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Jun5-04, 03:16 PM
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Quote Quote by relative_sceptic
So which one is older, when they return to the same time frame?

Remembering that the travelling twin will experience identical accelerations/decelerations in returning to earth as the twin who originally returned to earth. The travelling twin will just spend longer in the uniform states of motion in between the periods of acceleration/deceleration.

Okay, this is something that, in my opinion, gets overlooked to often when discussing this type of thing, and it leads to a lot of unnecessary confusion.

The first thing to realise is that acceleration does not cause time dilation from the view of the unaccelerated frame. (IOW if you are observing an object that is accelerating, you only have to deal with its velocity at any given moment to determine its time dilation, the fact that it is accelerating has no effect.

Now in the accelerated frame, things are different, If you are in this frame, three factors control how you will measure other objects time rates: the magnitude of the accelleration, the direction the object is with respect to the acceleration, and the distance of the the object.

The second thing is that time dialtion is not something that anyone experiences it is something that you measure as happening in other frames.

Now in your situation, from the Earth Frame, the twin that turned around last ages the least becuase he undergoes the longest duration of time dilation.

From either twins view, Earth time runs fast when they turn around and start heading back (when they are an accelerated frame once they retrun to uniform motion towards the Earth, they will once more measure time running slow on Earth). Since the twin that turns around last is further from the Earth when he turns around, he will measure Earth time as running that much faster during this time. He will return to Earth expecting more time as having passed on Earth than his brother will.