£21.95.'Aber es ist ein wichtiger Unterschied zwischen allem, was vor 1933 geschah, und dem, was dann kam: Alles frühere zog an uns vorbei undüber uns hin, es beschäftigte uns und regte uns auf, und den einen oder andern tötete es oder ließ ihn verarmen; aber keinen stellte es vor letzte Gewissensentscheidungen. Ein innerster Lebensbezirk blieb unberührt. Man machte Erfahrungen, man bildeteÜberzeugungen: Aber man blieb, was man war. Keiner, der, willig oder widerstrebend, in die Maschine des Dritten Reichs geraten ist, kann das ehrlich von sich sagen.' (Sebastian Haffner) 1
IntroductionWerner Heisenberg (1901Heisenberg ( -1976) is probably best known for his discovery of the uncertainty relations in quantum mechanics and their physical interpretation (1927). En route to quantum mechanics itself, he had earlier had the decisive insight that quantum-mechanical variables do not commute (1925). This formed the basis for the creation of matrix mechanics immediately afterwards by Born and Jordan, appearing in mature form in the 'Dreimännerarbeit' with Heisenberg. 2 This was one of the two paths along which modern quantum mechanics was discovered.Some of his other achievements in theoretical physics deserve to be mentioned. Barely 20, and well before the introduction of electron spin, Heisenberg made the revolutionary proposal to allow half-integral quantum numbers in the context of the Zeeman effect (1921). He made various other significant contributions in the period 1920-1925 that led to the establishment of quantum mechanics, and once the new theory had arrived, he was the first to derive a number of important consequences. He was awarded the 1932 Nobel Prize 'for the creation of quantum mechanics, the application of which has led, among other things, to the discovery of the allotropic forms of hydrogen' (official citation). Similarly, Heisenberg provided the quantum-mechanical explanation of the occurrence of para-and ortho-helium (1926). His explanation of ferromagnetism (1928) stands out, too, but his most important work of the period is the foundation of quantum field theory with Pauli (1929). A few years later he became one of the founders of theoretical nuclear physics with a detailed proposal on the interactions between protons and neutrons in a nucleus. 3 Heisenberg had certain other ideas that were somewhat off focus when he proposed them, but which in later developments by others turned out to be influential. This applies to his suggestion to replace quantum field theory by an S-matrix (1942), an idea that came to dominate elementary particle physics in the sixties, to his proposal of a minimal length (1936), which much later found its correct incarnation in lattice field theory as well as in quantum gravity, to his description of superconductivity as a phase transition in the late forties, eventually leading to the modern idea of 0 Supported by a fellowship from the Royal Netherlands