Obituary for Frieder Lenz
Frieder Lenz was born on 12 November 1942. In 1962, he began studying physics at the University of Freiburg. He completed his doctorate in 1971 under the supervision of the renowned nuclear physicist Hans Marschall. His dissertation already made a significant impact: he laid the foundations for a model-independent analysis of elastic electron-nucleus scattering, which went on to have a lasting influence in the field.
Between 1972 and 1976, Frieder Lenz held postdoctoral positions at MIT (Boston) and SIN (Villigen). During this time, he not only established his reputation but also forged long-standing scientific relationships that would shape decades of fruitful collaboration. Among his close collaborators were Ernie Moniz and John Negele from MIT, Justus Koch from Boston University, Koichi Yazaki from the University of Tokyo, and Michi Hirata from Hiroshima. These distinguished scientists were united by a deep mutual friendship, centred around the intellectual and personal presence of Frieder Lenz.
In 1976, Frieder Lenz completed his habilitation in Freiburg. The subsequent decade saw him leading the theory group at SIN, while also lecturing at ETH Zurich. It was during this period, as a postdoctoral researcher, that I had the privilege of joining his group. SIN (Swiss Institute for Nuclear Research, now the Paul Scherrer Institute, PSI), located near Zurich, was one of only three “meson factories” worldwide, alongside LAMPF (Los Alamos) and TRIUMF (Vancouver). These were proton accelerators designed to produce intense pion beams with energies of several hundred MeV. Nuclear physics was on the cusp of a new era in the study of strong interactions. Researchers began to realise that nuclear structure could be probed not only through classical nucleonic particle-hole excitations but also via subnucleonic degrees of freedom within nucleons.
At SIN’s energy scales, this shift manifested in the Delta resonance (spin 3/2, isospin 3/2, 1232 MeV), which appeared prominently in pion-nucleus scattering data. Frieder Lenz and his collaborators developed a theory to describe the production and propagation of this extremely short-lived Delta resonance within the nucleus. The focus of study moved from pion-nucleus interactions to Delta-nucleus interactions and the dynamics of Delta-hole states, which could also be excited electromagnetically. His group produced state-of-the-art multi-particle calculations which remain relevant to this day. Notably, the Delta-nucleus potential—extracted from pion-nucleus scattering data and including a spin-orbit term identified for the first time—continues to be employed in the analysis of high-energy heavy-ion collisions.
This enduring relevance is particularly remarkable considering the working conditions for theoretical physicists half a century ago. I vividly recall an era without PCs or the internet, reliant instead on a central computing facility. The only personal data storage consisted of long trays of punch cards or thick stacks of continuous printer paper. Each test run of a programme could take an entire day. Preprints were physically posted across the globe and only became available in libraries after a substantial delay. Yet the cumbersome nature of numerical computation fostered deeper analytical approaches—one had to fully explore a problem analytically before resorting to numerics. This nurtured formal and mathematical skills, which Frieder Lenz exemplified with exceptional mastery.
During his tenure at SIN, Frieder Lenz emerged as a leading figure in medium-energy nuclear physics. Meanwhile, revolutionary developments in high-energy physics were reshaping our understanding of the strong interaction, culminating in the emergence of quantum chromodynamics (QCD). Quarks, gluons, asymptotic freedom, and confinement became the dominant concepts, gradually eclipsing the focus on nucleons and mesons. This transition was already evident in Lenz’s work toward the end of his time at SIN. Together with colleagues, he constructed a solvable many-body model with confinement but without van der Waals forces and explored its properties. The field’s interest was shifting from traditional nuclear physics towards fundamental questions in strongly interacting quantum field theories.
