# Active Reading Solution: Four-Displacement

- The horizontal component of the vector tells us the distance traveled.

- The vertical component tells us the time elapsed, multiplied by
*c.* In relativistic units *c=1* so this is numerically equivalent to the time elapsed in whatever units you are measuring. For example, if the vertical component of D**R** is 4 light-years, then the journey took four years.

- The direction of that vector relates to the speed of the journey. As we have discussed before, speed=1/slope, ranging from a vertical line (speed=0) to a 45
^{o} line (light). No journey could have a slope below that 45^{o} line.

Note that all of these components are measured in your coordinate frame (presumably the one in which you drew the *x* and *t* axes). Observers in different reference frames would measure different components for D**R** because they would measure different distances and times for the journey.