Yang Chen Ning–The Most Influential Physicist of Our Time

Passing through the Old Gate, you can see the early buildings of Tsinghua University. Visitors from all over the country and all over the world come here to take in the atmosphere of Tsinghua University as a century-old institution. In the northwest corner of these buildings stands the Science Building, the location of the Institute for Advanced Study where Yang Chen Ning serves as honorary director.

In order to advance the study of basic science in China, Tsinghua University established the Institute for Advanced Study in 1997, with Yang Chen Ning as the honorary director. Five years later, Yang decided to return to China and to his roots on the campus of the University – “Tsinghua Garden”, as it had been known to him.

Recalling his experience studying abroad and anticipating the future of his educational dream after returning to China, Yang composed a poem called Guigen (Back to My Roots), in which he wrote:

China transforms and Tsinghua shoulders heavy responsibilities,

Students chase their dreams while I guide their path.

Fruitful Achievements – World-Renowned Physicist

On the four sides of a small marble cube that sits in the office of the Science Building, 13 major contributions that Yang has made to physics are inscribed. It was a present from Tsinghua University for Yang’s 90th birthday.

Yang is best known as a member of the Chinese Academy of Sciences and a Nobel laureate. Some have described him as the third all-round physicist of the 20th century after Einstein and Fermi – a man possessed of a vast range of knowledge and talents. Others see him as a respected and humble elder – a man with whom one could argue about anything. In the field of physics, Yang’s name is always associated with the two theories: Parity Non- Conservation in Weak Interaction, and Yang-Mills Non- Abelian Gauge Theory.

In the 1950s, particle physics was gaining momentum in the United States. At the time, scientists believed that parity was “conserved”, meaning that a particle’s mirror image had exactly the same properties as itself. But the Theta-Tau puzzle has made scientists wonder whether this was just an exception to Parity Conservation or a breakthrough to overturn it. The work of Yang Chen Ning and Tsung-Dao Lee solved the conundrum. Together in June 1956 they published an article in Physical Review, which proved that previous Beta decay experiments were not complex enough to confirm parity conservation. By putting forward several new experiments that could be used to verify parity non- conservation, Yang and Lee blazed a trail for further research into this theory.

Many physicists recall October 1957 as a time of excitement and legend. In that year, at the age of 35, Yang won the Nobel Prize in Physics. Yang and Lee thereby became the first Chinese laureates. The significance of the award lay not only in the academic achievement, but also in the boost it provided to the self-belief of a nation. Before that, the scientific talent of the Chinese had been questioned. Ching-Wu Chu, a distinguished physicist specialized in superconductivity and a member of the US National Academy of Sciences, was in high school at the time. He spent his spare time reading every news report he could find about Yang, and talked earnestly to his classmates about “parity non-conservation” – a subject on which they could understand nothing.

Tsu-Teh Chou, a professor of physics at the University of Georgia, was dining at a tiny Chinese restaurant in Liverpool, England, 12 years later, and overheard both the chef and the owner talking proudly about Yang’s achievements.

In fact, the Yang-Mills Non-Abelian Gauge Theory has a much bigger academic influence than Parity Non- Conservation in Weak Interaction, earning him a reputation as a master in physics. According to Nie Huatong, former dean of the Institute for Advanced Study of Tsinghua University, “When Mr. Yang and Mr. Mills wrote their paper in 1954, ‘Yang-Mills’ was not recognized as a theory of physics, but only as a mathematical structure that might be useful for physics. By 1972, however, this very simple and beautiful mathematical structure was formally recognized as a fundamental structure of physics, and eventually laid the foundation for the weak interaction.” The Yang-Mills theory is seen as a work that has reshaped the development of physics and modern geometry since the second half of the 20th century. The Franklin Institute presented the 1994 Bower Award and Prize for Achievement in Science to Yang, its citation saying, “This theoretical model, already ranked alongside the works of Newton, Maxwell, and Einstein, will surely have a comparable influence on future generations.”

