference proceedings. In our opinion, however, this process needs further investigation. A brief survey of the papers of the conference proceedings will now be given and some of the dynamics of the interaction between the two disciplines just mentioned will be pointed out. The first section deals with the interrelationship between mathematics and physics from a more general perspective. J. Lützen discusses in what way one could describe this interrelationship as an application of mathematics to physics. To capture its features Lützen introduces the following image: The application of mathematics to physics cannot be compared to the use of a stone for opening up an oyster, but rather to the use of a screwdriver on a screw. Just like a screw and a screwdriver, mathematics and physics can hardly be perceived independently of each other. To underpin this claim Lützen gives a series of examples: how central parts of Liouville’s mathematics was inspired by and based on physical problems; how the development of non-Euclidean geometry can be seen as part of a physical investigation; how a geometrical form led Heinrich Hertz to the introduction of the concept of “Massetheilchen” into his mechanics; how Schwartz’s work on the theory of distributions was inspired by physics. J. Šebesta takes a different approach to the topic. From the perspective of a theoretical physicist, he points to the usefulness of mathematics for physics. He differentiates between two roles of mathematics with respect to theory-building in physics: mathematics as a tool and its epistemological role. He illustrates his scheme by sketching numerous examples ranging from the time of Galileo Galilei and Johannes Kepler to the 20th century and drawn from diverse physical fields. In doing so, he points to various shifts in the relationship between mathematics and physics with respect to the creation of physical theories, e. g. an increasing distance from the empirical basis or the resort to new, hitherto unused mathematical concepts and results. He suggests putting this down to a fundamental asymmetry between empirical and theoretical cognition. The local level of the dynamics of the interaction between mathematics and physics is the focus of the second section, which consists of three papers, the first two paying special attention to the institutionalization of theoretical (and mathematical) physics and to the denomination policy. In the first paper, K.-H. Schlote and M. Schneider compare 6 Introduction
the developments at three German universities (Jena, Halle-Wittenberg, Leipzig) during the period from 1815 to 1945. During the first half of the 19th century the philosophical foundation of the interrelationship was of central importance to Jakob Friedrich Fries in Jena. Later on questions of optics closely connected to the construction of instruments dominated the research of physicists in Jena. The later development has to be seen in the context of the local optical industry (Carl Zeiss, Otto Schott), since in Jena industry and university were closely linked with respect to finance and personnel. The development at Halle university from 1895 onwards illustrates how theoretical and experimental research were interwoven in the research of theoretical (and also experimental) physicists. In Leipzig one can witness the early establishment of a strong tradition in mathematical physics which delayed the institutionalization of theoretical physics. It was only in the 1920s that Leipzig succeeded in becoming one of the leading centres of quantum mechanics, atomic and molecular physics by appointing Peter Debye, Werner Heisenberg and Friedrich Hund. The mathematician Bartel Leendert van der Waerden took on the role of their mathematical advisor. Thus, a wealth of dynamics on various levels could be captured by this research. In the second paper, K. Reich gives a survey of the development of theoretical physics at Hamburg university from the time of its foundation to 1959 when the plan for a particle accelerator (DESY) took shape. She concentrates on Wilhelm Lenz, who initiated a small research group, and some of his students and collaborators (Ernst Ising, Wolfgang Pauli, Werner Theis, Hans Jensen, Erwin David). With respect to the interrelation of mathematics and physics a couple of features are mentioned, e. g. the mathematicians’ support (in particular that of Wilhelm Blaschke) in establishing a professorship for theoretical physics in 1921, their commitment to lecturing on relativity theory (Erich Hecke, Blaschke, Emil Artin) and Pauli’s presence in Artin’s lecture course on hyper-complex systems (algebras). Reich also points out developments related to the rule of the National Socialists and thus touches upon influences from the political context. This thread also plays a prominent role in the third paper – J. Ritter’s paper on Oswald Veblen, Luther P. Eisenhart and the Princeton School in the 1920s and early 1930s. Veblen’s activity in military research during the First World War led not only to a re-orientation of his research to Introduction 7 differential geometry in order to integrate mathematics and physics in a new synthesis, but also to a re-organization of his research as a collective enterprise – both aiming at raising the US, and in particular Princeton, to a world-class scientific power. Ritter describes how mathematics and physics are re-modelled in this process by showing their essential unity and by drawing on lessons from the past to inspire new geometry and physics. The