A Conservative Revolutionary (I)1一個保守的革命者（上）

弗里曼·戴森 譯/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.

很高興有機會來給我的老朋友和同行弗蘭克·楊（即楊振寧）唱唱贊歌。我演講的題目叫《一個保守的革命者》。演講結束時，你們會明白這個題目的含義。

我最喜愛的一本書是為慶祝弗蘭克60大壽，在1983年出版的《述評版楊振寧論文選（1945—1980）》。這是弗蘭克的論文選集，他給每篇論文都寫了一份說明，用以解釋各篇論文的寫作背景。限于篇幅，該書只收錄了他全部論文的三分之一。他親自敲定了哪些論文入選，這種選擇遠比一個由專家委員會挑選出來的集子更能體現他的思想和性格。入選的論文有些很重要，有些并不重要。有些是專業性的，有些是普及性的。但篇篇都是瑰寶。他沒有試圖在500頁紙上，盡可能多地塞進艱深的科學內容。他在這500頁紙上，努力向我們展示一位偉大科學家的精神，并且極為出色地做到了這一點。這些論文向我們展現了他的科學成就，同時也展示了他個人的奮斗。它們向我們展示了他取得成就的深刻緣由，展示了他對培育他的中國文化所感到的驕傲，展示了他對自己的中國老師和美國老師所懷有的崇敬，展示了他對數學形式美的熱愛，還展示了他將平凡的實驗物理世界和抽象的群與纖維叢世界相結合的能力。他很明智地將長達80頁的說明匯總在一起，置于該書的開頭部分，而不是附在各篇論文中。因此，這些說明可以連起來閱讀。它們以學術自傳的形式，向我們講述了弗蘭克的人生故事。這個自傳是經典之作。它以干凈洗練的文字描述了他的生平事跡。它淡淡地揭示出那些給他的工作帶來靈感并讓他取得現在這些成就的強烈情感與忠誠。

在弗蘭克這本書中，篇幅最短卻又最為光彩奪目的一件瑰寶，是對費米的那兩頁描述。它是為費米和他合寫的一篇論文所做的導讀，這篇論文收錄在費米的一本論文集中。1946年至1949年，弗蘭克在芝加哥大學師從費米。他從費米那學到的物理知識比從其他任何人那里學到的都多，而費米的思維方式也在弗蘭克的腦海中留下了難以磨滅的印象。弗蘭克寫道：“我們了解到，物理學不應該是專業人士的一門學問。物理學是要徹底從頭開始搭建的，一塊磚一塊磚地砌，一層一層地壘。我們了解到，抽象工作出現在細致的基礎工作之后，而不是出現在它之前。”從1954年發表的那篇卓越的楊—米爾斯合寫論文的題目中，可以看出費米腳踏實地的精神。如今，人們一談到這篇論文，就稱之為那篇引入了非阿貝爾規范場的論文。可是，它的題目并沒有提到非阿貝爾規范場。它的題目是《同位旋守恒與同位規范不變性》。“如何理解同位旋守恒”這個物理問題出現在先，而“非阿貝爾規范場”這個抽象的數學概念出現在后。倘若由費米來處理這個問題，他會用這種方式，弗蘭克在處理這個問題時也采用了這種方式。費米是偉大的，因為他既懂得如何計算，又懂得如何傾聽大自然的聲音。終其一生，弗蘭克很好地兼顧了自己在抽象數學方面的天賦和費米對物理細節的實事求是。

請允許我在這里簡短地插入一個關于費米的故事，這是和弗蘭克毫無關系的題外話。我不是費米的學生，但我有幸在學術生涯的關鍵時刻和費米共處了20分鐘。我從這20分鐘內學到的東西，比我在20年時間里從奧本海默那學到的還要多。1952年，我以為自己找到了一個關于強相互作用的好理論。我組織了一幫康奈爾大學學生和博士后，用這個新理論計算介子—質子的散射。我們的運算結果和費米在芝加哥大學回旋加速器上測得的截面非常吻合。于是，我得意地從伊薩卡趕到芝加哥，給費米看我們的結果。費米很客氣也很友好，但對我們的結果沒有多大興趣。他說：“計算有兩種方式。我喜歡第一種方式，就是有個清晰的物理圖景。第二種方式要有嚴格的數學架構。你哪種都沒有。”他和我的對話就此結束，也宣告了我們這種理論的終結。后來的結果證明，我們的理論不可能成立，因為它沒有將矢量相互作用考慮在內。費米憑直覺就看出這個理論必定是錯誤的。在這20分鐘時間里，他的常識幫我們省掉了好幾年無謂的計算。弗蘭克無須吸取這個教訓，因為他在芝加哥大學讀書期間，就已經掌握了費米的常識。

論文選集出版后的15年，弗蘭克并沒有閑著。1995年又出了一本書，這次不是他自己寫的，而是他的朋友們向他的70華誕獻上的賀禮，書名是《楊振寧——20世紀的一位偉大物理學家》。這本書包含了一些個人的贊譽和回憶，都隱含在書中的專業文章里。它描述了弗蘭克多年來（至今仍在）如何積極參與幫助中國取得科學上的發展和繁榮。