When studying the properties of solids we always say that electrons are in (stationary) energy eigenstates. The theory of conduction for example (with conduction bands and stuff) follows from the assumption that electrons are in energy eigenstates but why are they in such states in the first place? What prepared the electrons in energy eigenstates? And if they are not in energy eigenstates how is it possible that the assumption of them being in energy eigenstates works so well?
Example: I have a block of aluminum. It has never had the energy of its electrons measured by me nor anybody else: it just came out of production. If I use it as a conductor it has a resistance. Assuming its electrons are in energy eigenstates I can calculate its resistance. Why am I allowed to assume that its electrons are in energy eigenstates? If nothing ever measured the energy of the electrons of the aluminum they might be in a much different state.