In 1986, Frieder Lenz was appointed to the Chair of Theoretical Physics III in Erlangen. Two years later, I was fortunate to join him there through a Fiebiger Professorship. My initial impression was that the SIN theory group had simply relocated to Erlangen. The familiar flow of visiting scholars continued unabated, and many new ones joined. Two significant changes reflected the transition from a research institute to a university environment: direct collaboration with experimental groups diminished, and greater emphasis was placed on teaching. Frieder Lenz’s research evolved rapidly in a more theoretical direction, spurred by the success of QCD as the theory of strong interactions. His focus shifted from medium-energy nuclear physics to gauge field theories, quarks, and gluons. He developed a profound interest in fundamental theoretical issues such as spontaneous symmetry breaking, the Higgs mechanism, confinement, and light-cone quantisation. Nevertheless, his motivation and primary concern always lay with the physical, not merely the formal-mathematical, aspects.
In particular, Frieder Lenz initiated important studies on symmetries in gauge field theories within quantum mechanics. As a warm-up, Abelian theories (QED, Abelian Higgs models) were reconsidered from a novel perspective, followed by significant developments in QCD in axial gauge. He and his collaborators made notable progress in clarifying symmetry structures by carefully distinguishing between physical, global symmetries and local gauge symmetries. They showed, for example, that the relevant symmetry in quantum electrodynamics is a global residual gauge symmetry he termed “displacement symmetry.” Unlike the conventional textbook interpretation, this symmetry remains unbroken in the Higgs phase but is spontaneously broken in the Coulomb phase, with the photon acting as the associated Goldstone boson. The widespread explanation of photon masslessness via local gauge invariance and spontaneous breaking through the Higgs mechanism does not hold up under quantum mechanical scrutiny, as it depends on an unjustified reversal of the order of gauge fixing and quantisation.
The Chair of Theoretical Physics III consistently attracted excellent undergraduate and doctoral students, many of whom went on to pursue successful academic careers in Germany, the United States, and beyond. A particular boost came from a structured graduate programme (Graduiertenkolleg) that Frieder Lenz initiated in collaboration with Klaus Rith’s experimental chair and two theoretical colleagues from Regensburg, Wolfram Weise and Ernst Werner. The programme was renewed multiple times, operating for many years even in collaboration with successors at the partner institutions. A highlight of each academic year were the workshops held in Waischenfeld or Kloster Banz, where a small number of distinguished speakers were invited to give extended lectures without time constraints. Each speaker was assigned two doctoral students responsible for preparing lecture notes, which often resulted in publications within the Lecture Notes in Physics series and provided excellent opportunities for close interaction between students and faculty.
Beyond these workshops, regular exchange with the University of Regensburg was fostered through joint study days alternating between Erlangen and Regensburg, involving all doctoral and diploma students and faculty. Weekly commuting during term time enabled deep collaboration and even led to university changes prior to graduation, as well as to the development of an accelerated study programme after the graduate college concluded.
Teaching activities at the Chair were enriched through structured seminars during term breaks, often involving summer visitors. Frieder Lenz typically selected a contemporary topic, and participants would educate one another through informal blackboard talks over the course of a week.
In the final years of his active career, Frieder Lenz turned his attention to quantum field theory in accelerated frames (keywords: Rindler space, Unruh effect). He also began to offer highly popular lectures on general relativity, marking a broader shift within particle theory towards gravity—a shift reflected in the appointment of his successor, Thomas Thiemann, and the renaming of the Chair to “Quantum Gravity.” For those who never had the opportunity to attend one of his lectures, his (apparently only) recorded public lecture from 2005, Einstein’s Space-Time, delivered at the Collegium Alexandrinum of FAU, is highly recommended (available here).
Frieder Lenz will be remembered for his charisma, kindness, openness, and, above all, his infectious enthusiasm for physics. He always taught using chalk and blackboard—never with slides, scripts, or projectors. Students regarded him as strict but fair in examinations, a reputation he valued. Attempts by colleagues or visitors to persuade him to compromise on solving difficult problems were invariably met with his firm motto: “no cheap thrills.”
On a personal note, I will always remember Frieder Lenz as a true universal scholar in the classical sense, for whom every lunchtime conversation at the institute was a joy and a profound enrichment—often reaching far beyond the confines of physics.
Frieder Lenz passed away on 4 April 2025 at the age of 82.
Written by Prof. Michael Thies, FAU Erlangen-Nürnberg