# Twin Paradox on time linearity and aging during acceleration

I think I understand the observed final result of the twin paradox.

My question is about the time perception and aging (physical impact of time dilation) of the twin when she decelerates and turns around.

When one twin travels near light speed while another stationary, they cannot observe who is actually traveling and their perception of time is the same.

Things changed when one of the twins decelerates, turn back, and accelerates back to the stationary twin.

And the result is one is aged much faster, while the order does not.

So, if we observe from the perspective of the moving twin:

• When she is moving away, time is linear and aging occurs normally.
• When she have turned back and moving backward, time is still linear and aging occurs normally.

So that implies, when she decelerates, turn back, and accelerates, either:

• her time suddenly "speed up" and she ages at a rapid speed, or
• she will perceive that the turnaround takes a long time until her physical aging matches the result of the twin paradox.

Which would be more likely or based on her perception, it is one and the same?

Also, if that is true, then it also means that the time taking her to reach 0.95c (for example) would take forever (in her perception).

In the twin paradox, when the twin met again, are they still traveling at close to light speed relative to each other?

What will happen when the twin meet and the moving twin decelerates?

UPDATE: as pointed out by @tfb, I got it backward.

So from the viewpoint of the stationary twin, when the traveling twin turns around, she will observe the traveling twin "freezes up" and take a very long time to turn around.

While on the other hand, the traveling twin does not feel the effect of time dilation and her perception of time is linear.

Then, a follow-up question is, can the deceleration/acceleration of the traveling twin measurable? Or due to the difference in frame of reference, the concept of "measuring" is meaningless because the time-frame-of-reference is variable (depends on which frame of reference you are observing from)?

• You have things backwards: the twin who undergoes acceleration experiences less proper time: a geodesic is the longest curve between two timelike-separated events. – tfb Mar 1 '17 at 22:40
• Ar, you are right. Let me correct the question – unional Mar 1 '17 at 22:41