驚聞李政道先生仙逝，他和楊先生關於弱相互作用中宇稱不守恒原創權一輩子的爭論，又一次浮出水麵。謹以7年前寫一篇小文澄清一些事實，作為對大師的悼念。https://blog.wenxuecity.com/blog/frontend.php?act=articlePrint&blogId=61334&date=201703&postId=7443
**********************************
大家知道，李政道和楊振寧兩位物理大師，因為發現弱相互作用中的”宇稱不守恒“而榮獲諾獎；而後因誰提出了原創思想，而爭論了一輩子。對此，李先生說這源自他1956年4月的想法。然而要客觀看待曆史，就不能隻看一個人怎麽講。本文簡要介紹一下1956年4月的羅切斯特會議對提出“宇稱不守恒”的重要作用，以提供一個更加客觀的線索。
關於宇稱的曆史背景，當時要解決的是Tau-Theta之謎，即Tau和Theta這兩種粒子倒底是不是一回事。兩種粒子其他性質都相同，唯獨表現出不同的宇稱。在1956年4月的羅切斯特會議上，這個問題經過認真討論。2008年《波蘭物理學報》以物理史家的第三者視角【1】，對此作了詳細的介紹：
Nearly one hundred ninety physicists participated in the Sixth Annual Rochester Conference on April 3th–7th, 1956. One of its main topics was the rapidly growing field of the new elementary particles. The session on“Theoretical Interpretation of New Particles” was chaired by Oppenheimer...The introductory talk was delivered by Yang who gave a summary of experiments and several propositions to explain the tau–theta puzzle.
此次會議，設置了“新粒子的理論解釋”這一分會，美國的原子彈之父奧本海默，是這個分會的主席。楊振寧做了引言報告，總結當時關於tao-theta之謎的一些結果。事實上，在此之前李、楊已經發表了兩篇文章試圖解釋這個現象，當然都尚未涉及宇稱不守恒。但是在1956年4月初的羅切斯特會議上，通過大家的討論，碰撞出了質疑”宇稱守恒“的火花。比如根據另一位大牛費曼的回憶：
“It was during that discussion that the idea of parity nonconservation was first seriously discussed in large audience. Richard Feynman, who was a participant, gave a lively recollection of the event [2]: “I was sharing a room with a guy named Martin Block, an experimenter. And one evening he said to me, ‘Why are you guys so insistent on this parity rule? Maybe the tau and theta are the same particle. What would be the consequences if the parity rule were wrong?’
和費曼同屋的Martin Block，是一位當時尚沒有多少名氣的實驗物理學家，卻也提出了對宇稱不守恒的質疑。（他後來成為了西北大學物理係主任http://www.aspentimes.com/news/obituaries/founder-of-aspen-winter-physics-conferences-dies/）。費曼的回憶也提到了他代表Block向李政道提問：”如果宇稱守恒定則錯了，結果會怎樣？“對此李政道給了一個很複雜的回答，連費曼都沒有聽懂。可見當時李此時對這個答案也不確定。
‘So the next day at the meeting . . . I got up and said, ‘I’m asking this question for Martin Block: What would be the consequences if the parity rule was wrong?’Lee, of Lee and Yang, answered something complicated, and as usual I didn‘t understand very well.
At the end of the meeting Block asked me what he said, and I said I did not know, but as far as I could tell, it was still open — there was still a possibility. I didn’t think it was likely, but I thought it was possible . . . ”.
網上還可以查到這次會議最後一天的總結發言【3】：
“The last day of the conference was devoted to participants sharing their conclusions on the θ-τ puzzle. Frank Yang gave an introductory review. After several talks had been given, Robert Oppenheimer, the chairman of the session, was ready to close the session when several prominent physicists chose to make statements. Murray Gell Mann （蓋爾曼）presented a list of approaches to the problem which he had considered, but without designating his choice. Richard Feynman （費曼）brought up Block's suggestion in the form of the θ and τ mesons being the same particle but with no definite parity. Frank Yang （即楊振寧）told the meeting that he had looked into several aspects of the nonconservation of parity without reaching a conclusion.”