Princeton scientists who were part of the research group pursued different strategies of mathematization for different problems. They offered rigour and completeness to existing theories (Eisenhart working on Weyl’s and Einstein’s theories), modifying exist- ing theories on the basis of mathematical demands (Tracy Y. Thomas on the same topic), and putting forward their own new physical theories (Veblen/Hoffmann on relativity). After the Second World War this unity between mathematics and physics for which the Princeton group strove was lost, ironically due to the work of Princeton geometers of the post-Veblen period. By focusing on Veblen and showing the links to work done outside Princeton as well as the contacts with scientists outside Princeton, by focusing on institutional as well as mathematical-technical aspects of the interaction of mathematics and physics, Ritter integrates various perspectives in his study and is able to give a very rich analysis. The individual element of the interrelations between mathematics and physics is the focus of the articles of the third section. Each of them is centred on a prominent scientist whose work and impact is analysed with regard to those interrelations. E. Scholz investigates what Weyl’s research tells us about Weyl’s varying conception of the interrelation between mathematics and physics. He identifies three forms of applying mathematics to physics in Weyl’s research: firstly, mathematical contributions with a mainly speculative and a priori claim of recognition for physics, secondly, the analysis of concepts regarding the foundation of physics, and thirdly the use of mathematics in a structural function or rather as the essence of what Scholz calls “the symbolic construction of reality”. These forms do not characterize different periods of Weyl’s thought with one following the other, but rather can be found in varying degrees in Weyl’s work – thus giving a complex picture of his conception of the interrelationship. At the same time Scholz’s analysis shows the stimulating impact of physics on various fields of mathematical research. The general theory 8 Introduction
of relativity, the rising quantum mechanics and unified field theories caused Hermann Weyl to engage in very different ways in theoretical physics. Although at times he was in close contact with physicists, discussing his ideas, he never became the founder of a school or a centre of research. The reception of the special theory of relativity in France and the role of Henri Poincaré within this process in combination with a broader view on the state of theoretical physics in France at the turn to the 20th century form the core of S. Walter’s contribution. Walter points to the striking differences in this reception between France, Germany and some other countries which is manifested by a quantitative analysis of publications. He then analyses Poincaré’s activities in physics in the decades around 1900. This qualitative analysis shows not only Poincaré’s intellectual and institutional dominant position in French science and his great impact on theoretical physics in France, but also reveals some of his controversial judgements on the studies of other physicists, which quickly turned out to be misjudgements. Poincaré launched his own theory of relativity mainly before 1905 and in 1912 he recognized the philosophical significance of the Einstein-Minkowski theory of relativity. The Einstein-Minkowski theory first received strong support from French physicists in their publications in 1911. The change to this theory is connected with the physicists Paul Langevin, who became Poincaré’s successor, Jean Perrin and Ernest Maurice Lémeray as well as with mathematicians like Émile Borel and Élie Cartan, and thus to a generational change. With the emergence of relativity and quantum theory in the first half of the 20th century the interrelationship of mathematics and physics entered a new phase. The problem of indeterminism was one aspect that was at the heart of this process in the 1920s. It entered physics especially in the work of James Clerk Maxwell and Ludwig Boltzmann on thermodynamics in the 19th century and gave rise to lively discus- sions especially among German physicists and mathematicians in the 1920s. In this context R. Siegmund-Schultze calls our attention to von Mises’ contribution to the description of physical processes by applying probabilistic and statistical methods that was published in 1920 and is nearly forgotten today. He assesses the impact of von Mises’ contribution as largely a methodological-philosophical one in the discussion of the Introduction 9 role of indeterminism in physics which weakened the postulate of determinism. In this connection, he also re-evaluated von Mises’ role that P. Forman attributed to him in his well-known work which links the discussions about indeterminism with social conditions of the “Weimar” Republic. In his research, Richard von Mises touched upon some topics that only later became prominent with the further progress in quantum physics. This, in turn, had also some influence on his research in the late 1920s which includes his contributions to ergodic theory, the theory of stochastic processes, the statistical applications in physics, as well as his changing opinion on the problem of indeterminism. From a more general point of view, this analysis elucidates the difficulties arising from the description of complicated physical processes that consist of the movement of many separate