His groundbreaking contribution to theoretical physics made Yang a world-renowned physicist. Freeman Dyson, one of the founders of quantum electrodynamics and an internationally respected physicist, believed Yang was the preeminent architect of 20th-century physics after Albert Einstein and Paul A. M. Dirac. At Yang’s 70th birthday party, Samuel C. C. Ting, another Nobel laureate, said, “When we think of the major milestones in physics during the 20th century, three things come to mind... the first one is relativity (Albert Einstein), the second is quantum mechanics (Paul A. M. Dirac), and the third is gauge theory (Yang Chen Ning).”

Fallen Leaves Return to Their Roots – An Unbreakable Bond with Tsinghua

Chen Fong Ching, former director of the Institute of Chinese Studies at the Chinese University of Hong Kong, once summed up Yang’s life as follows: “His great achievements in physics represent only half of Mr. Yang’s life – the other is his attachment to China. The two are inseparable.” The blossoming of his patriotic sentiment would not have been possible without the nurturing of Tsinghua.

To Yang, Tsinghua was a childhood paradise. In 1929, he moved to Tsinghua with his parents because his father, Yang Ko Chuen, was recruited as a professor of mathematics at Tsinghua University.

As a child, Yang was full of enthusiasm. Despite the upheaval and turmoil in the world outside Tsinghua, both at home and abroad, Yang lived a peaceful and fulfilling life inside the school: He watched movies in the auditorium with his parents; he formed a cheerleading squad in the stadium with friends, cheering for the athletes of Tsinghua; he went skating on the frozen lotus pond during the cold winters... Yang later joked that he was not a well-behaved child when he was young. He climbed every tree in Tsinghua, but his father never corrected him too much for his unruly behaviors.

When the full-scale war against Japanese aggression broke out in 1937, Yang Chen Ning’s family left Tsinghua. By March 1938, they found themselves in Kunming. That same year, Yang was admitted to the Department of Physics of National Southwest Associated University after completing one semester of his 11th grade. After graduation, he transferred to the Graduate School to pursue his master’s degree. During his postgraduate studies, Yang developed a strong interest in symmetry principle, statistical mechanics, and many-body problem. According to him, it was the two papers he wrote in National Southwest Associated University that really solidified his academic interest. One was his bachelor’s thesis, “Molecular Spectroscopy and the Principle of Symmetry”, under the guidance of Mr. Wu Ta-You, and the other was the “Transformation of Order-Disorder” under the guidance of Mr. Wang Zhuxi.

After establishing his academic direction, Yang took the examination for the 6th Tsinghua University government- funded program for studying in the United States. In the spring of 1944, the examination results were announced, and Yang was the only student admitted to the physics major, which allowed him to study for his doctorate at the University of Chicago. After receiving his doctoral degree, Yang continued to devote himself to theoretical physics. He achieved notable success, but due to the precarious Sino- US relationship at that time, Yang could not return to his motherland.

In 1971, under the influence of Ping-pong Diplomacy, Sino-US relations eased to some extent. As a result, Yang set out from the United States through Canada and made his way back to China. He was the first Chinese scientist to return to China after the relationship between the two countries eased. By actively promoting cultural exchanges and mutual understanding between the two peoples, Yang opened the door to personnel exchanges and scientific and technological cooperation between the two countries. Yang also had a chance to make a return visit to his alma mater.

Since then, Yang has visited Tsinghua University many times, always concerned with strengthening science in his motherland, especially in the field of basic science research. His efforts eventually led to breakthroughs in the field at Tsinghua University. In May 1993, Tsinghua University recognized a significant challenge to be faced – to establish a truly world-class university, it must have first-class science majors. This required both funds and staff to support basic theoretical research in a relaxed but structured environment. Wang Dazhong, then Tsinghua’s president, laid great emphasis on the system and research model of the Institute for Advanced Study at Princeton University in a report by the Faculty of Science. It was imperative to establish an institute for advanced studies.

The development of the discipline needed leaders and builders who were well-known in scientific research worldwide, and there was no more suitable candidate than Yang Chen Ning. Therefore Tsinghua University offered a sincere invitation to Yang, hoping that he would steer the building of the Institute of Natural Sciences at Tsinghua University and give a strong boost to science majors in the University.

