I've recently learned about general relativity from my Physics professor, where English is his second language. Needless to say, there are parts of it that don't make sense, so I'm hoping someone can explain this next question.
I was given the following scenario (which I modified slightly): Consider an astronaut who has a twin on a planet, and relative to each other, they are stationary. Their speed relative to the inertial reference frame of the Universe, however, is unknown. The astronaut travels to some arbitrary point in space and back for what he thinks is a year, but upon arrival he notices that his twin has aged 10 years. This happens because he approached the speed of light on his trip which warped the effect of time on him, but obviously not the planet.
Question: Why would this happen? If the astronaut approaches the speed of light, he is doing so relative to the speed of the planet. However, from the perspective of the astronaut, the planet would be moving nearly the speed of light. Given this logic, his twin should've only aged a month.
Possible answer: I've considered that this is because of the speed of the astronaut based on the inertial reference frame of the two bodies. If this is right, then I have another, slightly more complex problem.
Scenario 2: Consider 3 bodies that are stationary relative to each other. Body 1 remains stationary, but bodies 2 and 3 group together and take off with the same relative speed and orientation. At an arbitrary distance, body 2 heads back to body 1 at a speed half of what it traveled previously.
Problem: According to the inertial reference frame of the 3 bodies, body 1 should be aging faster than body 2, which should be aging faster than body 3. However, taking the inertial reference frame set by bodies 2 and 3 after they finished accelerating, body 3 should be aging faster than body 2.
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