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I got a feeling that learning quantum mechanics is easy and hard. Most of time, it is easy to "accept" all the concepts given in the book by simply looking at their mathematical interpretation. But it is hard if you really take it serious to try to understand everything from the physical knowledge learned in classical point of view.

One problem I am learning is about the (quantized) angular momentum. The classical picture of angular momentum is pretty straightforward. Though the quantization of the angular momentum in quantum physics is not a super hard concept to accept (at least from math used to solve the PDE of Schrodinger), I still got stuck on some aspect especially on the z component of the angular momentum. My textbook emphasizes that angular momentum does not have a prefer direction. If that's the case, what's the point to select a special axis z for problem analysis. Or I ask which direction is really the z axis pointing to? I was told that the electron could have many different angular momentum pointing different direction and all those angular momenta are precessing about a same axis called z axis. It is confusing why they all precessing about the same direction?

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# I About precessing directon of angular motion in quantum mechanics

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