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I am still not persuaded with the explanations of the twin paradox which I can find over the internet, so I am posing an extended version:

Consider triplets - one travelling to the west, one to the east and one staying. Given the fact, that the two of the triplets travel at different directions, the regular explanation will not be suitable. Can sb. elaborate?

Vojtěch
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    Special relativity is suited just fine to describe multiple travelers moving in any directions. Can you edit the question to elaborate what you think the "regular explanation" is and why specifically you think it fails for your example. – Paul T. Jun 28 '21 at 20:38
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    Why is the regular explanation not suitable? If you analyze the situation by pairs of people you find that both who traveled aged less than the one which stayed. If the trajectory of the travelers was symmetrical, then both travelers aged exactly the same. – Lucas Baldo Jun 28 '21 at 20:39
  • Try to understand the simple case (twins) first –  Jun 28 '21 at 20:46
  • You can have as many siblings as you like, try septuplets- the principles are exactly the same. One of them remains in a single rest frame- all the others switch frames. The switching causes the asymmetry which results in different rates of ageing. – Marco Ocram Jun 28 '21 at 20:52
  • Related, possible duplicate. – rob Jun 28 '21 at 20:55
  • I'd like to add here that differential aging of the "moving" twin is not just speculation, it's actually observed in experiments: every day at the LHC and other particle accelerators, in airplanes (e.g. the Hafele-Keating experiment), and in the GPS system. – Eric Smith Jun 29 '21 at 00:18
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    I don't think there are any good "twin conundrum" questions left at this point in time . . . currently 741 "results" on this site alone. Is there any solution to this or do we just accumulate more bad questions & answers ad infinitum? – m4r35n357 Jun 29 '21 at 08:29

1 Answers1

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A spacetime diagram on rotated graph paper allows ticks to be read off.
Essentially, follow the bouncing light-signals (arranged so that the principle of relativity is satisfied), which bounce off parallel mirror worldlines [not shown].
All light-clock diamonds (which visualize the ticks of a wristwatch) have the same area.

Here is one for the problem you pose.

The light rays broadcast by the initially-forward-traveler are shown, which can be used to compare what the other observers see with their eyes to compare the remote clock-face with the local clock-face (associated with the Doppler effect).

The most time elapses on the inertial path from separation to reunion.

robphy-RRGP-triplet

This diagram is taken from my earlier answer https://physics.stackexchange.com/a/402711/148184 from What if two twins flew off in opposite directions and were reunited in a perfectly symmetric way, would they have aged same?

A more elaborate explanation is at another answer https://physics.stackexchange.com/a/434193/148184 from Question about Special Relativity similar to twin clock experiment

UPDATE:

For something with asymmetric motions, go to https://physics.stackexchange.com/a/508935/148184 from Equivalence of two definitions of proper time in special relativity

robphy
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