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<blockquote data-quote="Umbran" data-source="post: 7547295" data-attributes="member: 177">I understand.&nbsp; One can naively think, 'Well I can use any frame of reference I want.&nbsp; I could use Twin A's frame, or Twin B's frame.&nbsp; Each can think the other is moving.&quot;That's why I introduce a frame that is not dependent on the Twins, and measure *both* of them relative to that.&nbsp; In my example, the reference frame in which Earth is at rest*.&nbsp; It happens that one of my Twins stays at rest relative to Earth, and the other does not.I could have both twins leave Earth, and zoom around and come back, even passing either other, if you want.&nbsp; Using Earth as the stationary frame of reference, I can tell you which one ages more.Yes.&nbsp; Remember that this is (falsely) called a paradox, because at some point, the twins are the same age, and later, they are not.&nbsp; So, for some period of their lives, they have the same clock.&nbsp;&nbsp; You can measure everything with respect to *that* frame, where both have the same clock.&nbsp; Then, either one or both can move, and you have a reference.*If you want, call it the Sun at rest, allow Earth to orbit, the difference is negligible when the travel is on the order of light years.&nbsp; It all comes out in the wash.</blockquote>

[QUOTE="Umbran, post: 7547295, member: 177"] I understand. One can naively think, 'Well I can use any frame of reference I want. I could use Twin A's frame, or Twin B's frame. Each can think the other is moving." That's why I introduce a frame that is not dependent on the Twins, and measure *both* of them relative to that. In my example, the reference frame in which Earth is at rest*. It happens that one of my Twins stays at rest relative to Earth, and the other does not. I could have both twins leave Earth, and zoom around and come back, even passing either other, if you want. Using Earth as the stationary frame of reference, I can tell you which one ages more. Yes. Remember that this is (falsely) called a paradox, because at some point, the twins are the same age, and later, they are not. So, for some period of their lives, they have the same clock. You can measure everything with respect to *that* frame, where both have the same clock. Then, either one or both can move, and you have a reference. *If you want, call it the Sun at rest, allow Earth to orbit, the difference is negligible when the travel is on the order of light years. It all comes out in the wash. [/QUOTE]

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