As a PhD candidate Leendert followed Professor Sybren de Groot from Leiden in 1964. In 1968, he obtained his Ph.D. cum laude with the dissertation On the covariant derivation of macroscopic electrodynamics from electron theory. This work, for which he was awarded the Royal Dutch / Shell Prize in 1974, he elaborated into the book Foundations of Electrodynamics, together with his dissertation advisor De Groot. It serves as a completion of the program of Lorentz by reconciling the derivation of the macroscopic Maxwell equations with relativity theory and quantum theory. In particular, it provided a definitive answer to an old question about the correct form of the energy-momentum tensor.

Rigorous treatment of electromagnetic phenomena and their statistical or quantum-mechanical averaging remained a recurrent theme for Leendert and his students in the context of retarded Van der Waals forces, molecules and plasmas in external fields and in the proximity of walls, optical and dielectric phenomena, and electron gases. For him, fundamental thoroughness went hand in hand with physical relevance. He also showed an interest in mathematical and methodological aspects. Already in the days of punch cards, he combined his own great analytical skills with an intensive use of the powerful computers at the University. More recently, he was an early advocate of the possibilities of Mathematica.

Leendert was a gifted teacher. In the nineteen nineties, he was even voted professor of the year in the science faculty. His meticulously prepared lectures were crystal clear. For detailed questions by students he always had extra explanatory material at the ready to be shared immediately. To end his lectures, he used to share a piece of history of science. He was resolute about examples to clarify physical concepts: “quantum optical entanglement” instead of “Schrödinger’s cat.” His physics expertise as well as his didactical qualities were also prominent in his role as advisor. His questions were clear and he checked the calculations of his Ph.D. candidates meticulously, sometimes also offering an alternative approach. Long after his retirement he continued to remain involved in physics, showing others the way with his publications and his phenomenal memory. He also got involved in High School physics: for many years he initiated projects for students’ final profile assignments in physics, often based on issues currently in the news.

Leendert’s great enthusiasm, based on a deep knowledge, not only of physics but also of literature, history, painting, and music, he loved to share with his friends, colleagues, and students. All of them have experienced their association and collaboration with him as a privilege and an inspiration.

Anne Kox, Thijs Michels and Giuseppe Salvati
Translation: Henriette Schatz