© David Kaplan.
This movie represents an A-B potential (from the previous figure), looking from the top, at every point in space. The black crosses represent the A and B directions, and the red lines start off pointing in the A direction. A wave—a packet of energy that we could also interpret as a particle—is shown as a coherent wiggling of the red line from the A position to somewhere between A and B. Because it doesn't cost potential energy for the arrow to point in different directions, the wave can be as long and gentle as one likes. In particle language, this corresponds to a particle with no mass-energy, or no mass.