# A History of Methods of Approximation in modern Chemistry and Physics (ca. 1870–1970)

Project of Dr. Christian Joas

A continuing thread in Christian Joas’s research is the history of methods of approximation in both chemistry and physics: What do scientists do when they cannot solve an equation exactly?

The exact calculation of the behavior of even the very simplest interacting many-body systems (such as a molecule, consisting of nuclei and electrons interacting through Coulomb forces) in practice often turns out to be extremely tedious or flat-out impossible. Ingenious methods have been developed to circumvent this difficulty, allowing the approximate calculation of the properties and dynamics of such systems, both by hand or on computers. Many of these methods, especially since the advent of quantum theory, rely on the so-called perturbation theory, a technique initially developed in 19th century celestial mechanics.

The project studies both the history of perturbation theory in physics and chemistry, as well as several specific cases of often highly-influential approximation methods, such as the Born-Oppenheimer approximation, the random-phase approximation, so-called semiclassical approximation methods, or the iterative numerical solution of the gap equation in the physics of superconductors on early transistorized computers.

This project has collaborations with Alexander S. Blum (Max-Planck-Institut für Wissenschaftsgeschichte, Berlin), Johannes Knolle (Max-Planck-Institut für Physik komplexer Systeme, Dresden) und Gerges Waysand (Laboratoire Souterrain Bas Bruit, Rustrel, France).