Submitted by AutoModerator t3_zf37c4 in askscience
nivlark t1_izc6htj wrote
Reply to comment by Be_Cool_Bro in Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science by AutoModerator
An observer always measures their own time to be normal, but will see time running slower for a second observer who is in motion relative to the first. And this is reciprocal: for the second observer, their time is normal and the first observer's clock is running slow.
Space has to be involved because otherwise there are contradictions: consider observers A and B, where A is stationary on Earth and B is travelling at high speed to a distant planet. A will measure B's journey to take a certain amount of time, but because B's clock is running slower, they would conclude that the journey took less time.
If both observers were to agree on the distance B had travelled, then they would have to disagree on their speed. The solution is that they don't agree on the distance: not only does time run slower in moving reference frames, lengths are measured to be shorter. From B's perspective, the planet is approaching them and so the distance to it shrinks. Their clock measured fewer ticks, but the distance they travelled is also less, and so they can still agree with A about their speed.
Be_Cool_Bro t1_izce672 wrote
This made perfect sense to me. Thank you.
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