On Heisenberg's Discovery

Giridhari Lal Pandit

My main aim here is to present a general methodological principle, which I propose to designate as Heisenberg-Einstein context principle, as it emerges from their dialogue on physical theory, becoming their most important meeting point in the interpretation of quantum mechanics. I regard as context-principles those which can serve as guiding principles in theory-finding and theory-testing within physics. I think that they must be judged by their impact in making physics open-ended. As regards Heisenberg-Einstein dialogue, three important questions can be raised. The first question is which particular aspect of physical theory were they both concerned with. The second question is whether there emerged from their dialogue some general principle, as a guiding principle of sufficient methodological interest, on which they both showed significant agreement, resolving some of their methodological differences by removing some of their shared philosophical misunderstandings concerning physical theory. This gives rise to the third question whether their dialogue resulted in a significant impact on their respective positions on the interpretation, or foundations, of quantum mechanics. While answering the last two questions in the affirmative, I shall argue for the thesis that in their dialogue they shared a rather deep concern with the fundamental aspects of physical theory — with its mathematical formalism. Taken in this aspect, a physical theory, as and when it becomes mature after it has passed tests, contributes to context-building in theory-finding and theory-testing, generating new frontiers of discovery in physics and astronomy. In a nutshell, I am here concerned with the problem of reconnecting observation with theory, and discovery, or rather frontier of discovery, in physics with the core-context of discovery, trying to bring out the significance of the Heisenberg-Einstein context principle.

Mara Beller

Malcolm S. Longair

The discovery of the spin of the electron by Uhlenbeck and Goudsmit was a major advance in the understanding of physics at the atomic level. Its discovery coincided with the development of both matrix and wave mechanics and its incorporation into the scheme of quantum mechanics and statistics led to deeper understanding of the underlying structure of quantum mechanics. Almost immediately, Heisenberg and Jordan used the new scheme of matrix mechanics to derive the expression for the g -factor which Landé had derived empirically from a very close study of the anomalous Zeeman effect. An important consequence of these developments was that the different approaches of matrix and wave mechanics were brought together. In particular, the discovery of spin as a new quantum number suggested the possibility of understanding systems containing more than one electron. Heisenberg's analysis of the helium atom was to pave the way for the full incorporation of spin into quantum mechanics and quantum statistics. Spin and the Landé g -factor The story of the discovery of the spin of the electron by Uhlenbeck and Goudsmit (1925a) was told in Sect. 8.5. As discussed in that section, their discovery was based upon empirical studies of the regularities observed in the anomalous Zeeman effect, inspired by the intricate analyses of Landé. Although based originally upon the classical concept of a rotating electron, electron spin is a purely quantum mechanical property intrinsic to the electron.