Spacetime diagrams are a way to represent the trajectory of objects in space and time.

Figure 3a shows the motion of a particle in one spatial dimension, x.
For each time t, the position x is shown as a point on the graph. As time
passes, the particle traces out its *world line* in spacetime. In
the real world, space has three dimensions (x, y, z) plus time, resulting
in four dimensional spacetime. Since it is hard to draw four dimensions
on a flat surface, we content ourselves with representing two spatial dimensions
(x and y) plus time in figure 3b.

There is a helpful analogy
between spacetime and ordinary space.

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