In February 2023, Professor Lee Chang—the longest-serving faculty member at Tsinghua University—departed this world at the age of 98, leaving behind a lifetime of devotion to physics and education. He was a witness and a major contributor to the physics research and education in China over the last 70 years. As a physicist, his research in the 1950 s on steady states of electron–positron systems and their annihilation transitions earned international recognition as the foundational work of positron annihilation spectroscopy [24]. In the 1980 s, he and his colleagues pioneered a new technique called spinor calculus to formulate the helicity amplitudes used in non-Abelian gauge theories [25]. Known in the literature as “Chinese Magic”, this work received in 2013 the Chinese Physical Society’s highest honor, the Zhou Peiyuan Award. As an educator, his teaching career spanned the entire history of the People’s Republic of China. Always in a race against time, he never lost his enthusiasm or passion for teaching, and his lifelong devotion inspired generations of young scholars.

Beginning in late 2023, the AAPPS Bulletin launched a series of papers in honor of Professor Lee Chang, showcasing the wide-ranging research of his students. This memorial collection pays tribute to his enduring legacy and celebrates the countless memories of his extraordinary 98-year life as a distinguished physicist and educator.

Chang was born in Tianjin in 1925. He graduated from Fu Jen Catholic University in 1946, ranking first in the School of Science. After spending several years as a research assistant at Shandong University and Fu Jen Catholic University, he moved to Cornell University in the United States in 1948 to pursue his Ph.D. degree. In early 1949, responding to a friend’s call to serve the newly founded People’s Republic of China, he returned to Tianjin to teach in the Department of Physics at Peiyang University (now Tianjin University).

In 1952, Chang was selected to study in the Soviet Union, beginning his preparation by studying Russian at the Beijing Russian Language Institute. The following year, he entered Leningrad State University for postgraduate studies in theoretical physics under the supervision of V. A. Fock—an academician of the USSR Academy of Sciences best known for his pioneering work on the Hartree–Fock method. Chang’s research focused on electron–positron many-body systems. He successfully integrated the stationary states and annihilation transitions of multi-electron–positron systems, under the influence of external fields such as nuclear and solid-state periodic potentials, into Fock’s theoretical framework [24]. His results demonstrated how these external fields modify electron–positron interactions during annihilation, revealing the natural width of the annihilation line, the two-photon angular correlation, and multi-electron correlation effects. This groundbreaking work laid the theoretical foundation for the study of positron annihilation in complex multi-electron systems.

In 1957, after returning to China, Chang began his long association with Tsinghua University. He played an instrumental role in founding the Department of Engineering Physics and went on to serve as its deputy head and director of the Nuclear Physics Teaching and Research Group. From 1957 to 1963, Chang devoted himself primarily to teaching and training students, leaving little time for scientific research. In 1963, however, he was granted the opportunity to visit the Niels Bohr Institute in Copenhagen, Denmark, for one year, together with Fujia Yang. During his time there, Chang collaborated with a senior Japanese theoretical physicist, Ziro Koba, on studies of high-energy particle collisions. Because perturbation theory could not adequately describe high-energy nucleon–nucleon scattering, they adopted an alternative approach by parameterizing the S-matrix and determining its various parameters based on unitarity and symmetry principles [26]. Their results provided important insights for interpreting experimental data.

Chang’s teaching and research were largely suspended during the Cultural Revolution (1966–1976), apart from a short period teaching a solid-state physics class. When he resumed teaching in 1978, his lectures on quantum mechanics were met with great enthusiasm. Many of his former students in this class went on to become leading figures in China’s condensed matter community, including Binglin Gu, Bangfen Zhu, Shoushan Fan, and Senfang Sui, all later elected as academicians of the Chinese Academy of Sciences.

In 1979, Caltech and Tsinghua University established a cooperative relationship, and R. Vogt, the head of the Department of Mathematical Physics and Astronomy at Caltech, invited Chang for a one-year visit. From 1980 to 1981, Chang conducted research within Caltech’s particle theory group, which included distinguished theoretical physicists such as Richard Feynman and Michael Gell-Mann. During his stay, he contributed to the group’s research on quantum chromodynamics (QCD), co-authoring a paper on the π-meson form factor with three colleagues [27], and also independently pursued studies on photon–photon collisions. This year marked a significant reconnection with his own research, which had been interrupted for over a decade.