particles. C. Lehner puts special emphasis on the changes in theoretical physics which took place in the first third of the 20th century. They led to a new assessment of theoretical physics. Theoretical physicists became much more engaged with mathematics than before. Discussing the work of Pascual Jordan and his visionary program of a unified quantum field theory, Lehner tries to demonstrate that a new generation of physicists launched a new idea of theoretical physics. Comparing Jordan’s approach to the research done by contemporaries like Erwin Schrödinger, Paul Dirac, and John von Neumann, he gives an impression of the richness in the theoretical variety that a study of this process offers. Lehner attempts not so much to sketch some features of the new stage of theoretical physics as to stress mainly the peculiarities of Jordan’s situation. Formed by the unique Goettingen atmosphere of the 1920s, Jordan seems to have regarded mathematics and physics as essentially not distinct. Jordan went beyond what might be seen as the typical task of a mathematical physicist, “the precise elaboration of existent physical theories” and put forward far-reaching ideas about the foundations of quantum physics. He realized that new paths had to be taken and he worked on them in quantum field theory in particular. His contributions to this field were, however, not well received by his contemporaries. He never reached a solid mathematical ground and abandoned this line of research in 1929. Lehner points out that, taken from a wider perspective, the two characteristic features of modern theoretical physics, radical positivism and fundamental universalism, were at odds in Jordan’s 10 Introduction
research. With regard to the interplay of mathematical and theoretical physics, the changing roles of visualization and theoretical models elaborated in this context are also worth mentioning. Many questions and problems regarding the interrelations between mathematics and physics that arose in the realm of individual research activities also emerged in the context of analysing the development of concepts and theories. These topics are treated from this broader point of view in the last section. The section partly deals with the same fields of physics as the papers sketched above. J. Lacki looks in detail at the foundation of quantum mechanics. Sketching the historical situation in the mid 1920s, he points to the four different formalisms that existed in quantum mechanics: wave mechanics, matrix mechanics, q-numbers, and operator calculus. Each used different types of mathematical objects and followed a varying logical order of presentation. He is able to identify a common feature of the representatives of these four formalisms: mathematicians and physicists alike lacked both a geometrical intuition and an insight into the linear structure of their respective issues and problems. The decisive step taken to change this was initiated by the physicist Fritz London. Striving for a better understanding of the equivalence of matrix mechanics and wave mechanics, he studied the analogy between the transformations of variables which occur in solving the quantum mechanical problem and the classical canonical transformations of coordinates. London realized the linearity of the operations, stressed the importance of the linear structure and referred to the publications of mathematicians like Salvatore Pincherle and Paul Levy. Today these publications count as pillars in the early history of functional analysis. There followed an interesting interplay between quantum mechanics and functional analysis including the theory of linear spaces that found its first important outcome in von Neumann’s foundation of quantum mechanics which was based on the theory of operators in Hilbert space. In addition to the intensive mutual influences between these fields, it is noteworthy that the essential impulse towards the structural property was given by a physicist. Investigating the construction of the relativistic wave equation of the electron, H. Kragh gives an interesting instance of the interplay between mathematics and physics in the late 1920s. At that time the theory of the Introduction 11 electron that the physicists were searching for had to be compatible with both the new quantum mechanics and special relativity and, in addition to that, was also to include the spin of the electron. The solution to this task was a result of Dirac’s work. His crucial step was the reduction of the physical problem to a mathematical one. Then Dirac got his results by “playing around with mathematics” and by disregarding physics. On the one hand, the introduction of Dirac’s relativistic wave equation meant that physicists had to leave their path as far as the mathematical description of the problem was concerned. On the other hand, the objects introduced by Dirac, like special operators and matrices, stimulated a lot of new mathematical research on Clifford algebras and operator theory. Thus, both disciplines profited in this process. Dirac himself appreciated mathematics as a useful tool for physical research. Later on, Dirac pointed to the vague concept of mathematical beauty as a strong motivation for him. Kragh, however, claims that this concept did not influence Dirac’s way of research on the relativistic wave equation. One of Dirac’s ideas, the famous delta-function, also forms a starting point of K.