**Time as the Fourth Dimension**. Physics deals with events that occur in space and time. Each event occurs at a certain location in space and at a certain point in time. The location in space can be defined by three physical coordinates, which correspond to the three spatial dimensions, or variables. The point in time can also be defined by a coordinate. In this sense, time is the fourth dimension, or fourth variable, of physics.

Galileo was the first mathematician to think of time geometrically. By using line segments to represent time intervals, he was able to use geometric proofs in physics. Time as a fourth dimension takes on a special importance in Einstein's theory of relativity. In this theory, space and time cannot be separately considered but must be thought of as a single four-dimensional entity. This entity is called space-time, or the space-time continuum.