学几何的人跟相对论有缘，总有机会被拽回来再看。这次确实长进了。两本好书：
S.W.Hawking & G.F.R. Ellis
The large scale structure of space-time.
R.K.Sachs & H. Wu
General Relativity for Mathematicians.
前一本是一时冲动用美元买的，当时有人劝我忍忍吧也许过几天你就不想要这本书了。现在看来还算值，能学到点东西花钱也就算了。第二本是大一的时候看到图书馆展销外文书，随便买了几本。最初版本的GTM，印刷的不行。有些地方都已经字迹模糊了。又拿出来看，突然觉得写得太好了。伍洪熙真是功德无量啊。
对一个已经懂得黎曼几何的人来说，貌似几句话就可以知道相对论是什么东西了。毕竟场方程理解起来没有问题。问题还有两个（1）Stress-Energy tensor的物理意义，要不然总是觉得有点空 。虽然知道这个也不能解决数学问题。（2）这个理论和“观察”的关系，总归是需要研究一些具体的模型，需要和经典的东西建立一点联系，需要讨论一下观察（比狭义相对论的观察困难太多了，伍的observers一章解决我很多问题）。
伍的书上有两段名言抄一下：
Weinberg， in Gravitation and Cosmology,
"Physics is not a finished logical system. Rather, at any moment it spans a great confusion of ideas, some that survive like folk epics from the heroic periods of the past, and others that arise like utopian novels from our dim premonitions of a future grand synthesis. The author of a book on physics can impose order on this confusion by organizing his material in either of two ways: by recapitulating its history, or by following his best guess as to the ultimate logical structure of physical law. Both methods are valuable, the great thing is not to confuse physics with history, or history with physics."
这段有点意思但不是太懂，
Einstein: "As for as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality."
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