Role of the Observer
In the double slit experiment, there is interference between the fired particles when both slits are opened. Yet, when an observer checks to see what's actually going on by looking at the slits as the particles pass through them, the interference disappears. In other words, the particles act like particles when they are observed, yet they act like waves when they are not observed. Why does this happen, and what does it mean? Certainly, particles don't have a consciousness with which to deduce, "Oh snap, they're watching me--I better go straight through and not make a scene."