This is the final volume of Heisenberg's Collected Works. It contains his papers on a (nonlinear) unified theory of elementary particles, as well as his contribution to superconductivity and multiparticle production. Especially interesting is the first group of papers, which is split intotwo sections dealing with, firstly, the formulation of the famous nonlinear spinor equation and, secondly, its applications. Among others the reader willfind a thorough discussion of Heisenberg's collaboration with W. Pauli on these matters. Illuminating annotations to the various sections in this volume have been provided by H. Koppe, R. Hagedorn and the editors.

vii FOREWORD TO THE ENGLISH EDITION The lectures which I gave at the University of Chicago ix It is an unusual pleasure to present Professor Heisen- in the spring of 1929 afforded me the opportunity of re- berg's Chicago lectures on "The Physical Principles of viewing the fundamental principles of quantum theory. the Quantum Theory" to a wider audience than could Since the conclusive studies of Bohr in 1927 there have attend them when they were originally delivered. Pro- been no essential changes in these principles, and many fessor Heisenberg's leading place in the development of new experiments have confirmed important consequences the new quantum mechanics is well recognized by those of the theory (for example, the Raman effect). But even who have been following its growth. It was in fact he who today the physicist more often has a kind of faith in the first saw clearly that in the older forms of quantum theory we were describing our spectra in terms of atomic mecha- correctness of the new principles than a clear understa- nisms regarding which we could gain no definite knowl- ing of them. For this reason the publication of these C- cago lectures in the form of a small book seems justified. edge, anq who first found a way to interpret (or at least describe) spectroscopic phenomena without assuming Since the formal mathematical apparatus of the quan- the existence of such atomic mechanisms.

Werner Heisenberg is one of the greatest scientists of our century. His work extends over a period of fifty years, ranging from turbulence theory, the establishment of quantum mechanics, its fundamental applications in atomic and solid state physics, to the theory of cosmic ray phenomena and of elementary particles. He recognized early the epistemological significance of the new discoveries. He was able to place the radical changes in the foundation of physics of this century in the historical context of natural philosophy. His thoughts on language as the medium to grasp scientific truth, artistic truth, religious truth, truth in general, reached many auditors and readers, scientists as well as non-scientists. In the 75 years of his life the political and social structure of his home country, of Europe and the world over underwent drastic changes. He grew up in Imperial Germany, made his great contributions to quantum mechanics during the period of the Weimar Republic and was engaged in nuclear physics when the potentates of the Third Reich tried to discredit relativity and quantum theory as "degenerate" science; in World War II he participated in the German effort to develop a nuclear reactor. After the war, he devoted himself mainly to the physics of elementary particles. In addition, he acted in many official capacities: for the promotion of research, the reconstruction of science in the Federal Republic of Germany, and the advancement of international collabora tion."

Die theoretische Deutung der Ouantenstruktur der Materie hat unsere Vor- stelluhg von der Wirklichkeit tiefgreifend veriindert. Der entscheidende Schritt dazu wurde von Werner Heisenberg vor fast einem halben Jahrhundert getan. Die neue Ouantenphysik hat in vielen physikalischen und philosophischen Arbeiten ihren Nieder- schlag gefunden; sie hat ganz neue Gebiete erschlossen, wie insbesondere die Atomkem- physik und die Physik der Elementarteilchen. Wemer Heisenberg hatte und hat noch an der Erforschung dieser neuen Gebiete wesentlichen Anteil. Sein Hauptinteresse in den letzten Jahren galt dabei der Entwicklung einer einheitlichen fundamentalen Feldtheorie der Materie. Das Lebenswerk Heisenbergs ist in so hohem MaRe Teil der heutigen physika- lischen Literatur geworden, daB - damit verglichen - der Versuch einer gesonderten Darstellung in einem Festband immer als unzureichend oder uberflUssig erscheinen muB. Die in diesem Band gesammelten Arbeiten versuchen deshalb nicht, einen Oberblick uber sein Lebenswerk zu geben, sondem sind im wesentlichen Arbeiten aus dem augenblick- lichen Interessengebiet der Autoren, thematisch allerdings 5.0 ausgewiihlt, daB sie mittel- bar oder unmittelbar mit Arbeiten Heisenbergs in Verbindung stehen. AuBerdem wurde durch die Art der Darstellung angestrebt, uber den Fachmann hinaus einen groReren Leserkreis anzusprechen. Die meisten Beitriige haben deshalb mehr allgemeine und grund- siitzliche Erorterungen zum Inhalt.