The twin paradox is not a symmetrical problem , as one twin has to change direction in order to get back to earth . Thus , this is not a valid argument . Your original statement was This is why when travelling very fast , or when in a very powerful gravitational field , outside events seem to pass quicker . This scenario is symmetric , the twin paradox is not . Apples and oranges . When you are traveling without acceleration relative to some inertial frame of reference I , the problem is perfectly symmetrical . There is no way to tell if you are traveling at a constant speed or if you are standing still . Hence , the effects observed in the two frames your rest frame I ' and the frame I have to be the same . That is the essence of the special theory of relativity , as it ' s core statement is , that all inertial frames of reference are equally valid to describe a phenomenon . You ' re working on the assumption that since time dilates symmetrically , that there must be some absolute time . Could you pinpoint the statement where I implied this assumption ? However , it is not the instantaneous velocities that matter it is the change in velocities . That is absolutely wrong . Special relativity does not deal with acceleration . The Lorentz Transformations only depend on relative velocities . The general Lorentz transformations can be written ast ' tvxc 2 x ' x 1 x v v v 2 tvwhere the bold expressions are vectors . As you can see , a change in velocity or acceleration is not involved .