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Physics and Philosophy

1955 to 1956
University of St. Andrews

These lectures discuss the theoretical and empirical results brought about with the development of quantum physics, at the same time giving an insight into the extent to which such results go beyond the scope of pure science and affect fundamental philosophical conceptions of reality and of the place of man in it. The latter theme is investigated both theoretically and historically. From the former perspective, Heisenberg focuses on the structural links between physical theories and their philosophical premises. He pays special attention to the interaction between ontological and epistemological assumptions at stake in the axiomatic postulations of every physical theory. In the context of his historical reconstruction, Heisenberg establishes some fundamental links between quantum physics and ancient and modern developments of the philosophical idea of a ‘fundamental’ substance or structure of matter in Western thought.

This interaction between a theoretical approach and a historical perspective also structures the presentation of the main notions and results of quantum theory, which are always discussed in their relation to classical physics.

Heisenberg’s presentation is preceded by an introduction by Prof. F. S. C. Northrob, isolating three central questions the lectures intend to answer: What do the experimentally verified theories of contemporary (quantum) physics affirm? How do they require man to think of himself in relation to the universe described by such theories? How is this new way of thinking, which is a creation of the Western world, going to affect other parts of the world?

The first three chapters, ‘An Old and a New Tradition’, ‘The History of Quantum Theory’ and ‘The Copenhagen Interpretation of Quantum Theory’, discuss the origin of quantum physics and the fixation of its main concepts and results during the first quarter of the twentieth century, especially in the Copenhagen School, to which Heisenberg himself belonged and which is taken to have produced a consistent account of the apparently contradictory dualism between classical and quantum physics. The phenomenon of radiations emitted by a black body, to the explanation of which quantum physics was originally connected, is discussed, as is Planck’s law of heat radiation, Einstein’s interpretations of the photoelectric effect and of the observations on the specific heat of solid bodies, and Bohr’s explanation of the stability of the atom by appeal to Planck’s hypothesis. The concepts of a ‘probability wave’, indeterminacy (Heisenberg) and of complementarity (Bohr) are introduced and explained. The third chapter in particular deals extensively with the concept of a probability function, expressing both the indeterminacy due to uncertainty relations at an ontological level, and an epistemic concept of uncertainty linked with the error theory.

Chapter 4 is devoted to ‘Quantum Theory and the Roots of Atomic Science’, and establishes philosophical links between the Greeks’ conceptions of matter, with a special reference to the Pythagoreans and to Plato, and the concept of an ‘elementary particle’ in modern physics as a ‘mathematical form’ (as a solution to some eternal law of motion of matter). ‘The Development of Philosophical Ideas since Descartes …’ (chapter 5) continues the investigation of the links between the history of philosophy and the history of physics, focusing on Descartes and Kant. Chapters 6 and 7 investigate in depth the issue of the relations between different scientific conceptual systems, with respect to the consistency and flexibility of their mathematical formalizations, and offer a brief presentation of Einstein’s theory of relativity.

Chapter 8 discusses and replies to three different lines of argument against the Copenhagen interpretation of quantum theory, while the next two chapters, ‘Quantum Theory and the Structure of Matter’ and ‘Language and Reality in Modern Physics’, investigate both the ontology of the new theory, which is linked to the Aristotelian notion of ‘potential’ and to the history of the search for a ‘unification principle’ in modern physics, and the status and limits of its linguistic apparatus. Finally, the concluding chapter is devoted to ‘The Role of Modern Physics in the Present Development of Human Thinking’.

Chiara Tablet