- #1
kelly0303
- 579
- 33
Hello! I am reading about spin-orbit coupling in Griffiths book, and at a point he shows an image (section 6.4.1) of the vectors L and S coupled together to give J (figure 6.10) and he says that L and S precess rapidly around J. I am not totally sure I understand this. I know that in the presence of spin-orbit coupling ##L^2## and ##S^2## are conserved, so the magnitude of L and S must be constant but ##L_z## and ##S_z## are not good quantum numbers, so they must change. I assume that this precession around J is the only configuration consistent with these requirements. However, I am not sure I understand why does the precession need to be fast. ##L_z## and ##S_z## would be conserved only if you have no precession at all, so you would have consistent results no matter how fast they precess. Where is this "speed" of precession coming from? Thank you!