I read that quantum particles (like photons) travel as waves of probability, but when they encounter other quantum particles, the probability waves break down, and the photons assume properties of particles. An example given of this is: if you fire a single photon from point A, to a point B which is located on a solid surface, the photon will travel as a wave and in effect take all possible paths from A to B (loops, zig-zags, etc). And the photon will ultimately take the most probable path, and end up at point B on the solid surface. What I fail to understand is the double-slit experiment. Consider the example above. If you take that same solid surface, and cut two slits into it, and repeat the experiment, why would the wave of probability not disappear as soon as the photon hits the solid surface? Why does the wave continue to show interference on the other side of the surface just because there are two slits in the surface? If what I said above is correct (the emboldened part), the solid surface is made of particles, so the wave should break down on this side of the wall.