## Illustration 5: Understanding Spacetime Diagrams

 Animation 1 Animation 2 Animation 3

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, as in kinematics, 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.  What has been done is to multiply time by the speed of the woman.  Therefore, velocity*time is plotted versus position.  This converts the unit of time into meters.

v / c =