Time Dilation and Differential Aging

[Doc Al]

When Einstein demonstrated how time dilation happened, he always said it was "uniform motion v". Here as if you were saying, only accelerating SIGNIFICANTLY would produce the effect of time dilation while uniform motion would not. Why?

For two frames in relative motion, the time dilation "effect" is completely symmetric. Each measures the other's clocks as running slow (and being out of synch). In order for the twin to return to return to earth, so that his age can be directly compared to his brother's, he must accelerate. That breaks the symmetry, since the traveling twin cannot remain in a single inertial frame. But it's the relative speed, not the acceleration, that produces the differential aging.

 

[JesseM]

When Einstein demonstrated how time dilation happened, he always said it was "uniform motion v". Here as if you were saying, only accelerating SIGNIFICANTLY would produce the effect of time dilation while uniform motion would not. Why?

Only when acceleration is involved can they move apart and then reunite at a single spot to see whose clock is behind. As Doc Al said, if they are not in a single spot, then because they disagree about simultaneity, they can disagree about what their clocks read "at the same moment", so they can disagree about whose clock is behind. Take a look at my thread An illustration of relativity with rulers and clocks to see how two frames moving inertially can each say the other frame's clocks are running slow, but they'll never disagree about what two clocks passing next to each other at a single location in space read at the moment they pass.