Spreading out

What happens if the hole in the piece of card, blocking out all other paths, is very small?

Given a narrow gap...

Arrange the waypoints so that the only paths that can be explored pass through a narrow gap (a slit in two dimensions). Drag the detector up and down. Notice the changes in brightness.

This effect is a surprising prediction—putting a narrow gap between source and detector results in an interesting spread of brightness at the detector. It's not only straight-through. You get brightness either side as well, and you might even have noticed alternating brighter and dimmer bands as you move away from the straight-through position. These are all real. Try it out with a small enough slit, and you can see these alternating bands. Without a laboratory, try touching your first finger and thumb, in front of a bright light.

Reasoning about photons exploring space goes on giving. Another prediction using the same ideas.