-H. Peters’ article. Peters covers the use of the delta-function and other generalized functions in quantum mechanics as well as in quantum field theories later on and the establishment of a mathematical rigorous theory of these functions. He draws our attention to a new and unusual point of view, culminating in the hypothesis that mathematical and phenomenological rigour (mathematische und phänomenologische Strenge) are closely correlated. According to Peters, phenomenological rigour is the rule that the mathematical linking of (physical) facts has to be connected only with the facts themselves and not with any causes in the back. From this point of view, Peters interprets the process of the rigorization of mathematical concepts and methods used in a physical theory as a process that promotes a focusing on real observable facts. Hence, the installation of mathematical rigour removes imaginary magnitudes within the physical theory and makes it more realistic – a total reversal of the common interpretation of the interrelations between mathematics and physics. Nevertheless, further investigation has to sharpen the definition of the concept of “phenomenological rigour” and to test whether this hypothesis holds true in other contexts. The genesis of concepts as well as their development including their transition into new physical fields form an important aspect of the 12 Introduction
interrelations between mathematics and physics. A. Borrelli analyses in detail how the concept of angular momentum emerged in mechanics, electrodynamics, quantum mechanics, and quantum field theory as well as its adaptation from mechanics to quantum mechanics. Her analysis deals with the development in France, Great Britain and Germany in the period from 17th to the early 20th century. She explains why the various concepts of angular momentum which differ are nevertheless perceived as the “same” physical concept. Her research shows that there existed a close correlation between certain aspects of physical and mathematical practices. Borrelli does not stop here, but also embeds the development in wider, philosophical and technological contexts. The concept of angular momentum is shaped and constantly supported by ongoing interactions and unresolved tensions between these different fields. Like other such concepts, it emerged from a co-evolution of mathematics and physics. As mentioned above, there exist some fields in physics, like the theories of electricity, magnetism, heat, or colour, that for a long time developed on an empirical-experimental basis without any connection with mathematics. A mathematization of these fields took place only in a late stage of their development. F. Steinle examines this process with respect to the genesis of the electromagnetic field theory in the works of Michael Faraday, William Thomson, and Maxwell. He starts by giving three common features which can be ascribed to such a process in general: Firstly, the mathematical analysis starts with a specific hypothesis about a hidden mechanism at a microscopic level and uses methods from other physical fields, especially from mechanics. These methods might have to be modified and adapted to the special case under investigation. Secondly, experiments are designed, specified and evaluated on the basis of these mechanical speculations. They serve either to correct and refine the hypothesis or to improve the data basis and to determine quantitative trends. Thirdly, the scientists who are engaged in this process are acquainted both with mathematics and physics and contribute to empirical as well as to mathematical research. Steinle then shows that this pattern does not apply to his case. In his case, the experimental investigations and the forming of qualitative concepts by Faraday was completely separated from the later proper mathematization done by Thomson and Maxwell. This mathematization Introduction 13 included the development of new mathematical concepts and methods and the difficult process of adjusting Faraday’s more qualitative concepts to those ones. Although Faraday knew little about mathematics, some of his concepts were also of a mathematical character – so much so that Maxwell characterized him later as a “mathematician of very high order”. Faraday tried to introduce an appropriate system of reference that was to handle the various dependencies. This system showed similarities with the geometry of position. Furthermore, Faraday’s experiments did not have any relation to microphysical speculations and an explorative character. Steinle then shows how Thomson and Maxwell drew upon Faraday’s results in their mathematization of electrodynamics, and why in Maxwell’s view Faraday could be called a mathematician. Steinle’s study opens up a new view on the relations between mathematics and experimental physics which introduces new facets of their interaction and points to the role of experimental physics in theory-building. To sum up, the papers in the conference proceedings display the many different ways and circumstances how the two disciplines, mathematics and physics, and their practitioners come together, how they “meet” and how many different forms these “meetings” can take. The picture of the interaction which is drawn here is a multi-facetted one, a kind of mosaic. In order to understand this interaction more thoroughly, more research on the dynamics at all levels and in various contexts (philosophical, technological, social, economical, etc.) is required. This could give us a useful basis for a comparison with other processes of mathematization in other disciplines.