Yang had long been waiting for this moment. Despite having spent so many years abroad, his pulse continued to beat to the future of China. Recalling his ambitious dream winning a Nobel Prize he had harbored since the age of 12, and the solemn commitment made when he was leaving to study abroad that one day he would return, Yang could not suppress his excitement.

After careful preparation, the Institute for Advanced Studies of Tsinghua University was formally established on June 2, 1997, with Yang Chen Ning as the honorary director. Speaking at the inauguration, Yang anticipated that the institute would make a significant contribution to international science and technology in the coming 10, 20, or 50 years.

Stay Loyal to Where I Came from – For the Next Generation of Scientists

After returning to Tsinghua, Yang immediately threw himself into a compressed and busy work schedule, maintaining his vigorous attitude towards scientific research and teaching. In the fall semester of 2004, the 82-year-old Yang still lectured in General Physics to four freshman classes. He had two classes and one office hour session per week, prepared exam questions for the mid- term and final exams, and was present in the examination room. His role was exemplary for the students.

According to students who attended Yang’s classes at that time, the classroom was always fully occupied with hundreds, or even close to a thousand students for each lecture. Those who were lucky enough would get a seat, while others huddled in the hallways or on the windowsills of the classroom. Those who could not squeeze into the classroom also gathered around the back door or the windows of the classroom to catch a glimpse of Yang lecturing.

In addition to his teaching, Yang was always ready to promote basic science in China and develop future talent. In 2009, there was a renewed discussion about the concern expressed by the renowned physicist Hsue- Shen Tsien – why Chinese universities could not foster outstanding talent on a par with the masters of the early 20th century. Chen Xi, then vice-minister of education and former secretary of the CPC Committee at Tsinghua University, came to Tsinghua to investigate how to better train basic researchers, and paid a special visit to Yang. Yang put forward many constructive suggestions outlining his expectations for the younger generation. Soon after, Tsinghua University set up a Physics Class, part of the Tsinghua Xuetang Program, aiming at training world-class physicists.

One of Yang’s main concerns is to communicate with the next generation of scientists, hoping to inspire more young people with an interest in the discipline to contribute to the development of scientific fields in China. At the International Science Master Forum during the 15th annual meeting of the China Association for Science and Technology, in response to a senior high school student’s question about “how to cultivate and maintain innovative spirit and curiosity”, Yang answered that research on the frontiers of science generally goes through both painful times and happy times. “I think there are two very important mindsets that you must maintain simultaneously: The first is don’t give up no matter what, and the second is if you really feel that you can’t go on any more, you need to find yourself a new direction,” he said.

Looking back on a relationship with Tsinghua University that has lasted for more than 90 years, Yang felt he had returned to begin a new career. “When I came back to Tsinghua, I started a new journey,” he said. “My main goals here are to help Tsinghua train young people. Tsinghua has some of the best young people in China. It is a worthwhile mission to do something to help them and lead them towards the correct path in study and research. Of course, older people are calmer in general, and I have some hopes that when I look back, I have done something meaningful in my new career.”

Now in his 90s, Yang strolls around the campus with a walking cane. Memories of the past are evoked as he passes by each scene on campus. Locust trees and ginkgo continue to flourish by the roadsides, while bright young students pass him by. Yang seems to see his own reflection in the students – the bright young man who was full of scientific enthusiasm.

Yang has spent his whole life in the ivory tower of academia, which has allowed him to maintain a certain level of simplicity and innocence. He recalls his youth at Tsinghua, his school years at NSAU, his academic journey in the United States, and his life back in Tsinghua since 2003. “My life has been a circle, where I started out from a point, travelled a long way, and finally returned to where I came from.”

At the age of 98, Yang Chen Ning has returned to the starting point of his life journey, completing a voyage of almost a century in what seems like the blink of an eye. In his poem Thoughts in the 90s, Yang wrote, “My life is full of hopes and devotion with goals, principles, and no regrets.” He has always been the boy who loved to climb trees, and he has spent his whole life climbing the “tree” that leads to the palace of science. “Being down-to-earth and simple” is Yang Chen Ning’s expectation for China’s endeavors in science, and also his life creed. It represents a belief that one of the most influential physicists of our time has practiced for his whole life.

Writers: Liu Shutian, Zhang Liwen

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