In 1981, following his return from Caltech, Chang was appointed Head of the Department of Engineering Physics. In June 1982, Tsinghua University reinstated the Department of Physics, transferring the solid-state physics, nuclear physics, and theoretical physics programs from the original Department of Engineering Physics to the newly reestablished department. Chang served as the first head of the reinstated Department of Physics until he stepped down in 1984.

In 1983, Chang invited Professor Tai-Tsun Wu from Harvard University, a distinguished theoretical physicist, to visit Tsinghua University. During his visit, Wu introduced the helicity amplitude method for studying multiple bremsstrahlung processes and \({e}^{+}{e}^{-}\) multiphoton annihilation in massless quantum electrodynamics (QED) and posed the question of whether this method could be extended to non-abelian gauge fields such as QCD. In response, Chang organized a small research group to explore the problem and developed a new technique—spinor calculus, later known as “Chinese Magic”—to formulate helicity amplitudes for non-abelian gauge fields [25]. In a chapter of “Gauge/String Duality for Weak Coupling”, Edward Witten commented on this helicity amplitude approach, noting that the helicity amplitudes in the perturbative expansion of gauge field theory exhibit a surprisingly simple analytic structure, a simplicity that may provide new insights into the foundations of gauge theory.

In the late 1980 s, in order to help graduate students transition more effectively from studying fundamental theories to engaging in scientific research, Chang decided to write a textbook titled Frontier Problems of Quantum Mechanics with Professor Molin Ge of Nankai University. Covering a broad range of advanced topics, the project took many years to complete. The first edition of the book was published in 2000, followed by an expanded second edition in 2012. Using this textbook [28], the Department of Physics at Tsinghua University launched the course Frontiers in Quantum Mechanics in 2000. Chang personally taught the course for many years (later joined by a young faculty member Jiazhong Hu), becoming the oldest active lecturer at Tsinghua University until his passing.

Beyond his research and teaching, Chang was also deeply devoted to science popularization, particularly in introducing outstanding international works of popular science to Chinese audiences. His translations, published by Tsinghua University Press, include A Brief History of Time: From the Big Bang to Black Holes by Stephen Hawking (distinct from another well-known Chinese translation of the same title), as well as Einstein’s Toys: Exploring the Secrets of the Universe and Gravity and Fearful Symmetry: The Search for Beauty in Modern Physics by Anthony Zee.

The process of writing the textbook inspired Chang to explore new research directions well into his seventies. Together with graduate student Rong Lü, he investigated macroscopic quantum tunnelling in magnetic particles and achieved significant results. In 1998, he organized a research group on Bose–Einstein condensates (BEC) at the Institute for Advanced Study, mentoring many young students from both the Institute and the Department of Physics as they entered the emerging field of ultracold atomic physics and launched their academic careers. Throughout this period, Chang remained actively involved in research, co-authoring six papers published in internationally recognized journals and supervising a doctoral student through to completion. Remarkably, in 2007, at the age of 82, he published a Physical Review Letters paper as first author—a testament to his enduring dedication and scholarly vitality [29].

The nine papers in this special collection of the AAPPS Bulletin, dedicated to Professor Lee Chang, stand as a testament to the enduring reach of his legacy in teaching, scientific research, and the nurturing of future generations of scholars. These papers encompass a wide range of topics, including information scrambling [30], quantum breakdown model [31], ultracold Fermi polarons [32], topological materials [33], macroscopic quantum tunnelling [34], non-Hermitian physics [35], exact Bethe ansatz solutions [36], novel computational approaches [37], and strong-coupling diagrammatic theory [38]. As guest editors, we express our heartfelt thanks to all the contributors for their generous support and participation in this special collection.

The Editorial Team of the collection “Lee Chang: A Legendary Physicist and Educator”.

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