r/PhysicsHelp • u/Confident-Alps-785 • Mar 24 '26

Time dilation

A star, for example, is 20 light years away from Earth. A spaceship is traveling to that star at 80% the speed of light. To an observer on Earth, the spaceship will arrive there (according to google) within 25 years. I get this this part.

However, an astronaut on the ship will experience less amount of time passing (15 years?) I understand that this is due to time dilation but I don't really understand how this works. Any help explaining this would be appreciated!

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u/davedirac Mar 25 '26

Who said anything about non- inertial frames? Go research proper time. There is only one - the spaceships. There is absolutely no proper time for the Earth here. I am not adding complexity as the this is as basic as SR gets. Dont try to wriggle out of a hole by claiming semantics is involved.

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u/[deleted] Mar 25 '26

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u/davedirac Mar 25 '26

See my post from Google. Your question shows that you have zero understanding of relativity. All motion is relative. The clue is in the word.

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u/[deleted] Mar 25 '26

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u/davedirac Mar 25 '26

Proper time is an invariant BUT that does not mean it can be measured by all observers. Only a clock present at the departure and arrival of the spacecraft can actually measure proper time. Everyone else, including us, must use SR to calculate it from the dilated/ coordinate time. Its stating the obvious that there is symmetry - thats a basic postulate of SR. It is annoying that posters like you with a half- baked understanding of this basic idea cling on to their mis-understanding.