Consider an infinite well of lenght L. We measure the position of the particle and obtain x=L/2. After the measurement, what is the probability of finding the particle in a given eigenstate? My solution: WhaaaaaAaaT ?! Neither Gasiorowicz and Griffiths talk about the probability of the particle being in a certain wave function! I tought the wave function was that one thing that was determinate in QM. Worst, the one clue I have is contradictory to the statement of the problem; it is when Griffiths says that if we measure the position of a particle, then its wave function must crumble into a dirac delta about this position so that another measurement performed right after will be guarenteed to return the same position. A Dirac delta function is no eigenstate of the particle in the box, hence a contradiction. (unless the answer to the problem is in fact 0% for that very reason, but I doubt it.) Eidt: I kinda recall someone on this forum saying how the wave function can be any linear combination of the eigenstates, but once a measurement is made, it must settle into one of those, but I can't find back the thread.