Part I
Allgemeine Entwicklungen | General developments
Examples and Reflections on the Interplay between Mathematics and Physics in the 19th and 20th Century
Jesper Lützen 1 Introduction ...... 18 2 Application of mathematics to physics ...... 18 2.1 The nature and development of mathematics ..... 18 2.2 The nature of physics ...... 20 2.3 Encounters of mathematics and physics ...... 21 3 The mathematics of Joseph Liouville. Early and mid-19th century ...... 21 3.1 Electro-statics and conformal mappings ...... 22 3.2 Spectral theory of a class of integral operators ..... 23 3.3 Heat and Sturm-Liouville theory ...... 25 3.4 Electrodynamics and differentiation of arbitrary order 26 4 Heinrich Hertz’s Principles of Mechanics. Late 19th century . 29 4.1 Pure mathematics? Neo-humanism ...... 29 4.2 Heinrich Hertz’s mechanics ...... 30 4.3 Physics influences mathematics: Reduced compo- nents of a vector ...... 31 4.4 Mathematics influences physics: Massenteilchen . . . 31 5 The theory of distributions. Early and mid-20th century . . . 33 5.1 Mathematical autonomy and structures ...... 33 5.2 Distributions: a functional analytic generalization of functions ...... 34 5.3 Early δ-functions ...... 35 5.4 Generalized solutions of partial differential equations 36 5.5 Generalized Fourier transform ...... 37 5.6 Distributions between structuralism and applications 37 6 Conclusion ...... 38 7 Bibliography ...... 38 18 Part I. Allgemeine Entwicklungen | General developments
1 Introduction
From the 17th century and till this day mathematics and physics have been closely linked. However, the nature of this link has been considered differently by different mathematicians, physicists and philosophers. I shall begin this paper with a discussion of the nature of the interaction between the two subjects and continue with some examples of such interactions from the 19th and 20th century. The examples will be taken from my earlier research on Joseph Liouville, Heinrich Hertz and the prehistory of the theory of distributions.
2 Application of mathematics to physics
The interaction between mathematics and physics is usually described as an application of mathematics to physics. This asymmetrical description of the relationship is justified in the sense that mathematics provides a precise and quantitative language to describe (understand) physical phenomena and even to predict new phenomena. And indeed, applica- tions of mathematics to physics are more common than applications of physics to mathematics, although one can find examples of the latter. Still, the relation between the two subjects is often misrepresented as an interaction between two independent fields similar to a Stone Age man’s application of a stone to crack an oyster. Here the stone and the oyster have developed rather independently of each other and nothing would prevent the one from existing without the other. However, as I shall try to argue in this paper, the application of mathematics to physics is much more like the application of a screwdriver to a screw. Here the screw and the screwdriver have developed together and the one would be unthinkable without the other. Similarly, mathematics and physics have developed in close and continuous interaction and their present form is clearly marked by this close connection.
