Hi everyone. I am now learning the perturbation theory in QM. and I have encountered something that puzzles me. from http://en.wikipedia.org/wiki/Perturbation_theory_(quantum_mechanics [Broken]) says, "...Since the overall phase is not determined in quantum mechanics, without loss of generality, we may assume <n(0)|n> is purely real. Therefore, <n(0)|n(1)>=-<n(1)|n(0)>, and we deduce..." What do you mean by "the overall phase is not determined in quantum mechanics" ? I especially emphasize "in quantum mechanics". Does this mean that we cannot find a specific value for the phase by quantum calculation? I appreciate your help!