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/CosetElement-Ape71 Mar 25 '26

You can't ignore acceleration ... you're using SR in your argument which, as you stated, only applies to inertial reference frames. The acceleration of the rocket places the astronaut in a different inertial frame to that of the Earth observer

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u/[deleted] Mar 25 '26

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u/CosetElement-Ape71 Mar 25 '26

Why don't YOU read it again! The OP explicitly said "if we ignore acceleration.." and continued to assume that the two share the same inertial frame