Principles of General Relativity
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AP Physics 2 › Principles of General Relativity
A student on the Earth observes a spaceship moving at a considerable fraction of the speed of light. As a result of the spaceship's motion, the student observes that the clocks aboard the spaceship are running slower than when the spaceship was on the Earth. When a student aboard the spaceship observes the student's clock on the Earth, what effect will he notice on the rate of the Earth-bound clock?
The Earth-bound clock appears to run slower
The Earth-bound clock appears to run faster
The Earth-bound clock runs at the same rate
The Earth-bound clock first runs fast, then slow
The Earth-bound clock first runs slow, then fast
Explanation
For the observer on the spaceship, the Earth appears to be moving, and special relativity tells us that any observer moving at constant speed can consider him/herself stopped. Therefore, the clock on the Earth experiences the same slowing from the spaceship's viewpoint as the spaceship's clock from the Earth viewpoint. It's a little bit like looking at someone from far away. They look small due to the distance, but you do not look big to them, you also look smaller.