every one. I was disputing some weird behaviour of the act of measuring some observables quantities with my friend. But I still don't think what he said is strictly true. He said" each observable has its own eigenstate, and when you measure it, it will collapse to that eigenstate giving the corresponding eigenstate. I think some observables do share same eigenstate e.g. for a free particle with zero potential. They don't share same eigenstate if V is not zero. I think when you know its momentum then by uncertainty principle, the uncertainty of x will be infinite, then the wave function will be spreaded everywhere. Therefore it has a well define wavelength, and you have well defined k for wavenumber , hence a well defined energy by p square / 2m.

He said because the particle is not in a infinite square well, the energy can't be quantised, and hence can not have a well define energy, which I think is wrong.