Shau-Jin Chang

Shau-Jin Chang (1937–2025)

Emeritus Professor of Physics and former Associate Department Head Shau-Jin Chang (張紹進), 88, passed away on January 25, 2025, at his home in San Jose, CA. A theoretical particle physicist, he joined the Illinois Physics faculty in 1969 and retired in 1999. He published nearly 100 research papers on theoretical particle physics and is the author of the critically acclaimed book Introduction to Quantum Field Theory, published in 1990.

Chang was born in Suqian, Jiangsu Province, China, on January 7, 1937. He escaped to Chongqing with his parents during the Japanese invasion of China during World War II. This period was deeply embedded in his memory and years later inspired his daughter Iris to write the historical bestseller The Rape of Nanking.

Chang arrived in Taiwan with his mother in 1951. He attended the Department of Physics of National Taiwan University after placing first in the joint college entrance examination. After graduating with honors in mathematics and physics, he received a master's degree in physics from Tsinghua University in Taiwan. In 1962, he enrolled at Harvard University to study physics under Professor Julian Schwinger, receiving his doctorate in 1967.

From 1967 to 1969, Chang held a postdoctoral research appointment in high-energy theoretical physics at the Institute for Advanced Study in Princeton, NJ. Following this, he joined Illinois Physics in 1969. Chang was a Sloan Fellow from 1972 to 1974 and was elected a Fellow of the American Physical Society in 1976.

Much of Chang’s scientific life involved the applications of quantum field theory to elementary particle physics. After early papers studying the quantization of higher-spin particles, he became a pioneer in the theory of very high-energy interactions in the infinite-momentum frame. This work was both important and timely, because, starting around 1968, exciting discoveries were being made at the Stanford Linear Accelerator Center (SLAC) that showed the reality of the quark (parton) constituents of nucleons. The full exploitation of the data being produced by the high-energy electron-nucleon collisions required novel theoretical techniques, and the infinite-momentum frame provides simplifications that allow a clean modeling of parton effects. Chang’s co-authors at this time included Shang-Keng Ma, Lochlainn O’Raifeartaigh, R (Doug) Rajaraman, Roger Dashen, Francis Low, Sam Treiman, and Tung-Mow Yan. Yan was Chang’s contemporary as a student of Julian Schwinger. Chang also collaborated with David Campbell, who would later serve as head of Illinois Physics.

Shau-Jin's 1975 paper, “Chiral confinement: an exact solution of the massive Thirring model,” co-authored with Stephen D. Ellis and Ben Lee, has had impact far beyond particle physics. It reveals how a particle obeying the Dirac equation can be trapped in a domain wall where some scalar field changes sign. This work is the progenitor of entire fields of applications of topology to physical systems. It directly and indirectly led to the discovery of fractionally charged quasiparticles in condensed matter systems, topological anyons (with possible applications to quantum computing), and the conducting surface states in topological insulators. It also hints at why the weak interactions only interact with left-handed particles; this might be due to our universe being such a domain wall and all matter comprising the trapped particles.

In the late 1970s through the 1990s, Chang’s work took a new direction. After a collaboration with Nathan Weiss on the classical instability of the field equations in quantum chromodynamics (QCD) and the realization that the subsequent motion might be chaotic, Chang began a systematic and highly cited exploration of chaos in classical mechanical systems and the related notion of iterated maps. A remarkable byproduct of this work is a beautiful theorem in pure mathematics that generalizes to n dimensions the classical theorem of Poncelet on polygons that simultaneously inscribe and circumscribe a pair of conic sections.

Chang was a very interactive and valuable member of the department. Illinois Physics Professor Gordon Baym remarked that, “Shau-Jin was the person I could turn to for wisdom on almost any question in physics.” Chang was a mentor who took his students seriously and was dedicated to teaching. He won the Illinois Physics Best Teacher Award many times. He received the Distinguished Alumni Award from the Department of Physics, National Taiwan University.

Chang’s daughter Iris summed it up at his retirement party in 1999:
“My father, too, represented an ideal for me to aspire to. He is perhaps one of the most idealistic people I know—one of the rare individuals on this earth motivated solely by the pursuit of knowledge, rather than personal ambition … Money, power, social status— all of these things meant nothing to him unless he could enjoy a quiet intellectual life, doing the two things he loved most: physics and the nurturing of young minds. I consider him blessed because he found that life at the University of Illinois.

Father also possessed a strong sense of justice. I always believed that had he not become a physicist that he would have made an excellent judge. He had a keen sense of what was fair, as well as the uncanny ability to perceive an issue from all different points of view. But he also had a deep compassion for others, an intuitive understanding of human weakness, and a genuine sympathy for the underdog, a feeling that extended even to animals. ... Nothing distressed him more than to see a helpless creature hurt, just as nothing infuriated him more than to see a blatant abuse of power.

Perhaps what I admire most about my father is that he never lost that childlike wonder for the world. Rare in a world of cynics, Father has retained his fascination for the universe and for all the mysteries that lie within. For him, education is a lifelong endeavor. … He is what Einstein would have considered the quintessential intellectual— the person who learns for the same reasons a child wants to learn ... for love, curiosity, and the sheer thrill of discovery.”

Chang is survived by Ying-Ying Chang, his wife of 60 years, as well as his son Michael Chang, a computer engineer. His late daughter Iris Chang passed away in 2004.