It may be helpful to visualize this situation using spacetime diagrams. For a given observer, the t -axis is defined to be a point traced out in time by the origin of the spatial coordinate x, and is drawn vertically. The x -axis is defined as the set of all points in space at the time t = 0, and is drawn horizontally. The statement that the speed of light is the same for all observers is represented by drawing a light ray as a 45° line, regardless of the speed of the source relative to the speed of the observer.