Hello, as far I understand wavefunction collapse (or decoherence or whatever stops particles from acting as waves) is initiated through something referred to as a measurement. These measurements are thought of as interactions. Now the question I have is, why are some interactions measurements while others are not without any obvious physical differences? For example, photons when used as a measuring tool do collapse the wavefunction permanently, but photons exchanged between parts of a particle do not. (molecules have shown interference patterns while they definitely require electromagnetism to stay together). Also the influence of other molecules does seem to collapse the wavefunction (the screen that the particles are fired towards), but not always since mirrors do not. From all the wavefunction collapse mechanisms that I've heard of, they all involve electromagnetism, but not all electromagnetic effects seem to be able to collapse the wavefunction. Why? If the answer to this is actually still unknown, is there any sort of list out there that shows exactly which kinds of interactions have been shown to be able or unable to collapse wavefunctions?