Apologies if the question has been asked, I didn't see it in my search but maybe I missed it. I was wondering if there is a formal definition of when/where a quantum experiment begins (as opposed to where it ends, i.e. with the collapse of the wave function), and whether it matters. For example, with a double-slit experiment, we talk about firing "one electron at a time" towards the slit, and we say the experiment ends when we measure that the electron has hit a particular area of the screen. But it would seem that not just its path but also the presence of that electron is conditional, i.e. that prior to the measurement, the electron was in a superposition of states fired-and-flying-towards-the-slit and not-fired-yet. The electron comes from the electron gun which is being heated. From what I understand that means the electron in question was in a superposition of states jumped and not-jumped the barrier in the heated filament due to the flow of electrons making the electrical current in the filament. Those electrons themselves are presumably in a superposition of states of passing and not passing through the filament, and so on. In other words it seems like prior to the measurement, it's not just the landing position of the electron on the screen that's in the superposition of states, but that, tracking backwards, everything else that makes the chain is fuzzy until a certain point (if it exists). I.e. if there is a such thing as the "rise" of the wave function, can we say that, assuming you are the experimenter, if your first observation is that your finger is pushing the "on" button, and your next observation is that you see is a quick flash on the screen, the theory doesn't tell you what happened in between? That seems somewhat counter-intuitive. (As to whether this matters, my intuition is that the longer the parts of the chain from the end to the start we include in the observation, the more unique and less reproducible/repetitious that sequence is, and so less relevant for making predictions, but would appreciate hearing your thoughts.) Thanks!