A Conservative Revolutionary (I)1一個(gè)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(jī)會(huì)來(lái)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(gè)保守的革命者》。演講結(jié)束時(shí)，你們會(huì)明白這個(gè)題目的含義。

我最喜愛(ài)的一本書(shū)是為慶祝弗蘭克60大壽，在1983年出版的《述評(píng)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(xiě)了一份說(shuō)明，用以解釋各篇論文的寫(xiě)作背景。限于篇幅，該書(shū)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(yuǎn)比一個(gè)由專家委員會(huì)挑選出來(lái)的集子更能體現(xiàn)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(yè)性的，有些是普及性的。但篇篇都是瑰寶。他沒(méi)有試圖在500頁(yè)紙上，盡可能多地塞進(jìn)艱深的科學(xué)內(nèi)容。他在這500頁(yè)紙上，努力向我們展示一位偉大科學(xué)家的精神，并且極為出色地做到了這一點(diǎn)。這些論文向我們展現(xiàn)了他的科學(xué)成就，同時(shí)也展示了他個(gè)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(duì)培育他的中國(guó)文化所感到的驕傲，展示了他對(duì)自己的中國(guó)老師和美國(guó)老師所懷有的崇敬，展示了他對(duì)數(shù)學(xué)形式美的熱愛(ài)，還展示了他將平凡的實(shí)驗(yàn)物理世界和抽象的群與纖維叢世界相結(jié)合的能力。他很明智地將長(zhǎng)達(dá)80頁(yè)的說(shuō)明匯總在一起，置于該書(shū)的開(kāi)頭部分，而不是附在各篇論文中。因此，這些說(shuō)明可以連起來(lái)閱讀。它們以學(xué)術(shù)自傳的形式，向我們講述了弗蘭克的人生故事。這個(gè)自傳是經(jīng)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(lái)靈感并讓他取得現(xiàn)在這些成就的強(qiáng)烈情感與忠誠(chéng)。

在弗蘭克這本書(shū)中，篇幅最短卻又最為光彩奪目的一件瑰寶，是對(duì)費(fèi)米的那兩頁(yè)描述。它是為費(fèi)米和他合寫(xiě)的一篇論文所做的導(dǎo)讀，這篇論文收錄在費(fèi)米的一本論文集中。1946年至1949年，弗蘭克在芝加哥大學(xué)師從費(fèi)米。他從費(fèi)米那學(xué)到的物理知識(shí)比從其他任何人那里學(xué)到的都多，而費(fèi)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(xiě)道：“我們了解到，物理學(xué)不應(yīng)該是專業(yè)人士的一門學(xué)問(wèn)。物理學(xué)是要徹底從頭開(kāi)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(xiàn)在細(xì)致的基礎(chǔ)工作之后，而不是出現(xiàn)在它之前?！睆?954年發(fā)表的那篇卓越的楊—米爾斯合寫(xiě)論文的題目中，可以看出費(fèi)米腳踏實(shí)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(guī)范場(chǎng)的論文。可是，它的題目并沒(méi)有提到非阿貝爾規(guī)范場(chǎng)。它的題目是《同位旋守恒與同位規(guī)范不變性》?！叭绾卫斫馔恍睾恪边@個(gè)物理問(wèn)題出現(xiàn)在先，而“非阿貝爾規(guī)范場(chǎng)”這個(gè)抽象的數(shù)學(xué)概念出現(xiàn)在后。倘若由費(fèi)米來(lái)處理這個(gè)問(wèn)題，他會(huì)用這種方式，弗蘭克在處理這個(gè)問(wèn)題時(shí)也采用了這種方式。費(fèi)米是偉大的，因?yàn)樗榷萌绾斡?jì)算，又懂得如何傾聽(tīng)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(shù)學(xué)方面的天賦和費(fèi)米對(duì)物理細(xì)節(jié)的實(shí)事求是。

請(qǐng)?jiān)试S我在這里簡(jiǎn)短地插入一個(gè)關(guān)于費(fèi)米的故事，這是和弗蘭克毫無(wú)關(guān)系的題外話。我不是費(fèi)米的學(xué)生，但我有幸在學(xué)術(shù)生涯的關(guān)鍵時(shí)刻和費(fèi)米共處了20分鐘。我從這20分鐘內(nèi)學(xué)到的東西，比我在20年時(shí)間里從奧本海默那學(xué)到的還要多。1952年，我以為自己找到了一個(gè)關(guān)于強(qiáng)相互作用的好理論。我組織了一幫康奈爾大學(xué)學(xué)生和博士后，用這個(gè)新理論計(jì)算介子—質(zhì)子的散射。我們的運(yùn)算結(jié)果和費(fèi)米在芝加哥大學(xué)回旋加速器上測(cè)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(fèi)米看我們的結(jié)果。費(fèi)米很客氣也很友好，但對(duì)我們的結(jié)果沒(méi)有多大興趣。他說(shuō)：“計(jì)算有兩種方式。我喜歡第一種方式，就是有個(gè)清晰的物理圖景。第二種方式要有嚴(yán)格的數(shù)學(xué)架構(gòu)。你哪種都沒(méi)有。”他和我的對(duì)話就此結(jié)束，也宣告了我們這種理論的終結(jié)。后來(lái)的結(jié)果證明，我們的理論不可能成立，因?yàn)樗鼪](méi)有將矢量相互作用考慮在內(nèi)。費(fèi)米憑直覺(jué)就看出這個(gè)理論必定是錯(cuò)誤的。在這20分鐘時(shí)間里，他的常識(shí)幫我們省掉了好幾年無(wú)謂的計(jì)算。弗蘭克無(wú)須吸取這個(gè)教訓(xùn)，因?yàn)樗谥ゼ痈绱髮W(xué)讀書(shū)期間，就已經(jīng)掌握了費(fèi)米的常識(shí)。

論文選集出版后的15年，弗蘭克并沒(méi)有閑著。1995年又出了一本書(shū)，這次不是他自己寫(xiě)的，而是他的朋友們向他的70華誕獻(xiàn)上的賀禮，書(shū)名是《楊振寧——20世紀(jì)的一位偉大物理學(xué)家》。這本書(shū)包含了一些個(gè)人的贊譽(yù)和回憶，都隱含在書(shū)中的專業(yè)文章里。它描述了弗蘭克多年來(lái)（至今仍在）如何積極參與幫助中國(guó)取得科學(xué)上的發(fā)展和繁榮。