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<blockquote data-quote="freyar" data-source="post: 7548488" data-attributes="member: 40227">Umbran's explanation is spot-on, but I did want to offer a comment on the acceleration part. It's not the acceleration that makes the difference in aging, but it isn't possible for the traveling twin to go away and then come back to the earth to make a comparison to the stay-at-home without accelerating, at least in our universe. There has to be a change in inertial frame somewhere.On the other hand, you could imagine a universe where one direction is actually a circle that's small enough to travel around.&nbsp; Then the traveling twin could just travel around the whole universe to get back to the earth without accelerating. The earth-bound twin will still be older. But both twins are in supposedly equivalent inertial reference frames the whole time. The key in this example is that special relativity doesn't work across the whole universe when a direction is finite in extent. Basically, there is a preferred reference frame where the circle direction is static.</blockquote>

[QUOTE="freyar, post: 7548488, member: 40227"] Umbran's explanation is spot-on, but I did want to offer a comment on the acceleration part. It's not the acceleration that makes the difference in aging, but it isn't possible for the traveling twin to go away and then come back to the earth to make a comparison to the stay-at-home without accelerating, at least in our universe. There has to be a change in inertial frame somewhere. On the other hand, you could imagine a universe where one direction is actually a circle that's small enough to travel around. Then the traveling twin could just travel around the whole universe to get back to the earth without accelerating. The earth-bound twin will still be older. But both twins are in supposedly equivalent inertial reference frames the whole time. The key in this example is that special relativity doesn't work across the whole universe when a direction is finite in extent. Basically, there is a preferred reference frame where the circle direction is static. [/QUOTE]

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