Actually... it is! You can think of the barrier with two slits as a device that measures (a) whether the photon hit the barrier or missed barrier and additiona

Actually... it is! You can think of the barrier with two slits as a device that measures (a) whether the photon hit the barrier or missed barrier and additionally (b) when the photon hits the barrier, where did it hit the barrier. Notably, if the photon misses the barrier, it doesn't tell you where it missed. This is why, when the photon doesn't hit the barrier, it can end up in a superposition of passing through both slits. There's some classic thought experiment, I think it's [1], thinking about an atom decaying into a superposition expanding in a spherical shape, and how you can make barriers to shape the outgoing superposition. For example, to make an expanding half sphere, block off the other half sphere and focus on the times the decay didn't hit the barrier. 1: https://en.wikipedia.org/wiki/Renninger_negative-result_experiment#Renninger's_negative-result_experiment https://en.wikipedia.org/wiki/Renninger_negative-result_expe...