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费米的“直觉”与“武断”拯救了Freeman Dyson
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费米的“直觉”与“武断”拯救了Freeman Dyson
2009.02.25
科学家根据自己的经验和认识凭直觉所做的判断常常被外行贴上“武断”或者“专断”的标签。很多自以为具有探索精神的人甚至认为这些没有100%根据的凭直觉的判断缺乏逻辑，甚至是有悖于科学的自由探索精神。
我记得十年或者十多年前读到著名的理论物理学家Freeman Dyson在杨振宁先生的一个生日（大概是70岁）聚会上的讲话，谈到他自己在1953年与费米的一次会面中，如何接受了费米凭直觉所做的判断，放弃了自己当时正在做的研究，并改变研究方向，而至今仍感到幸运的故事。他在那个讲话中提到这个故事，是要说明杨振宁先生曾经在芝加哥大学的工作和学习，虽然并不是费米的入室弟子，但费米对于杨先生的科学思想和判断仍然有非常重要的影响。
Dyson的那篇讲话我是在《英语世界》杂志上看到的。过去20年我只偶然翻阅过一次《英语世界》，就读到这篇文章，应该算是鬼使神差，也说明《英语世界》是值得推荐的一个刊物，虽然我很少读。我在网上没有找到Dyson的这篇讲话，不过却发现2004年的《自然》（Nature）杂志上有Dyson专门回忆与费米会面的这一经历的一篇更为详细的短文章《Turning points -A meeting with Enrico Fermi》。所以把它找来放在这里，供有兴趣的朋友们参考。
Nature杂志《Turning points -A meeting with Enrico Fermi》原文链接：http://www.nature.com/nature/journal/v427/n6972/full/427297a.html
Dyson的故事说的是他刚出道的时候自己的一个理论计算结果能够很好地解释费米的一个重要的实验结果，于是兴高采烈地去芝加哥找费米讨论。可是费米看都没有看他的东西，就告诉他说他的计算不可能有意义。虽然费米当时并不真正知道正确的理论是什么，但费米给Dyson提供了很值得参考的几条理由。费米说做好的理论与计算有两种方式：
"There are two ways of doing calculations in theoretical physics", he said. "One way, and this is the way I prefer, is to have a clear physical picture of the process that you are calculating. The other way is to have a precise and self-consistent mathematical formalism. You have neither."
得知Dyson的理论中有近似，并且用到4个参数，费米又说：
"I remember my friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk."
Dyson的文章中说得很清楚，虽然他自己的计算结果与实验符合很好，他后来为了学生的学位问题也把计算结果总结出来发表了一篇《物理学评论》的论文，但他还是接受了费米的判断，自己很快就转行去做凝聚态物理的研究了。直到十几年后Gell-Mann发现了夸克，人们才知道Dyson当年的理论计算是基于不正确的理论。但是，费米的一席话拯救了Dyson，使他和他的学生们免于在“一个黑巷子中停滞不前”（getting stuck in a blind alley）。Dyson在文章开篇大有“听君一席话，胜读十年书”的味道。
One of the big turning points in my life was a meeting with Enrico Fermi in the spring of 1953. In a few minutes, Fermi politely but ruthlessly demolished a programme of research that my students and I had been pursuing for several years. He probably saved us from several more years of fruitless wandering along a road that was leading nowhere. I am eternally grateful to him for destroying our illusions and telling us the bitter truth.
按照我们科学网上有些人的看法，Dyson完全可以反过来指责费米是“武断的学霸”。因为费米自己没有充足的理由，也不了解Dyson的研究的细节，就直接宣判Dyson的研究是错误的，还拿了一些不是100%正确的说法来给自己的武断的“判断”做依据。Dyson完全可以选择坚持下去，以证明自己的理论计算是可靠的和正确的。可是，事实表明费米当年凭直觉所做的判断是正确的。
科学判断和任何日常生活中的判断一样，从来都不是基于100%可靠的证据。但是，科学判断和任何日常生活中的判断同样都存在对错的问题。没有人会说大科学家或者著名学者的判断和理解就一定会100%正确，可是如果根据这一点就去肯定和支持那些质疑他们的判断的“小人物”的意见，99%（或者十有八九）以上的情况会是适得其反的。
其实有经验的学者和科学家会常常用费米上面的所提到的标准来判断新的理论和计算结果，以及新的实验结果，以免将大量的精力甚至经费耗费在不值得深究的问题上。这样做也许偶尔会错过或者推迟某些方面的科学发现，但是更重要的是可以避免更多错误和浪费的发生。
我在与学生讨论问题时，常常会提到Dyson的这个故事。迄今为止，我们在研究中所做的判断的确是鲜有错误。人生苦短，将有限的精力浪费在追求看起来很美但其实不值得的课题上，还不如像我一样抽时间在科学网上写一些博客文章更好。
我对科学网上张志东先生的《激辩猜想》中提到的三维ising模型精确解的判断遵循的就是费米对Dyson讲的原则。或许我是错的，也未可知。
Dyson的故事，算是对吴宝俊小朋友的博文《忽不忽悠与是否改行无关！》的一个回答，以及对我自己前两天的《被改行的博士后忽悠了》博文的一个补充。
吴宝俊《忽不忽悠与是否改行无关！》博文链接：http://www.sciencenet.cn/m/user_content.aspx?id=216961
我的《被改行的博士后忽悠了》博文链接：http://www.sciencenet.cn/m/user_content.aspx?id=216791
我以前写过一篇有关Dyson的书《Scientist as Rebel》（《作为反叛者的科学家》）的博文。希望了解的朋友也可以去看一看。
《赞美民间科学家》博文链接：http://www.sciencetimes.com.cn/m/user_content.aspx?id=6945
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Essay
Nature 427, 297 (22 January 2004) | doi:10.1038/427297a
标题：
Turning points - A meeting with Enrico Fermi
作者：Freeman Dyson, Freeman Dyson is at the Institute for Advanced Study, Einstein Drive, Princeton, New Jersey 08540, USA.
