阿留 惊闻李政道先生仙逝，他和杨先生关于弱相互作用中宇称不守恒原创权一辈子的争论，又一次浮出水面。谨以7年前写一篇小文澄清一些事实，作为对大师的悼念。https://blog.wenxuecity.com/blog/frontend.php?act=article... » 2024-08-06 12:23:58 ( 3687 reads)
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惊闻李政道先生仙逝，他和杨先生关于弱相互作用中宇称不守恒原创权一辈子的争论，又一次浮出水面。谨以7年前写一篇小文澄清一些事实，作为对大师的悼念。https://blog.wenxuecity.com/blog/frontend.php?act=articlePrint&blogId=61334&date=201703&postId=7443
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大家知道，李政道和杨振宁两位物理大师，因为发现弱相互作用中的”宇称不守恒“而荣获诺奖；而后因谁提出了原创思想，而争论了一辈子。对此，李先生说这源自他1956年4月的想法。然而要客观看待历史，就不能只看一个人怎么讲。本文简要介绍一下1956年4月的罗切斯特会议对提出“宇称不守恒”的重要作用，以提供一个更加客观的线索。
关于宇称的历史背景，当时要解决的是Tau-Theta之谜，即Tau和Theta这两种粒子倒底是不是一回事。两种粒子其他性质都相同，唯独表现出不同的宇称。在1956年4月的罗切斯特会议上，这个问题经过认真讨论。2008年《波兰物理学报》以物理史家的第三者视角【1】，对此作了详细的介绍：
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.
此次会议，设置了“新粒子的理论解释”这一分会，美国的原子弹之父奥本海默，是这个分会的主席。杨振宁做了引言报告，总结当时关于tao-theta之谜的一些结果。事实上，在此之前李、杨已经发表了两篇文章试图解释这个现象，当然都尚未涉及宇称不守恒。但是在1956年4月初的罗切斯特会议上，通过大家的讨论，碰撞出了质疑”宇称守恒“的火花。比如根据另一位大牛费曼的回忆：
“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?’
和费曼同屋的Martin Block，是一位当时尚没有多少名气的实验物理学家，却也提出了对宇称不守恒的质疑。（他后来成为了西北大学物理系主任http://www.aspentimes.com/news/obituaries/founder-of-aspen-winter-physics-conferences-dies/）。费曼的回忆也提到了他代表Block向李政道提问：”如果宇称守恒定则错了，结果会怎样？“对此李政道给了一个很复杂的回答，连费曼都没有听懂。可见当时李此时对这个答案也不确定。
‘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 . . . ”.
网上还可以查到这次会议最后一天的总结发言【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 （盖尔曼）presented a list of approaches to the problem which he had considered, but without designating his choice. Richard Feynman （费曼）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 （即杨振宁）told the meeting that he had looked into several aspects of the nonconservation of parity without reaching a conclusion.”
也就是说，质疑宇称守恒，恐怕是这个会议上集体讨论使与会者受到的启发，产生这一想法的人，绝对不止一个。盖尔曼、费曼和杨振宁这几位大牛，以及实验物理学家Martin Block，都考虑到了不守恒的可能性；杨还特别提到他从几个不同角度审视了宇称不守恒，但尚未得出结论。不但如此，李先生还直接从费曼替Block提的问题中获得了启发，"What would be the consequences if the parity rule was wrong?"，不可能不引起他更深入的思考。而李先生的回忆，也说宇称不守恒的想法产生于56年4月，印证了他在罗切斯特会议上受到的影响。
综上所述，这一质疑，其实并非李、杨二位的独创，而是在罗切斯特会议上，在大家的讨论中产生和逐步清晰的一条新思路。李先生受到了Block、费曼质疑的启发；尽管杨先生此时已经在往这个方向考虑了，但直到会议开始时，尚未敢豁然提出；大家的热议，才促使他在总结发言时表达了一些看法。我想，这些讨论，应该促进了杨、李进一步往不守恒的方向考虑。
但另一方面，物理学的质疑不能只是一句话，那样的话论点就成了光杆司令。必须有翔实理论和实验分析来支持。能抓住这一想法，在2个月内作出深入研究，并提出具体的实验验证方案，这才是李、杨两位的真正贡献，体现了他们作为物理大师的卓越眼光和能力。杨先生后来每次演讲，都提到问题的关键是最终想到了只有在弱相互作用中宇称才会不守恒，这样他们就不至于让理论违背大量其他相互作用中宇称守恒的事实。
所以从某种意义上说，这二位争论谁第一个提出”不守恒“的想法并无意义，因为他们其实都在这次会议上受到了的启发，也不止一位与会者产生了这样的质疑；关键是谁在论证中做出了决定性的贡献。而这一点，史家恐怕是永远弄不清楚的，因为即使李先生本人也用了”你追我赶“交替领先的描述【4】，在具体论证方面，很难讲谁的贡献更大。其实，这对历史而言也实在并不重要——真理，早已在两人的合作和吴女士的验证中获得了，而吴的实验验证，和他们的理论一样重要。
不但如此，此后杨、李基于”只有在弱相互作用中宇称才会不守恒“的关键假设，想查看一下到底有没有弱相互作用中宇称守恒的实验验证，吴女士还为李、杨的这篇论文提供了重要实验参考资料【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.”
正是有了这些实验资料，才促成了李杨二位论证”不守恒“的决心。所以，如果要我评诺贝尔奖，吴先生应该领40%的奖金，李、杨各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：“两个孩子十分好奇，他们肩并肩向着光跑去。有的时候一个在前面，有的时候另一个在前面。”
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多谢您雅临分享，您的记忆应该是比较准确的。其实李先生那时还是年轻，有点心急；其实上当时绝大部分物理学家都不相信宇称会不守恒，几乎没有人想投入真金白银和大量时间去实验验证一个看上去很不可能的理论猜想。因此他即便静等吴先生的消息，诺奖也不会跑了，根本不用去劝说其他实验物理学家。毕竟吴先生是第一个给他提供支持的实验物理学家，既然跟人家说好了，再找别人合作，怎么说还是有点不妥吧。
多谢平兄雅临同感！感觉李、杨二位真的不值得为此争执，两人都已经成功得不能再成功了，还不满足吗，非要一争高下？如果讲点佛家的”不分别“，继续合作下去，也许本来可以谱写更多的辉煌。可见顶级科学家读点佛学，当是开卷有益。: )