Assume a period of uniform relative motion (no acceleration) of a spaceship in reference frame, B, going 80% of light speed observed from my reference frame, A, when it passes by Earth (my references frame).
When it passes Planet X which, say, is 20 light years away from Earth, 25 years will have elapsed in my reference frame (A). But also from my reference frame, I will judge 15 years elapsed time on the spaceship's clock which is in reference frame, B.
Two Questions:
1); Since uniform motion is relative, the perspective from the spaceship, i.e., reference frame B, is that my reference frame (A) is moving at 8O% light speed while the spaceship is stationary. So, shouldn't the spaceship's clock (reference frame B) now show the elapsed time of 25 years from when Earth is at the spaceship's location until Planet X arrives at the spaceship's location. And, also from the spaceship's perspective, i.e., reference frame B, shouldn't the elapsed time on Earth (reference frame A) now be judged only 15 years?
2); How can the clocks in each reference frame (A and B) run slower than the other?
In order to "see" what's actually going on here, an explanation in plain English to assist the mathematics would be helpful.
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