Per Christian Hemmer

Per Hemmer was an internationally recognized physicist focused on statistical physics. For 60 years he played a crucial role in creating and inspiring the group of statistical physics in Trondheim, at the Norwegian University of Science and Technology (NTNU, previously NTH). His contributions as a scientist, teacher, and administrator are truly impressive.

Hemmer was born in Arendal in southern Norway, got his master’s degree in technical physics at NTH in 1956, and his Dr.techn degree from the same university in 1959. In his thesis he proved that a heavy particle in an infinite chain of light ones (in a suitable limit) obeys the diffusion equation. This elegant proof was noted in the international community, and in 1961 Hemmer went to Rockefeller University in New York where, with Mark Kac and George Uhlenbeck, he worked on the one-dimensional many-particle model with strong short-range repulsion and weak long-range attraction. In the appropriate limit they showed that the equation of state is that proposed by Van der Waals. This opened a whole new field of research, in equilibrium but also in non-equilibrium, where the 3D version of Van der Waals is the zeroth approximation, and successive corrections due to the finite relative range of repulsive and attractive forces can be systematically calculated. Hemmer made the first important steps here.

In 1966 Hemmer was appointed assistant professor at NTH and in 1969 full professor in theoretical physics. He was a guest professor at Rockefeller University; University of Nijmegen, the Netherlands; Universidad Autonoma, Madrid; and State University of New York at Stony Brook.

Through his career, Hemmer was interested in a variety of problems within statistical physics, broadly defined: equations of state for classical and quantum systems, renormalization theory, Markov processes, queuing theory, effective mass, and fiber bundles.

Highlights are papers with George Stell on fluids with several phase transitions, the derivation with Alex Hansen of a universal scaling law for burst events in fiber bundles, exact results on the two-dimensional Coulomb gas, and work on effective mass theory with Gaute Einevoll and others.

A major part of Hemmer’s research was done in inspiring and inspired collaboration with his master’s and PhD students. His students are unanimous in their praise of Hemmer’s qualities as a mentor. I can personally vouch for this: I was his very first PhD student, about 10 years before the concept of a “PhD student” was institutionalized in Norway. Hemmer had just returned from New York and his famous collaboration with Kac and Uhlenbeck, and I was an ignorant young brat. Hemmer systematically helped me build the basis for my own life as a scientist. He encouraged me to extend the work on fluids with long-range forces to mixtures, and we had a lot of fun playing with the distribution of Yang–Lee zeros. His openness and warmth set a standard for fruitful scientific collaboration that I, like all his students and collaborators, am deeply grateful for.

But Hemmer was not only a great mentor for his PhD students. He took the teaching of basic courses as a central responsibility. His series of textbooks testify to this: on thermal physics, on statistical mechanics, on quantum mechanics, and on solid-state physics. All of them in Norwegian.

In addition to his work as scientist and teacher, Hemmer accepted responsibilities as an administrator: locally as chairman of the institute, later as dean; nationally in the Norwegian Research Council; and internationally in Nordita and in IUPAP, where he was vice president from 1984–90.

We are many who are grateful to Per Hemmer for all he did and what he represented. He was a great Norwegian physicist, a constructive collaborator, and a warm friend.