## Section 2.5: Understanding Spacetime Diagrams

One of the most useful ways to visualize moving objects in special relativity is with spacetime diagrams. Restart.  Consider the animation of a woman walking (position given in meters and time is given in seconds). The motion of the woman in Animation 1 is rather ordinary and is plotted on a position versus time graph as well as in a data table. Notice that the speed of the woman can be determined by the slope of the position versus time graph (in this case 1 m/s).

Now consider what is being represented in Animation 2.  In this animation, time is plotted versus position. The graph is the same as Animation 1 with the axes flipped. This way to represent the motion of the woman is almost what physicists would call a spacetime diagram. Two things are missing: we want to treat the time on the same footing (as far as units) as the position, and we need to take into account the universal speed limit, c.

Animation 3 puts time on equal footing with position by multiplying time by the speed of the woman. Therefore, velocity if the woman times time is plotted versus position. This converts the unit of time into meters.

v /c =