副题：How one intuitive physicist rescued a team from fruitless research.
正文： One of the big turning points in my life was a meeting with Enrico Fermi in the spring of 1953. In a few minutes, Fermi politely but ruthlessly demolished a programme of research that my students and I had been pursuing for several years. He probably saved us from several more years of fruitless wandering along a road that was leading nowhere. I am eternally grateful to him for destroying our illusions and telling us the bitter truth.
Fermi was one of the great physicists of our time, outstanding both as a theorist and as an experimenter. He led the team that built the first nuclear reactor in Chicago in 1942. By 1953 he was head of the team that built the Chicago cyclotron, and was using it to explore the strong forces that hold nuclei together. He made the first accurate measurements of the scattering of mesons by protons, an experiment that gave the most direct evidence then available of the nature of the strong forces.
At that time I was a young professor of theoretical physics at Cornell University, responsible for directing the research of a small army of graduate students and postdocs. I had put them to work calculating meson–proton scattering, so that their theoretical calculations could be compared with Fermi's measurements. In 1948 and 1949 we had made similar calculations of atomic processes, using the theory of quantum electrodynamics, and found spectacular agreement between experiment and theory. Quantum electrodynamics is the theory of electrons and photons interacting through electromagnetic forces. Because the electromagnetic forces are weak, we could calculate the atomic processes precisely. By 1951, we had triumphantly finished the atomic calculations and were looking for fresh fields to conquer. We decided to use the same techniques of calculation to explore the strong nuclear forces. We began by calculating meson–proton scattering, using a theory of the strong forces known as pseudoscalar meson theory. By the spring of 1953, after heroic efforts, we had plotted theoretical graphs of meson–proton scattering. We joyfully observed that our calculated numbers agreed pretty well with Fermi's measured numbers. So I made an appointment to meet with Fermi and show him our results. Proudly, I rode the Greyhound bus from Ithaca to Chicago with a package of our theoretical graphs to show to Fermi.
When I arrived in Fermi's office, I handed the graphs to Fermi, but he hardly glanced at them. He invited me to sit down, and asked me in a friendly way about the health of my wife and our new-born baby son, now fifty years old. Then he delivered his verdict in a quiet, even voice. "There are two ways of doing calculations in theoretical physics", he said. "One way, and this is the way I prefer, is to have a clear physical picture of the process that you are calculating. The other way is to have a precise and self-consistent mathematical formalism. You have neither." I was slightly stunned, but ventured to ask him why he did not consider the pseudoscalar meson theory to be a self-consistent mathematical formalism. He replied, "Quantum electrodynamics is a good theory because the forces are weak, and when the formalism is ambiguous we have a clear physical picture to guide us. With the pseudoscalar meson theory there is no physical picture, and the forces are so strong that nothing converges. To reach your calculated results, you had to introduce arbitrary cut-off procedures that are not based either on solid physics or on solid mathematics."
In desperation I asked Fermi whether he was not impressed by the agreement between our calculated numbers and his measured numbers. He replied, "How many arbitrary parameters did you use for your calculations?" I thought for a moment about our cut-off procedures and said, "Four." He said, "I remember my friend Johnny von Neumann used to say, with four parameters I can fit an elephant, and with five I can make him wiggle his trunk." With that, the conversation was over. I thanked Fermi for his time and trouble, and sadly took the next bus back to Ithaca to tell the bad news to the students. Because it was important for the students to have their names on a published paper, we did not abandon our calculations immediately. We finished them and wrote a long paper that was duly published in the Physical Review with all our names on it. Then we dispersed to find other lines of work. I escaped to Berkeley, California, to start a new career in condensed-matter physics.
Looking back after fifty years, we can clearly see that Fermi was right. The crucial discovery that made sense of the strong forces was the quark. Mesons and protons are little bags of quarks. Before Murray Gell-Mann discovered quarks, no theory of the strong forces could possibly have been adequate. Fermi knew nothing about quarks, and died before they were discovered. But somehow he knew that something essential was missing in the meson theories of the 1950s. His physical intuition told him that the pseudoscalar meson theory could not be right. And so it was Fermi's intuition, and not any discrepancy between theory and experiment, that saved me and my students from getting stuck in a blind alley.
学术论剑
https://blog.sciencenet.cn/blog-176-217019.html
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