The Flash, a comic superhero, is able to run close to the speed of light. Two people, with synchronized watches, stand several kilometers apart and time the Flash as he runs a straight course. The Flash has his own stopwatch and uses it to independently time his run. The Flash also observes the speed at which the finish line moves toward him, and upon finishing his run he calculates the length of the course by multiplying this speed by the time on his stopwatch. Who measures (I) the Flash's proper time for the race, and who measures (II) the rest or proper length for the race?
A) Both the Flash and the two people measure (I) and (II).
B) Neither the Flash nor the two people measure (I) or (II).
C) The Flash measures (I) but the two people measure (II).
D) The two people measure (I) and Flash measures (II).
E) Only the Flash measures both (I) and (II).

Question

Quizplus · Accepted Answer

The Flash measures his own proper time because he is in the same inertial frame as his stopwatch. The two people, being stationary with respect to the race course, measure its proper length.

The Flash, a Comic Superhero, Is Able to Run Close