2.1 The nature and development of mathematics
Mathematics is often characterized as an a priori creation of the human intellect, untainted by external factors such as the behavior of nature. This view of mathematics is probably philosophically defendable, in particular if one adheres to a formalistic philosophy of mathematics LÜTZEN: Examples and Reflections on the Interplay between Math. and Physics 19 according to which mathematics is a game played within mathematical structures. In this game one starts from arbitrary (but hopefully consistent) axioms and then uses some (arbitrary) rules of inference to deduce theorems in a cumulative process. Such a view of mathematics has no need for external input. On the contrary, external input is more likely to be seen as potentially dangerous diverting factors that may lead the rigorous process astray. Yet, during the last half century a growing number of philosophers have called attention to the unsatisfactory nature of the formalistic philosophy of mathematics. In particular it has been noticed, that this philosophy cannot account for the development of mathematics because it gives no idea of the forces driving the development. In order to understand the dynamics of the development of mathematics, philosophers have turned to history. One of the first and probably the most famous of these historically inspired philosophers of mathematics is Imre Lakatos. In his Proofs and Refutations1 he describes the development of mathematics as a dialectic process. It starts with a conjecture, continues with a proof that establishes the conjecture or a variation of it as a theorem. Then often follows a refutation mostly in the form of a counterexample leading to a new refined conjecture, after which the process can repeat itself. With this description of the development of mathematics Lakatos takes exception to the idea of a cumulative development of mathematics. Still, he describes the development as a highly internal affair. The original conjecture may be suggested by external facts, but the subsequent dialectic process is described as a purely mathematical one. The internal nature of the process is highlighted by the closed confines of the classroom which Lakatos has chosen as the scene of his rational reconstruction. There are probably areas of very pure mathematics such as number theory (before RSA coding) that has developed according to a purely internal mathematical dynamics, but many (or most) of the mathematical theories that are useful in physics (much of analysis and geometry) have developed in close connection with physics. Of course there is also a great deal of purely internal mathematical dynamics in the development
1 Lakatos 1976 20 Part I. Allgemeine Entwicklungen | General developments
of applicable mathematical theories, but even a few occasional pushes from outside may contribute decisively to the overall course of the development. Thus the idea of a completely a priori field of mathematics that suddenly turns out to be applicable to the description of nature is often a myth.
2.2 The nature of physics
Similarly physics depends on mathematics. Of course it is reasonable to assume that nature exists independently of us and indeed there are physical phenomena that are describable without mathematics (e. g. phenomena of colors and Ørsted’s experiment). But physics as it has developed in western civilizations is mostly a mathematical description of the world. Even the entities that the physicist tries to describe are for a great deal of a mathematical nature. Basic concepts such as time, space and mass may arguably be said to be non-mathematical in nature, but their quantification relies on simple arithmetic. And the formation of more complex entities has required more complex mathematics. Take for example the concept of velocity. Just the idea of the velocity Δx/Δt of a uniform motion could only be formulated after one had overcome the Greek convention that only quantities of the same kind could have a ratio. And the precise formulation of the concept of an instantaneous velocity of a non-uniform motion developed hand in hand with differential calculus. Similarly, energy was only conceptualized in the mid-19th century after it had been noticed that most forces of nature could be written as the gradient of a potential function, and after it had been noticed that if one changed the sign on this function it was a quantity similar to the vis viva ∑ 1 2 2 mv . Indeed the sum of the two was often preserved (conservation of energy). Thus it required rather sophisticated mathematics to conceptualize the notion of energy that today we may consider a rather elementary physical notion. So when we say that mathematics is the language that allows us to formulate the laws of nature, we must also admit that the concepts whose properties are described by these laws are often saturated with mathematics from the beginning. So the idea of a non-mathematical LÜTZEN: Examples and Reflections on the Interplay between Math. and Physics 21 realm of physics to which mathematics can be applied is likewise often a myth.
