I know there have been a lot of other threads on this topic but I wanted to get some feedback on my thoughts on this. Time dilation is a result of Einstein's second postulate of SR, although the first postulate is what gives rise to the apparent twin "paradox". The common solution to the paradox is that the twin that travels to a star and back to earth experiences a force, but it seems like this is argued in a way to say that the twin traveling to the star is somehow the one that's "truly" in motion. It seems the more fundamental way to look at it is that since the twin who travels to the star feels a force, it's not that they are in "true" motion and therefore the twin back on earth is "truly" at rest, but that the twin that leaves earth is not in an inertial reference frame, so Einstein's first postulate doesn't apply to the twin that leaves, therefore their clock actually runs slower. There are similar paradoxes such as an observer on the ground and another in the train. The train passes the observer on the ground calculates that the clock on the train is running slower, and vice versa. It seems the solution to this is the same though. The person had to get on the train and accelerate, therefore it's not an inertial reference frame. For both of these scenarios, it seems then that it is the frame that experienced a force that will have the slower clock when each clock from each reference frame are compared later. One other scenario is two astronauts A & B are floating in space and say orbiting a star in opposite directions. B sees A go by him then they synchronize their clocks. This seems like a situation where it's impossible to say who's clock will actually be slower when they compare their clocks at a later time. Maybe their clocks will read the same when they meet again. Is my thinking correct on this?