What is the meaning of Mao estimation by Lu Jiaxi, the master of Mao estimation?

Scientists are not "fortune tellers" and cannot "predict" their own research results; However, it is also a taboo for scientific workers to "search" aimlessly. When conducting scientific research, I always pay more attention to the prediction of the final result, so as to better grasp the research direction as a whole. I am used to calling this prediction a "rough estimate", and I often warn my students and researchers like this: "A rough estimate is better than no estimate!"

I especially emphasize "rough estimate", which is related to a mistake I made when I was a student. I remember that in the third year of college (1933), Mr. District Jiawei, who taught physical chemistry, liked to test students. Once he made several examination questions, one of which was particularly difficult, and I was the only one in the class who basically worked it out. However, when the revised paper was handed out, I found that the teacher only gave a quarter of the score for that topic, and I felt very wronged because I just put the decimal point of the answer in the wrong place.

The teacher noticed that I couldn't figure it out in my mind, so he patiently enlightened me and said, "If a bridge is designed, a wrong decimal point will cause big problems and make big mistakes." I'm going to deduct three-quarters of your score today, which means you put the decimal point wrong ... "。

I understood the teacher's painstaking efforts to deduct points, and then I thought, how can we avoid improper mistakes such as misplaced decimal points? When I calmed down to check the cause of the error, I found that the problem was not just a momentary negligence, because my calculation result was obviously unreasonable in order of magnitude; If we can carefully compare and analyze the conditions given by the topic when solving the problem, we can find the mistakes in time and correct them. The fundamental reason why I make a "mistake" is that I don't have a goal "spectrum" to solve the problem in my mind.

Since then, I have always tried my best to put forward a simple and reasonable physical model according to the meaning of the question, that is, to roughly estimate the order of magnitude of the answer. If the calculation result is beyond this range, I will check the calculation process as soon as possible ... This effectively overcomes the mistakes caused by accidental negligence.