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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