Theoretical physicist Michel Gaudin, known for his influential works in the domain of integrable models, passed away on 4 August. Michel entered L’Ecole Polytechnique in 1951 and obtained his PhD in 1967 at the Université de Paris. In his PhD thesis he found a solution, given simultaneously in the famous paper by C N Yang, to the Fermi gas with delta-function interaction, introducing what was later called the nested or higher-level Bethe ansatz. Except for a year spent visiting Yang at Stony Brook in 1970, he spent all his scientific life at the fundamental research division of the Commissariat de l’Energie Atomique (CEA-Saclay).
Michel used only French as a scientific language and a significant part of his research remained unpublished. The publication of his book La fonction d’onde de Bethe in 1983 was thus a major event, remaining an inspiring and pedagogical source of a variety of original information that cannot be found anywhere else. Translated into Russian in 1987 and English in 2013, it is considered a classic in the field.
One of the most well-known of his findings is the so-called Gaudin determinant for the norm of the Bethe eigenstates, which now plays a fundamental role in the computation of correlation functions. In another essential work, following the method of C N Yang and C P Yang, Michel solved the thermodynamics of the Heisenberg–Ising ring, simultaneously with Takahashi. He also diagonalised a class of quadratic Hamiltonians in spin variables, now referred to as the Gaudin model, and considered various extensions, in particular a system comprising a single oscillator coupled to a collection of independent two-level atoms, which has become a classic in the cold-atom and Bardeen–Cooper–Schrieffer communities. In addition, his pioneering early works on random matrices with M L Mehta introduced elegant and powerful new methods that played a key role in many developments in random matrix theory, with applications ranging from nuclear physics to non-critical string theories.
An original and prolific personality, Michel was principally guided by a quest for elegance. Although he cared little for recognition, in 2019 he was awarded the prestigious Dannie Heineman Prize for Mathematical Physics.
Michel Gaudin was a man of high integrity and modesty whose life was completely devoted to science. He was known for his rigour and reclusive style of work, but when approached by his colleagues, he generously shared his knowledge and insights.