Biography of Hagen Kleinert

Hagen Kleinert (born 15 June 1941 in Festenberg near Breslau; died 7 March 2025) was a German theoretical physicist and longtime professor at Freie Universität Berlin. His work ranged from particle physics and path integrals to condensed matter, critical phenomena, membranes and applications of mathematical methods in further fields.

Education and early years

Kleinert grew up in Hanover and studied physics at Technische Hochschule Hannover. In 1963 he went to the United States, first to the Georgia Institute of Technology in Atlanta and later to the University of Colorado at Boulder. There he received his doctorate in 1967 under Asim Barut. After returning to Germany he completed his habilitation at Freie Universität Berlin in 1969.

Freie Universität Berlin

In 1969 Kleinert became associate professor at Freie Universität Berlin, and in 1976 full professor of theoretical physics. He held this position until his retirement in 2009. For decades he shaped research, teaching and international contacts there. He supervised doctoral students, brought guests to the department and gave talks at conferences and summer schools.

Path integrals and theoretical physics

Kleinert's work was unusually broad. One center was path integrals, a method used in quantum mechanics, statistical physics, polymer physics and the theory of condensed matter. With İsmail Duru he treated the hydrogen atom using path integrals in 1978. Later he made contributions to variational perturbation theory, critical exponents, gauge fields, defect lines, membranes and collective fields.

Books and recognition

Alongside many research articles, Kleinert wrote extensive monographs. His book on path integrals became a widely used work for researchers and students. In 2008 he received the Max Born Prize of the German Physical Society and the British Institute of Physics. In 2009 the West University of Timișoara awarded him an honorary doctorate for his lifetime achievements in physics.

Farewell

Hagen Kleinert died on 7 March 2025. Freie Universität Berlin honored him as a scientist connected with its Department of Physics for more than five decades. His memory remains linked with a form of theoretical physics that brought together mathematical precision, wide-ranging themes and international collaboration.