You may have heard that time is the fourth dimension. Indeed, in Einstein’s special and general theories of relativity, space and time are fused into a single entity, space-time, and represented in a four-dimensional mathematical arena. Roughly speaking, ordinary space gets plotted on the first three axes and time gets plotted on the fourth. This construction can be viewed as a generalization of the two-dimensional xy plane of Fermat and Descartes.
