A child on a playground swing is swinging back and forth (one complete oscillation) once every 4 seconds, as seen by her father standing next to the swing. At the same time, a spaceship is hurtling by at a speed close to the speed of light. According to special relativity (and ignoring the Doppler effect for this question), her mother on the spaceship finds that the time for one full swing is
A) less than 4 seconds when the spaceship is approaching the swing and more than 4 seconds when it is moving away.
B) less than 4 seconds.
C) equal to 4 seconds.
D) more than 4 seconds.

Question

Quizplus · Accepted Answer

Time dilation, a consequence of special relativity, implies that a moving observer (the mother in the spaceship) will measure a stationary process (the child swinging) to take longer than an observer at rest with respect to the process (the father).

A Child on a Playground Swing Is Swinging Back and Forth