John Wheeler proposed the idea of the “one-electron universe” — a single electron traveling backward and forward in time that could create an arbitrary number of clones of itself at any point in time. In this demonstration, you can see what our world would look like if Wheeler’s theory were true. Create a world line for one electron by clicking and dragging on the left graph, or you can click the left button to have a random world line drawn for you. Feel free to move forward and backward in time arbitrarily by dragging the slider on the left. On the right, you can see all the “clone” particles represented by this single electron, and as you slide your time slice up and down, the particles at that specific time will be highlighted. Particles moving forward in time — electrons — are colored yellow, while particles moving backwards in time — positrons — are colored blue. Note that the sampling rate has been lowered so you can see discrete instances of the particles at different times — in actuality, there would be a particle at precisely every point in time for the duration of the world line.