There is a statement in the Landau and Lifgarbagez volume on quantum mechanics that has had me puzzled for weeks now. They say (bottom of pg. 23): "If the electron was in the state psi_n [an eigenstate of f], then a measurement of the quantity f carred out on it leads with certainty to the value f_n. After the measurement, however, the electron is in a state phi_n different from its initial one, and in this state f does not in general take any definite value." They seem to be saying that if you make a measurement and get f_n and then repeat the measurement immediately, you will not in general get f_n again! But the relevant experiments I know of do not seem to display such behavior. In the Stern-Gerlach experiment, for example, if a beam of electrons is determined to be spin up and then passed through the apparatus again, all the electrons in the beam will continue to be spin up. What am I missing here?