A twin is on a spaceship, the other on the earth. first twin accelerates and get distant form the earth, we know he is moving. He dies in the spaceship and his son takes control of it. Now, for his son the system is linear, he does not know the spaceship is moving, and he has not a possibility to discover it. Halls of ivy said that: The statement "he has not a way to discover the truth, that his ship is moving" is incomplete. Relative to what frame of reference? His ship is moving relative to some frames of reference, not moving, relative to others (in particular, a person is alway "not moving" relative to his own frame of reference). It simply isn't true to say it is "the truth, that his ship is moving". It was good for a week, but i think there's a bug: Nobody can deny that the ship is moving, at least you can say it is or it is not relative to some frame of reference. But, Have you cancelled the acceleration? In a moment of time it happened, for any frame. And, motion depends from frame to frame, but cannot a frame of reference be wrong? the ship is definitely moving, relative to nothing.