Hello; In classical physics, measurement does not play a major role, but in quantum physics measurement plays an active role, correct? In the double-slit experiment, whether or not the path of the particle is measured - a detector finds which slit it passes through. I'm not questioning why this happens (well, I do, but that's not what I'm asking here). I want to know what kind of 'observation' or measurement specifically causes the particle's path to be altered, i.e. at what degree of measurement does this phenomenon (begin to) occur?