*Please wait for the
animation to completely load.*

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.

*Please wait for the
animation to completely load.*

Finally, we need to take into account the universal speed limit
of the speed of light. Restart.
In Animation 3 it made some sense to multiply the time by the speed of the
woman. For a true spacetime diagram, we multiply the time by the speed of
light. The unit of the y axis now becomes the amount of time it takes for
light to travel one meter or 3.33 X 10^{-9} seconds. Select
v / c to be zero and then press the set value and play button. Notice that the woman does not move
in space but moves in time. Now try a v / c of 0.9. What does her
"trajectory" or "worldline" on the spacetime diagram look like now? As
v / c
gets bigger (approaches 1) the trajectory of the woman on the spacetime diagram
approaches the line of v = c. This is the 45 degree line of slope 1 that
appears on the graph. Now try a v / c of -0.9. Since nothing can travel faster than the speed of
light, an object that begins at the origin is forced to have a worldline between
the two lines on the graph. The only object that can have a worldline on
either of those lines is light. If we let the woman move in two dimensions
her motion would be constrained to move within a cone which is called the
lightcone. The one's boundaries mark the possible worldlines that light can have
if it starts at the origin at t = 0 m.