Following up on a previous exposition of the twin paradox in special relativity, this time I am going to use an animated spacetime diagram to explain what it would be like from the traveling twin’s perspective.
In the animation below, the world lines begin with both twins at the same place at the same time, and one twin travels away from Earth before turning back, where all the acceleration happens.1 These are colored, while we also include a neutrally colored line which shows what counts as “now” from the perspective of the traveling twin.
If you understand what all the lines mean, then you can note that from the traveling twin’s perspective, time is slower for the twin on Earth, then skips through a lot of time very quickly while turning back, and then flows slower again. Even if less time passes for the twin on Earth from the traveling twin’s perspective during the outbound and inbound legs, the big skip in time while turning around more than makes up for the traveling twin ultimately experiencing less time.
Even if you do not understand what all the lines mean, the animation should still give a visceral sense of there being a big skip, more than what an equivalent still image might give you. I made this animation as a test case to confirm ideas I had about creating SVG animations, but I was struck by how much more obvious it made the time skip.
As before, I have the fuller treatment of the topic elsewhere. If you would like to use this animation for your own purposes, the file is available on Wikimedia Commons under the Creative Commons Attribution-Share Alike license.
Presumably the traveling twin would also like to start from and stop at Earth, but we can ignore the acceleration at the beginning and end since it does not affect how time is dilated for the traveling twin. This is due to the fact that both twins are at the same time and place when the journey starts and ends.↩︎