I am puzzled by the concept that a bound electron, when changing state and emitting a photon, takes no time for the transition. The electron can only exist in defined energy states and I've been told that it's meaningless to talk about a time dependent evolution of energy for the electron during the transition. How do we determine that there is no time dependent energy evolution- Can it be determined empirically (if so by what means) or only from the formalism of QM?
