Obituary for Prof. Dr. Frieder Lenz
Frieder Lenz was born on November 12, 1942. He studied physics at the University of Freiburg beginning in 1962. In 1971, he completed his studies with a doctorate under the renowned nuclear physicist Hans Marschall. He made his mark early on with his dissertation. He developed the foundations for a model-independent analysis of elastic electron-nucleus scattering, which was to have a lasting influence on this field. Between 1972 and 1976, Frieder Lenz was a postdoctoral researcher at MIT (Boston) and SIN (Villigen). Here, he not only made a name for himself, but also laid the foundation for a circle of top-notch scientists with whom he and his group would remain in close contact and mutual exchange for decades. Particularly noteworthy are Ernie Moniz and John Negele from MIT, Justus Koch from Boston University, Koichi Yazaki from the University of Tokyo, and Michi Hirata from Hiroshima. A close friendship united these three different and outstanding scientists, held together by the focal personality of Frieder Lenz.
In 1976, Frieder Lenz habilitated in Freiburg. He spent the following ten years as head of the theory group at SIN and as a private lecturer at ETH Zurich. It was here, as a postdoc, that I had the opportunity and privilege of working in his group for the first time. The SIN (Swiss Institute for Nuclear Research, now the Paul Scherrer Institute, PSI) near Zurich was one of three “meson factories” worldwide, competing with LAMPF (Los Alamos) and Triumf (Vancouver). These were proton accelerators for generating intense pi meson beams of several 100 MeV. Nuclear physics was just on the threshold of the modern era of strong interaction. It was observed that nuclei could be investigated not only by classical nucleonic particle-hole excitations, but also by excitations of the subnucleonic degrees of freedom of individual nucleons. In the energy range of the SIN, this manifested itself mainly through the delta resonance (spin 3/2, isospin 3/2, 1232 MeV), which was ubiquitous in the pion-nucleus scattering data. Frieder Lenz and his colleagues developed a theory to account for the generation of the delta resonance and its propagation through the nucleus despite its extremely short lifetime. The focus shifted from the pion-nucleus interaction to the delta-nucleus interaction and the dynamics of delta-hole states, which could also be excited electromagnetically. Frieder Lenz and his group presented state-of-the-art investigations with very complex multi-particle calculations that are still valid today. For example, the delta-core potential (including a spin-orbit term identified for the first time), which was obtained from the pi-core scattering data, is still used today in the analysis of high-energy heavy ion reactions. This sustainability is all the more remarkable given that the working conditions for theoretical physicists half a century ago were not comparable to those of today. I still vividly remember that time without PCs and the Internet, but with a central computer center. The only personal data carriers were long card index boxes full of punch cards or thick stacks of large-format continuous paper from the computer center’s printer. Each test run of a program took a whole day. Preprints were sent by mail all over the world and, with the corresponding delay, were laid out on shelves in the library. However, the fact that numerical calculations were so cumbersome with the existing computers also had a positive side: one was forced to analyze the problems much more deeply in order to resort to numerical methods only at the very end. This trained formal and mathematical skills that Frieder Lenz, in particular, mastered perfectly.
During his time at SIN, Frieder Lenz became one of the defining figures in medium-energy nuclear physics. At the same time, the understanding of strong interactions had developed revolutionarily from another direction, high-energy physics, towards quantum chromodynamics. Quarks and gluons, asymptotic freedom, and confinement were the fundamental concepts that increasingly displaced the focus on nucleons and mesons. Towards the end of his time at SIN, this shift was already reflected in the work of Frieder Lenz and others. They designed a solvable many-particle model with confinement but without van der Waals forces and investigated its properties. During this period, the interest of many nuclear physicists shifted from traditional nuclear physics to fundamental questions of strongly interacting quantum fields. In 1986, Frieder Lenz accepted a position in Erlangen as Chair of Theory 3. I was able to follow him there just two years later, taking up a Fiebiger professorship at his chair. My first impression was that the theory group had been relocated from SIN to Erlangen. The familiar stream of visitors remained, and numerous new ones joined them. Two changes reflected the difference between a research laboratory and a university: direct contact with experimental groups in research declined, and the focus shifted increasingly to teaching. Frieder Lenz’s field of work developed rapidly in a highly theoretical direction, driven by the success of quantum chromodynamics as a theory of strong interaction. Frieder Lenz shifted the focus of his work from medium-energy nuclear physics to gauge field theories, quarks, and gluons. He developed a keen interest in fundamental theoretical questions such as spontaneous symmetry breaking, the Higgs mechanism, confinement, and light cone quantization. However, his motivation and main focus always remained on the physical questions, not the formal mathematical ones.
