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# Spacelines and Timelines

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A *spacetime* is the set of all points of a space in all its moments of time, and it is depicted by the whole two-dimensional surface of a graph. A *spaceline* (space dimension) is the set of all points of a spacetime that coexist in a specific moment of time, i.e. all points that have the same time coordinate (isochronal line). The *timeline* of a spacepoint is the set of all points of a spacetime that have the same space coordinate as the spacepoint. This is also the timeline of a stationary object. Obviously, we can also define in a similar manner *spacesurfaces* and *spacevolumes*, and *timesurfaces* and *timevolumes*.

Thus a *spacepoint* (a space coordinate) defines a timeline (or a timesurface in a 2+1 spacetime, or a timevolume in a 3+1 spacetime), and a *timepoint* (a time coordinate) defines a spaceline (or a spacesurface in a 2+1 spacetime, or a spacevolume in a 3+1 spacetime). Finally, we can define the timeline of a moving object as the set of all points that satisfy the motion equation of the object.