Suppose you have a particle which is not in a momentum eigenstate. You then measure the momentum (say, in the x-direction) by two seperate apperati simultaneously. Do the results have to agree? I asked my professor about this and he said that quantum mechanics will tell you that you can't perform such an experiment, as one of your measurements is going to have to occur before the other one. He also said that this isn't a very satisfying answer, which is isn't. Can anyone elaborate further or is this just one of the things you have to accept about QM.