IHES
Tribute to Ivan T. Todorov
It is with great sadness that IHES shares the news of the passing of Ivan T. Todorov, a towering figure in mathematical physics and a long-standing visitor to the Institute, with 34 visits between 1967 and 2022. Read personal tributes to Ivan T. Todorov by Jürg Fröhlich, Permanent Professor of Physics at IHES from 1978 to 1982, and Junchen Rong, former postdoctoral researcher at IHES, below.
Tribute to Ivan T. Todorov by Jürg Fröhlich
I first met Ivan at Princeton. From 1968 to 1975, he was a frequent visitor at the Institute for Advanced Study. From 1978 onwards, our paths crossed multiple times at IHES, allowing me to get to know him well and appreciate his way of life. I cherish many fond memories of our encounters.
His connection with Princeton was fostered through shared scientific interests with Arthur S. Wightman (axiomatic quantum field theory) and Valentin Bargmann (group theory), while his ties to IHES were established through Louis Michel (particle physics, symmetry principles) and later deepened through collaborations with younger colleagues at the Institute. He was also a frequent visitor at CERN, various scientific institutions in Germany, and the Schrödinger Institute in Vienna.
Todorov’s main scientific interests revolved around axiomatic quantum field theory, conformal field theory, and symmetry principles in quantum (field) theory. He was particularly drawn to many aspects of conformal field theory and applications of Lie group theory, Kac-Moody algebras, and quantum groups. His research also reflected a profound interest in the structural connections between quantum (field) theory and mathematics, including number theory.
Ivan collaborated extensively with leading scientists from both the East—particularly in the Soviet Union—and the West. His collaborators included V. Bargmann, N.N. Bogoliubov, D. Buchholz, M. Flato, V. Kac, G. Mack, R. Stora, and many others, along with numerous talented younger colleagues, including PhD students from Bulgaria, some of whom later pursued their careers in France. Todorov was unique among scientists from behind the Iron Curtain in his ability to establish and maintain collaborations with colleagues from both East and West. Not only did he arrange research visits for himself at leading scientific centers around the world, but he also facilitated similar opportunities for his students in Bulgaria. In this respect, he reminded me of the late Ludvig D. Faddeev. His ties to France were reinforced by his younger brother, Tzvetan Todorov, who settled in Paris as a young man and later became a CNRS researcher, making significant contributions to comparative literature, literary criticism, history, sociology, and philosophy.
Ivan Todorov was an exceptionally hard-working, highly motivated, and disciplined scientist. I remember him as someone who was always on the move, constantly engaged in his work. Beneath his warm and unassuming demeanor, one could sense a strong, ambitious, and determined personality. In seminars, he took meticulous notes, written in beautiful handwriting and highlighted in multiple colors to emphasize key points. He enjoyed discussing not only science but also literature and broader cultural topics. He was sociable and engaging—I recall many lively dinners and animated conversations in his company, both at Bures-sur-Yvette and elsewhere.
To conclude, I want to make a remark on Todorov’s life journey, but without going into any details. I admire the courage and resilience he displayed in facing significant hardship throughout his life and suffering through some tragedies.
Zurich, February 27, 2025
Jürg Fröhlich is a former Permanent Professor of Physics at IHES. He is currently Emeritus Professor of Physics at ETH Zurich.
Tribute to Ivan T. Todorov by Junchen Rong
Prof. Ivan T. Todorov was a pioneering figure in conformal field theory (CFT) and a visionary in mathematical physics.
Working alongside other early trailblazers, he employed the mathematical framework of harmonic analysis on the SO(n,1) group — also known as the Euclidean conformal group — to derive the conformal partial wave expansion of Euclidean conformal Green’s functions. His groundbreaking work provided a rigorous mathematical foundation for Euclidean conformal field theory, establishing principles that have since become cornerstones in the field.
Todorov’s contributions continue to shape numerous areas of research, including the conformal bootstrap programme, the Lorentzian inversion formula, holography, the SYK model, and de Sitter space physics. Remarkably, his work emerged at a time when conformal field theory was still in its infancy — nearly a decade before the seminal breakthrough in two-dimensional CFT by A.A. Belavin, A.M. Polyakov, and A.B. Zamolodchikov.
Beyond his research, Todorov co-authored the influential book “Harmonic Analysis on the n-Dimensional Lorentz Group and Its Application to Conformal Quantum Field Theory” with V. K. Dobrev , G. Mack , V. B. Petkova , and S. G. Petrova. This work remains an invaluable resource, bridging the gap between the mathematical and physical aspects of CFT. I often return to it for inspiration, and, each time, I uncover fresh insights that deepen my understanding of conformal field theory.
As a young field theorist who deeply admires Prof. Todorov’s work, I am truly saddened by his passing. His visionary contributions will undoubtedly continue to inspire generations of quantum field theorists to come.
Junchen Rong is a theoretical physicist, and a former postdoc at IHES. He is currently working at the Centre de physique théorique at École polytechnique.
