I simply want this to be expounded once in this blog. I've alluded to the double slit experiment in some of my recent remarks on quantum theory and its philosophical overlay, and I even pasted an image of one hypothetical result of the experiment onto my most recent such discussion. But for this once I'll be explicit. Imagine that electrons, or photons, are little tiny particles, as Democritus might have imagined. Imagine shooting them (in Chicago-movie machine-gun fashion) at a wall that had two parallel slits in it, and that there is a screen behind the wall, such that each particle that gets through the slit leaves a mark on the screen. What would you expect to see on the other side of that wall? You'd likely expect to see marks directly behind each slit, corresponding top the shape and size of the split. Some portion of the screen that lay between the two slits would presumably be left unmarked. That's not what happens. Consider light first. Thomas Young d...