# Starting with a problem

Let's start with a particle, an opening in a barrier, and a screen on the other side. If we send a particle though the opening, we would expect it to strike the screen directly behind the opening. If the opening is much bigger than the particle, things work out pretty much as we would expect. On the other hand, if the opening is very small, things get odd. Instead of always finding the particle just behind the opening, I may find it off to either side. Is there a way to keep our intuition about particles and add to it some new concepts that allow us to make sense of these observations? The multiple paths interpretation indeed makes this possible. What one needs to add to our intuition of particles are the concepts of uncertainty, phase and amplitude. The uncertainty arises from not knowing what path the particle actually takes in some sense; phase and amplitude allow us to get past this uncertainty by combining the information from all the possible paths to make a prediction of where we will find the particle. For relatively large openings, the prediction is the familiar one of the particle being pretty much directly behind the opening. What is surprising is that for small openings, the prediction is that the particle may strike the screen almost anywhere.

You can see this for yourself using the interactive figure below. Imagine a stream of electrons moving from a source (bottom) through a screen with an opening of variable width (middle) to a screen (top). The curve shows the number of electrons hitting each point along the screen. You can vary the width of the opening by clicking on the "thinner" or "thicker" buttons. Notice that with a very thick (wide) opening, most of the electrons hit the screen in a localized area directly opposite the opening. With a thin opening, more or less the same number of electrons hit the screen at all locations along the screen.

Instructions
Click the "Thinner Slit" and "Thicker Slit" buttons to vary the size of the opening through which the electrons travel. Click the "Left" or "Right" buttons to watch the black arrow on the right change as a function of the position along the screen.  Press "Stop" to stop the movement.

The difference in behavior for thick and thin openings is an experimental observation, which has to be accounted for. The "Left" and "Right" buttons and the black arrow on the right will help explain how the experimental observations can be made sense of using the "explore all paths" idea. Don't worry about them right now. We'll return to this figure at the end of the exhibit, and you'll know then what these buttons and arrows represent.