奇迹笔记

朗道和费曼都是我很喜欢的理论家，朗道以一套极度干净和抽象风格写作的理论物理教材著称（他也有高中水平的入门书但以这套理论物理最有名），而费曼则以三本风趣通俗的物理讲义最为著名（他也有高级教材但以这套讲义最有名）。

但，千万别被他们骗了。朗道看上去很强调数学和严谨，但他最著名的工作是很直观的，很需要物理直觉的。比如：超流理论，相变理论，密度矩阵，准粒子概念等。做这些工作都必须读完他那么多卷书才能做？太夸张了，其实用不着的。

费曼呢，正好相反。看上去很通俗，但那只是他的伪装，他的数学很好，他的思维也很抽象，但费曼玩的太熟了，最后总是以一种最直观的图像跟大家吹（费曼自己说：There is always another way to say the same thing that doesn't look at all like the way you said it before.）。好像人人都能做出来似的，但其实那很可能只是误导。

科学发现的逻辑和科学工作被表述出来的逻辑很可能是不同的，我们可千万别被他们给骗了。

附送：郝柏林写的朗道百年

附送：费曼自述发现路径积分的过程

So that didn't help me very much, but when I was struggling with this problem, I went to a beer party in the Nassau Tavern in Princeton. There was a gentleman, newly arrived from Europe (Herbert Jehle) who came and sat next to me. Europeans are much more serious than we are in America because they think that a good place to discuss intellectual matters is a beer party. So, he sat by me and asked, "what are you doing" and so on, and I said, "I'm drinking beer." Then I realized that he wanted to know what work I was doing and I told him I was struggling with this problem, and I simply turned to him and said, "listen, do you know any way of doing quantum mechanics, starting with action - where the action integral comes into the quantum mechanics?" "No", he said, "but Dirac has a paper in which the Lagrangian, at least, comes into quantum mechanics. I will show it to you tomorrow."

费曼很八卦，在啤酒节上，教授问他最近在干吗？他说：在喝啤酒啊。然后马上反映过来这位欧洲逃难来的教授没有美国式的幽默，赶紧老实地和他讨论最近的工作。

Next day we went to the Princeton Library, they have little rooms on the side to discuss things, and he showed me this paper.....

Professor Jehle showed me this, I read it, he explained it to me, and I said, "what does he mean, they are analogous; what does that mean, analogous? What is the use of that?" He said, "you Americans! You always want to find a use for everything!" I said, that I thought that Dirac must mean that they were equal. "No", he explained, "he doesn't mean they are equal." "Well", I said, "let's see what happens if we make them equal."

欧洲教授说：你们美国人真是太实用了，什么东西都想找出个用的方法来！费曼说：我想狄拉克的意思是相等，就让它们相等好了。

So I simply put them equal, taking the simplest example......and just calculated things out by Taylor series expansion, out came the Schrödinger equation. So, I turned to Professor Jehle, not really understanding, and said, "well, you see Professor Dirac meant that they were proportional." Professor Jehle's eyes were bugging out - he had taken out a little notebook and was rapidly copying it down from the blackboard, and said, "no, no, this is an important discovery. You Americans are always trying to find out how something can be used. That's a good way to discover things!"

欧洲教授惊呼：不！不！这是了不起的发现，你们美国人总是想知道怎么用。这是发现问题的好途径。