I am having some trouble undestanding why speed would cause time dilation. Here's why: Twin A travels at near the speed of light away from twin B for one year from in twin A's frame of reference. When he returns to twin B, he has aged two years but twin B has aged significantly more. Right so far? What is the difference if it's twin B that does the travelling? Why would twin A now be the one to age faster? From twin A's frame of reference, nothing is any different, other than who experiences the changes in velocity to get up to speed. This would imply to me that it's not the speed that dilates time, but the changes in velocity used to get up to speed. This would be consistant with with what we know about gravity and time dilation and the fact that gravity and mass are one in the same thing. Am I missing something here? Do we see time dilation between the equator and the poles, acounting for any gravitational or centrifugal differences?