也就是說，質疑宇稱守恒，恐怕是這個會議上集體討論使與會者受到的啟發，產生這一想法的人，絕對不止一個。蓋爾曼、費曼和楊振寧這幾位大牛，以及實驗物理學家Martin Block，都考慮到了不守恒的可能性；楊還特別提到他從幾個不同角度審視了宇稱不守恒，但尚未得出結論。不但如此，李先生還直接從費曼替Block提的問題中獲得了啟發，"What would be the consequences if the parity rule was wrong?"，不可能不引起他更深入的思考。而李先生的回憶，也說宇稱不守恒的想法產生於56年4月，印證了他在羅切斯特會議上受到的影響。
綜上所述，這一質疑，其實並非李、楊二位的獨創，而是在羅切斯特會議上，在大家的討論中產生和逐步清晰的一條新思路。李先生受到了Block、費曼質疑的啟發；盡管楊先生此時已經在往這個方向考慮了，但直到會議開始時，尚未敢豁然提出；大家的熱議，才促使他在總結發言時表達了一些看法。我想，這些討論，應該促進了楊、李進一步往不守恒的方向考慮。
但另一方麵，物理學的質疑不能隻是一句話，那樣的話論點就成了光杆司令。必須有翔實理論和實驗分析來支持。能抓住這一想法，在2個月內作出深入研究，並提出具體的實驗驗證方案，這才是李、楊兩位的真正貢獻，體現了他們作為物理大師的卓越眼光和能力。楊先生後來每次演講，都提到問題的關鍵是最終想到了隻有在弱相互作用中宇稱才會不守恒，這樣他們就不至於讓理論違背大量其他相互作用中宇稱守恒的事實。
所以從某種意義上說，這二位爭論誰第一個提出”不守恒“的想法並無意義，因為他們其實都在這次會議上受到了的啟發，也不止一位與會者產生了這樣的質疑；關鍵是誰在論證中做出了決定性的貢獻。而這一點，史家恐怕是永遠弄不清楚的，因為即使李先生本人也用了”你追我趕“交替領先的描述【4】，在具體論證方麵，很難講誰的貢獻更大。其實，這對曆史而言也實在並不重要——真理，早已在兩人的合作和吳女士的驗證中獲得了，而吳的實驗驗證，和他們的理論一樣重要。
不但如此，此後楊、李基於”隻有在弱相互作用中宇稱才會不守恒“的關鍵假設，想查看一下到底有沒有弱相互作用中宇稱守恒的實驗驗證，吳女士還為李、楊的這篇論文提供了重要實驗參考資料【3】：
“The international expert in beta decay at that time was Chien-Shiung Wu and she was at Columbia. T.D. Lee went to see her to see if she knew of any experiment that tested the question of the conservation of parity in weak force interactions. She did not know of any but lent Lee a near-thousand page compilation of the results of experiments entitled Beta- and Gamma-Ray Spectroscopy.Yang and Lee went through that work over the course of two weeks and concluded that the conservation of parity in weak force interactions had never been tested. They subsequently wrote an article calling for the testing of the conservation of parity in weak force interactions. It was published in Physical Review in 1956.”
正是有了這些實驗資料，才促成了李楊二位論證”不守恒“的決心。所以，如果要我評諾貝爾獎，吳先生應該領40%的獎金，李、楊各30%，似乎更加公平合理吧？
【1】Andrzej K. Wróblewski，”THE DOWNFALL OF PARITY — THE REVOLUTION
THAT HAPPENED FIFTY YEARS AGO", ACTA PHYSICA POLONICA B, 39, page 254 (2008)
【2】R.P. Feynman, Surely You’re Joking, Mr. Feynman! The Adventures of a
Curious Character as told to Ralph Leighton, p. 247–248, W.W. Norton &
Company, New York–London 1985.
【3】http://www.sjsu.edu/faculty/watkins/paritynoncon.htm
【4】https://zh.wikipedia.org/wiki/%E6%9D%8E%E6%94%BF%E9%81%93#cite_note-7 李先生的比喻：“兩個孩子十分好奇，他們肩並肩向著光跑去。有的時候一個在前麵，有的時候另一個在前麵。”
更多我的博客文章>>>
