So my question arises from the whole time dilation thought experiment where one person stays on earth the other speeds off in a rocket at near light speed. Upon return of course the one in the rocket is younger, etc. The reason I bring this up is how do we determine who is actually moving, without an acceleration. I thought you need a force to determine the relative motion and since all the SR equations seem to only focus on velocity how do we know it isn't simply the earth moving away from the rocket ship. I mean I'm sure something explains this, but I'm just slightly confused where and how it is incorporated. I also figure that I should add, my knowledge of special relativity is very elementary as I have never studied the theory in-depth.