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.