Hi there, Let me start by introducing myself. I am a student who considers himself to be a very scientific thinker, that is to say, not one who believes in mystical thinking, but I am not studying physics. I am trying to learn more about this for my own edification (and I will try to be a respectful and nice guy ;)). I have read carefully about the double-slit experiment, but one question has bothered me to no end about it. What I read is that time and time again photons fired through two slits behave as a wave with an interference pattern even when fired one-at-a-time, but when observed behave as particles that land in two lines. Somehow, the act of observation interferes with the experiment. The question that I can't get out of my head -- and I imagine physicists have talked about this -- is the question of what, exactly, qualifies as an observer for purposes of interfering with the experiment. Apparently our methods of observation cause the wave function to collapse. But what about (and forgive my ignorance) mirrors? Flashlights? Strange materials? Other strange events? Dead cats? ;) What else collapses the wave function; that is, what else is an "observer" to satisfy the purposes of the strange way in which we have chosen to use that word? Thank you for your time!