2.3 Encounters of mathematics and physics
So rather than talking about application of mathematics to physics it would be better to talk about encounters of the two subjects. A piece of mathematics meets a piece of physics; they interact and out comes a new mathematical description (model) of this piece of physics. In most cases the interaction also leads to a development of the ingoing piece of mathematics as well as the more general view of physics. In the rest of this paper I shall give some examples from the 19th and 20th century of such meetings of mathematics and physics. In most of the cases I shall highlight the changes that mathematics underwent as a result of the meetings. Only in the case of Hertz’s mechanics I shall discuss the more obvious change of physical theory.
3 The mathematics of Joseph Liouville. Early and mid-19th century
During the period 1770 – 1840 when Paris was the undisputed center of mathematical research the view of mathematics was generally highly applied. It is symptomatic that the highest mathematical education of the period was given at an engineering school: the École polytechnique. At this school Joseph Liouville got his basic training in mathematics and physics. In accordance with the philosophy of the school he later emphasized that the most interesting results of mathematics are indebted to physics and in particular to mechanics. In fact he contributed to many applied areas of mathematics and mathematical physics in particular to potential theory and mechanics, in which fields one still mentions several theorems named after him2. However, rather than discussing Liouville’s contributions to applied mathematics I shall in this paper discuss some of his contributions to pure mathematics that were inspired by physics.
2 For a discussion of these and other aspects of Liouville’s mathematics and career see Lützen 1990. 22 Part I. Allgemeine Entwicklungen | General developments
3.1 Electro-statics and conformal mappings
In 1848 Liouville formulated a theorem which is now named after him: Liouville’s theorem: Any conformal (i. e. angle preserving) map from space into space is composed of displacements and inversions in spheres. An inversion in a sphere is a mapping that maps the inside of a sphere (with radius R) onto the outside and vise versa in such a way that a point at a distance r from the center is mapped into a point with a distance R2/r from the center and on the same radial line. Liouville had learned about this mapping from the young William Thomson (later enobled as Lord Kelvin) who visited Paris in 1845. While in Paris Thomson discussed electromagnetism with Liouville who asked him to write a paper on Michael Faraday’s new electromagnetic discoveries. The aim was to find out if Faraday’s field theory could be brought into line with André-Marie Ampère’s mathematical theory. When they met Thomson also explained his idea of what he called electrical images to Liouville. His idea was to use an inversion in a sphere to transform one electrostatic problem into another one that could more easily be solved. When he returned to Cambridge Thomson committed his ideas to paper in a series of letters to Liouville who published them in his Journal de mathématiques pures et appliqués also called Liouville’s Journal.3 To these letters Liouville appended a lengthy note of his own in which he systematically studied the mathematical properties of inversions in spheres.4 For example he explained how the important differential operators of physics transform under this mapping and he proved that the mapping is conformal. In a one page note of 1850 he finally announced that conversely all conformal mappings can be obtained as compositions of displacements and inversions in spheres5. He published the proof of the theorem in a note in his edition of Gaspard Monge’s Application de l’analyse à la géométrie.6 In this way Thomson’s research on electrostatics led Liouville to dis- cover his celebrated purely geometric result about conformal mappings.
3 Thomson 1845a,b, 1847 4 Liouville 1847 5 Liouville 1850a 6 Liouville 1850b LÜTZEN: Examples and Reflections on the Interplay between Math. and Physics 23
Many of Liouville’s other investigations in geometry was likewise inspired by physics in particular Hamilton-Jacobi mechanics. For example he found new properties of geodesics on various surfaces by considering them as trajectories of a point moving on the surface. In general his works on surface theory was strongly inspired by Carl Friedrich Gauss’ Disquisitiones generales circa superficies curvas7 which was in turn inspired by Gauss’ work as a surveyor.
3.2 Spectral theory of a class of integral operators
An even more remarkable but less celebrated theory that Liouville developed as a result of input from physics was a spectral theory of operators of the form