Suppose I want to measure the momentum of a quantum system. What I do is I take the momentum operator and expand my wavefunction in term of the eigenfunctions of that operator, then I operate on the wavefunction with the operator and the reusult of the measurment is that the wavefunction "collapses" into one of the eigenstates. What if instead I want to mesure the energy? Then I do the same thing, I expand the wavefunction in eigenstates of energy and apply the energy operator. My question. Why can we write the wavefunction as a sum of eigenstates in two diffrent ways depending on what we want to measure? What if I want to measure both energy and momentum at the same time, how do I write the wavefunction then? And If I expand the wavefunction in eigenstates of the momentum operator where do the information about the energy of the system go? I mean, allt he information about the system should be encoded in the wavefunction. But if I expand it in its eigenstates of momentum only, then I can only gain information about the momentum of the system or what?