Spatial effects
At modern sporting events, supporters of one team sometimes demonstrate their enthusiasm by conspiring to create a “human wave” in the bleachers. Assume that such a wave goes around the stadium a couple of times and think of the motion of each supporter, as a function of time. The fan stands up and sits down in a periodic fashion, and therefore follows a rhythm.
This simple phenomenon illustrates that a spatially extended system which exhibits temporal oscillations can sustain traveling waves.
The human wave is a one-dimensional phenomenon. In two spatial dimensions, traveling waves can take the form of target patterns or of spiral waves.