A,Conservative,Revolutionary,(I)1一个保守的革命者（上）
弗里曼·戴森 译/肖明波 Freeman Dyson
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 Franks “Selected papers 1945–1980 with commentary”， published in 1983 to celebrate his sixtieth birthday. This is an anthology of Franks 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 Franks 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 Franks 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 Fermis collected papers. Frank studied with Fermi in Chicago from 1946 to 1949. He learned more physics from Fermi than from anybody else， and Fermis way of thinking left an indelible impression in his mind. Frank writes， “We learned that physics should not be a specialists 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”. Fermis 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 Fermis 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 Fermis 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 Franks 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與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。
在弗兰克这本书中，篇幅最短却又最为光彩夺目的一件瑰宝，是对费米的那两页描述。它是为费米和他合写的一篇论文所做的导读，这篇论文收录在费米的一本论文集中。1946年至1949年，弗兰克在芝加哥大学师从费米。他从费米那学到的物理知识比从其他任何人那里学到的都多，而费米的思维方式也在弗兰克的脑海中留下了难以磨灭的印象。弗兰克写道：“我们了解到，物理学不应该是专业人士的一门学问。物理学是要彻底从头开始搭建的，一块砖一块砖地砌，一层一层地垒。我们了解到，抽象工作出现在细致的基础工作之后，而不是出现在它之前。”从1954年发表的那篇卓越的杨—米尔斯合写论文的题目中，可以看出费米脚踏实地的精神。如今，人们一谈到这篇论文，就称之为那篇引入了非阿贝尔规范场的论文。可是，它的题目并没有提到非阿貝尔规范场。它的题目是《同位旋守恒与同位规范不变性》。“如何理解同位旋守恒”这个物理问题出现在先，而“非阿贝尔规范场”这个抽象的数学概念出现在后。倘若由费米来处理这个问题，他会用这种方式，弗兰克在处理这个问题时也采用了这种方式。费米是伟大的，因为他既懂得如何计算，又懂得如何倾听大自然的声音。终其一生，弗兰克很好地兼顾了自己在抽象数学方面的天赋和费米对物理细节的实事求是。
请允许我在这里简短地插入一个关于费米的故事，这是和弗兰克毫无关系的题外话。我不是费米的学生，但我有幸在学术生涯的关键时刻和费米共处了20分钟。我从这20分钟内学到的东西，比我在20年时间里从奥本海默那学到的还要多。1952年，我以为自己找到了一个关于强相互作用的好理论。我组织了一帮康奈尔大学学生和博士后，用这个新理论计算介子—质子的散射。我们的运算结果和费米在芝加哥大学回旋加速器上测得的截面非常吻合。于是，我得意地从伊萨卡赶到芝加哥，给费米看我们的结果。费米很客气也很友好，但对我们的结果没有多大兴趣。他说：“计算有两种方式。我喜欢第一种方式，就是有个清晰的物理图景。第二种方式要有严格的数学架构。你哪种都没有。”他和我的对话就此结束，也宣告了我们这种理论的终结。后来的结果证明，我们的理论不可能成立，因为它没有将矢量相互作用考虑在内。费米凭直觉就看出这个理论必定是错误的。在这20分钟时间里，他的常识帮我们省掉了好几年无谓的计算。弗兰克无须吸取这个教训，因为他在芝加哥大学读书期间，就已经掌握了费米的常识。
论文选集出版后的15年，弗兰克并没有闲着。1995年又出了一本书，这次不是他自己写的，而是他的朋友们向他的70华诞献上的贺礼，书名是《杨振宁——20世纪的一位伟大物理学家》。这本书包含了一些个人的赞誉和回忆，都隐含在书中的专业文章里。它描述了弗兰克多年来（至今仍在）如何积极参与帮助中国取得科学上的发展和繁荣。
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