I'm wondering if someone could tell me what would be the result of the following thought experiment, to help answer a simple question I have about the Heisenberg Uncertainty Principle: (I am just an educated layman trying to figure it all out- hopefully my thought experiment isn't complete nonsense!) I perform a double slit experiment. The detector consists of some kind of photosensitive plate to detect an interference pattern. In addition I have a row of lights connected to regions on the detector, each of which will blink on whenever a photon lands in that part of the detector. A light source is used that fires photons one at a time through the slit (or maybe a beam splitter?) onto the detector at a slow enough speed that an observer can actually see the lights blinking on and off. I employ an observer to watch the lights blink on. The observer is placed on a comfortable couch for several hours to watch the lights. Eventually he or she falls asleep and stops observing it. My question: Will an interference pattern appear on the detector since the observer is no longer observing? My current understanding is that it won't, since by merely connecting the lights to the experiment, regardless of whether someone is observing the lights, the probability wave will 'collapse'. In that case, if the lights are removed from the experiment, aren't the individual atoms in the photosensitive plate sufficient to collapse the interference pattern? My basic question revolves around what exactly does it take to make the light behave like a particle and not a wave? Popular press articles make this out to be something of a mystery.