Chapter 9.4, Problem 9.1QQ

Suppose the observer O′ on the train in Active Figure 9.5 aims her flashlight at the far wall of the boxcar and turns it on and off, sending a pulse of light toward the far wall. Both O′ and O measure the time interval between when the pulse leaves the flashlight and when it hits the far wall. Which observer measures the proper time interval between these two events? (a) O′ (b) O (c) both observers (d) neither observer