I read about the wheeler experiment been done in Australia. But one thing I don't understand. If the particle (but let's use a photon) goes through the two slits (or one) and the screen is removed after it already went through, how is it measured? Does it stop after the measurement? My understanding is that when the photon hits the final screen and has no more to go it shows as a wave pattern. But when it interacts when it has still some way to go, it is a particle? So is it sort of hit by other light beams to see what slit it went through along the way to the screen? Let's take visible red light for example, under what conditions can it interact and behave like a particle? And why is it so difficult to get light to behave as a particle? Thanks!