You are in a rocket moving away from Earth at one-third the speed of light relative to Earth. A friend is on Earth, and an astronaut in another rocket is moving toward Earth at one-third the speed of light (in the Earth reference frame), on a path collinear with your path. If each of you records the duration of your journey in your three respective reference frames, which of you records the proper time interval?

You are in a rocket moving away from Earth at one-third the speed of light relative to Earth. A friend is on Earth, and an astronaut in another rocket is moving toward Earth at one-third the speed of light (in the Earth reference frame), on a path collinear with your path. If each of you records the duration of your journey in your three respective reference frames, which of you records the proper time interval?