A Conservative Revolutionary (I)1一个保守的革命者（上）

I am delighted to have this opportunity to sing the praises of my old friend and colleague Frank Yang. The title of my talk is “A Conservative Revolutionary”. The meaning of the title will become clear at the end of the talk.

One of my favorite books is Frank’s “Selected papers 1945–1980 with commentary”， published in 1983 to celebrate his sixtieth birthday. This is an anthology of Frank’s writings， with a commentary written by him to explain the circumstances in which they were written. There was room in the book for only one third of his writings. He chose which papers to include， and his choices give a far truer picture of his mind and character than one would derive from a collection chosen by a committee of experts. Some of the chosen papers are important and others are unimportant. Some are technical and others are popular. Every one of them is a gem. Frank was not trying to cram as much hard science as possible into five hundred pages. He was trying to show us in five hundred pages the spirit of a great scientist， and he magnificently succeeded. The papers that he chose show us his personal struggles as well as his scientific achievements. They show us the deep sources of his achievements， his pride in the Chinese culture that raised him， his reverence for his teachers in China and in America， his love of formal mathematical beauty， his ability to bridge the gap between the mundane world of experimental physics and the abstract world of groups and fiber bundles. He wisely placed the eighty pages of commentaries together at the beginning of the book instead of attaching them to the individual papers. As a result， the commentaries can be read consecutively. They give us the story of Frank’s life in the form of an intellectual autobiography. The autobiography is a classic. It describes the facts of his life in clear and simple words. It quietly reveals the intense feelings and loyalties that inspired his work and made him what he is.

One of the smallest and brightest of the gems in Frank’s book is a two-page description of Fermi， written as an introduction to a paper by Fermi and Yang that was included in a volume of Fermi’s collected papers. Frank studied with Fermi in Chicago from 1946 to 1949. He learned more physics from Fermi than from anybody else， and Fermi’s way of thinking left an indelible impression in his mind. Frank writes， “We learned that physics should not be a specialist’s subject. Physics is to be built from the ground up， brick by brick， layer by layer. We learned that abstractions come after detailed foundation work， not before”. Fermi’s practical spirit can be seen in the title of the great Yang-Mills paper published in 1954. Anyone speaking about the paper today would call it the paper that introduced non-Abelian gauge fields. But the title does not mention non-Abelian gauge fields. The title is “Conservation of isotopic spin and isotopic gauge invariance”. The physical question， how to understand the conservation of isotopic spin， came first， and the mathematical abstraction， non-Abelian gauge fields， came second. That was the way Fermi would have approached the problem， and that was the way Frank approached it too. Fermi was great because he knew how to do calculations and also knew how to listen to what nature had to say. All through his life， Frank has balanced his own gift for mathematical abstraction with Fermi’s down-to-earth attention to physical details.

Please let me digress here briefly to tell a story about Fermi that has nothing to do with Frank. I was not a student of Fermi， but I had the good luck to spend twenty minutes with Fermi at a crucial point in my career. I learned more from Fermi in twenty minutes than I learned from Oppenheimer in twenty years. In 1952 I thought I had a good theory of strong interactions. I had organized an army of Cornell students and post-docs to do calculations of meson-proton scattering with the new theory. Our calculations agreed pretty well with the cross-sections that Fermi was then measuring with the Chicago cyclotron. So I proudly traveled from Ithaca to Chicago to show him our results. Fermi was polite and friendly but was not impressed. He said， “There are two ways to do calculations. The first way， which I prefer， is to have a clear physical picture. The second way is to have a rigorous mathematical formalism. You have neither”. That was the end of the conversation and of our theory. It turned out later that our theory could not have been right because it took no account of vector interactions. Fermi saw intuitively that it had to be wrong. In twenty minutes， his common sense saved us from several years of fruitless calculations. This was a lesson that Frank did not need to learn， since he had already absorbed Fermi’s common sense during his years as a student in Chicago.

Frank has not been idle during the fifteen years since his selected papers were published. Another book was published in 1995， this time not written by Frank but by his friends， a festschrift to celebrate his seventieth birthday， with the title “Chen Ning Yang， a great physicist of the twentieth century”. This book contains， hidden among the technical contributions， a number of personal tributes and recollections. It describes Frank’s active involvement， continuing up to the present day， helping science to grow and flourish in China.

很高兴有机会来给我的老朋友和同行弗兰克·杨（即杨振宁）唱唱赞歌。我演讲的题目叫《一个保守的革命者》。演讲结束时，你们会明白这个题目的含义。

我最喜爱的一本书是为庆祝弗兰克60大寿，在1983年出版的《述评版杨振宁论文选（1945—1980）》。这是弗兰克的论文选集，他给每篇论文都写了一份说明，用以解释各篇论文的写作背景。限于篇幅，该书只收录了他全部论文的三分之一。他亲自敲定了哪些论文入选，这种选择远比一个由专家委员会挑选出来的集子更能体现他的思想和性格。入选的论文有些很重要，有些并不重要。有些是专业性的，有些是普及性的。但篇篇都是瑰宝。他没有试图在500页纸上，尽可能多地塞进艰深的科学内容。他在这500页纸上，努力向我们展示一位伟大科学家的精神，并且极为出色地做到了这一点。这些论文向我们展现了他的科学成就，同时也展示了他个人的奋斗。它们向我们展示了他取得成就的深刻缘由，展示了他对培育他的中国文化所感到的骄傲，展示了他对自己的中国老师和美国老师所怀有的崇敬，展示了他对数学形式美的热爱，还展示了他将平凡的实验物理世界和抽象的群与纤维丛世界相结合的能力。他很明智地将长达80页的说明汇总在一起，置于该书的开头部分，而不是附在各篇论文中。因此，这些说明可以连起来阅读。它们以学术自传的形式，向我们讲述了弗兰克的人生故事。这个自传是经典之作。它以干净洗练的文字描述了他的生平事迹。它淡淡地揭示出那些给他的工作带来灵感并让他取得现在这些成就的强烈情感与忠诚。