Credit: © NIELS BOHR ARCHIVE, COPENHAGEN

“Quantum mechanics is a difficult theory, the history of which is even more difficult.” Such is not the conclusion, but the starting point of a study by Olivier Darrigol, in which he sets out to give a simplified account of the complex history of quantum mechanics, and its early history in particular (Studies in History and Philosophy of Modern Physics 40, 151–166; 2009). His “simplified genesis”, Darrigol hopes, might serve both physicists and philosophers as a more direct approach to the foundations of quantum theory.

Darrigol considers the period from Max Planck's quantum hypothesis to the first complete mathematical formalism of quantum mechanics, Paul Dirac's transformation theory. During that time, spanning the first quarter of the last century, a number of great minds left their mark, as they investigated a broad spectrum of physical phenomena. Different schools emerged, in terms both of geographical location and of approach, and by the middle of the 1920s, two distinct formulations of quantum mechanics had emerged: matrix mechanics (developed in Germany by Werner Heisenberg, Max Born and Pascual Jordan, and by Dirac in England) and Erwin Schrödinger's wave mechanics.

Darrigol takes these two branches — their formal equivalence was established eventually — as the backbone of his 'simplified history'. He starts the story of matrix mechanics with the failure of classical electrodynamics to describe black-body radiation, leading to the work of Niels Bohr (pictured) on the model of atomic structure and the correspondence principle, and, in what Darrigol says may be regarded as a “necessary consequence” — Heisenberg's quantum mechanics. The developments that eventually led to wave mechanics, on the other hand, he traces back to Einstein's light-quantum hypothesis and its extension to matter waves, by Louis de Broglie.

Although this overall structure of Darrigol's 'brief history' might be, in itself, not surprising, it is in the selection of key contributions that he chooses to take a new path, so as to construct a coherent sequence of achievements where each step follows as a consequence of previous ones (anything but an easy task in the face of the multilayered history of the field). Also, convoluted derivations are replaced with shorter, more direct reasoning. This approach, Darrigol admits, does leave out important developments, and, in a sense, provides the kind of “linear, great-men accounts” which, in principle, should be avoided in historical writing. But in the light of the already existing large body of work covering the history of quantum mechanics, priority is given to a short and clear account that highlights conceptual connections and key features of quantum theory, in a way that facilitates capturing its foundations, as well as the philosophical stance of some of its fathers. A fuller history, Darrigol argues, would not alter much the basic constructive steps in his simplified genesis, or help to understand why quantum mechanics was born.