How can this happen if each
twin sees the other twin's processes running slow? The answer:
one twin changes from his original moving inertial frame to
another, totally different one (to make the return trip), which
destroys the symmetry between the processes that would otherwise
exist. The travelling twin is younger than the stay-behind twin
when their frames coincide again.
In a practical case, the
travelling twin has to change inertial frames three times... to
take off, to turn around, and to land again. The other twin is
always in the same inertial frame.
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