In particular, Frieder Lenz initiated important work on symmetries of gauge field theories in quantum mechanics. As a warm-up, Abelian theories (QED, Abelian Higgs models) were examined from a new perspective, and later QCD was further developed in axial gauge. New insights were gained in particular on symmetry aspects through careful differentiation between physical, global symmetries and local gauge symmetries. Frieder Lenz and his colleagues were able to show, for example, that the relevant symmetry for quantum electrodynamics is a global residual gauge symmetry, which he called “displacement symmetry.” Contrary to the usual discussion in textbooks, this symmetry is unbroken in the Higgs phase and spontaneously broken in the Coulomb phase, with the photon as the Goldstone boson. The widely accepted explanation of the masslessness of photons based on local gauge invariance and the Higgs mechanism as an expression of the spontaneous breaking of gauge symmetry is not tenable in quantum mechanics, but is based on an unjustified reversal of the order of gauge fixing and quantization.
Over the years, the Theory 3 chair attracted excellent students as diploma and doctoral candidates, a considerable number of whom went on to pursue careers at universities in the US, Germany, and other countries. The promotion of young talent benefited in particular from a graduate college that Frieder Lenz had established together with the experimental chair of Klaus Rith and two theoretical colleagues from Regensburg, Wolfram Weise and Ernst Werner. The funding was extended several times, enabling the graduate program to run for many years, also in collaboration with the successors at the partner chairs. The highlights of the academic year were workshops in Waischenfeld or Kloster Banz. A small number of renowned speakers were invited to give lectures lasting several hours on their fields of work without any time pressure. Two doctoral students were assigned to each lecturer with the task of preparing lecture notes. This resulted in useful collections of material that were later published as Lecture Notes in Physics. At the same time, this opened up good and intensive opportunities for contact between students and professors. Apart from the workshops, the exchange with the University of Regensburg was very intensive, also promoted by study days. These took place alternately in Erlangen and Regensburg, with the participation of all doctoral and diploma students from the chairs involved, together with the lecturers. During the lecture period, students commuted weekly between Erlangen and Regensburg. The intensive contacts led to some students changing universities before completing their doctorates and to a new, accelerated course of study after the end of the graduate college.
The teaching activities at the Theory 3 chair were supplemented by structured continuing education programs during the lecture-free period, in which the numerous summer visitors were integrated. Here, Frieder Lenz typically selected a current topic, and the participants taught each other with the help of informal blackboard lectures, typically during a week.
In the last years of his active career, Frieder Lenz devoted himself to quantum field theory in accelerated reference frames (keywords: Rindler space, Unruh effect). In addition, he increasingly enjoyed giving lectures on general relativity, which were extremely well attended by students. This heralded a general development in particle theory towards gravitation, which was also reflected in the appointment of Frieder Lenz’s successor, Thomas Thiemann, and the renaming of the chair to “Quantum Gravitation.” Anyone who did not have the opportunity to attend a lecture by Frieder Lenz is recommended to watch his (apparently only) recorded lecture from 2005 at the Kollegium Alexandrinum of the FAU entitled “Einsteins Raum-Zeit” (Einstein’s Space-Time) (for a wider audience) (fau.tv/clip/id/508).
Frieder Lenz will be remembered for his charisma, his friendliness, his sociability, and above all his infectious enthusiasm for physics. He always gave his lectures with chalk on the blackboard, without any aids such as scripts, slides, or projectors. Students found Frieder Lenz strict but fair in exams and tests, which made him very happy. Attempts by visitors or colleagues to persuade him to compromise on difficult problems were mercilessly rebuffed with the phrase “no cheap thrills.” Personally, I will always remember Frieder Lenz as a true polymath in the classical sense, with whom every discussion during lunch at the institute was a great pleasure and enrichment and could lead to many areas beyond physics.
Frieder Lenz died on April 4, 2025, at the age of 82.
by Prof. Michael Thies