The youthfulness of the traveling twin became famous but Einstein knew that, within special relativity, this youthfulness was non sequitur. Einstein had to deduce it from something (the situation was getting dangerous) so in 1918 he informed the gullible world that, during the turning-around acceleration of the traveling clock (twin), a HOMOGENEOUS gravitational field appears and the youthfulness of the traveling twin is a consequence:

http://sciliterature.50webs.com/Dialog.htm
Albert Einstein 1918: "A homogeneous gravitational field appears, that is directed towards the positive x-axis. Clock U1 is accelerated in the direction of the positive x-axis until it has reached the velocity v, then the gravitational field disappears again. An external force, acting upon U2 in the negative direction of the x-axis prevents U2 from being set in motion by the gravitational field. [...] According to the general theory of relativity, a clock will go faster the higher the gravitational potential of the location where it is located, and during partial process 3 U2 happens to be located at a higher gravitational potential than U1. The calculation shows that this speeding ahead constitutes exactly twice as much as the lagging behind during the partial processes 2 and 4."

This HOMOGENEOUS gravitational field is crucial - without it, the twin paradox is an absurdity, as Einstein explained in his 1918 paper. The problem is that the HOMOGENEOUS gravitational field itself is a much bigger absurdity - it extends from the traveling twin to the stationary twin, no matter the distance between them, and is generated by the turning-around acceleration of the traveling clock (twin), an acceleration which is even absent in some twin paradox scenarios! Actually "absurdity" here is a euphemism - Einstein's 1918 HOMOGENEOUS gravitational field is one of the greatest idiocies in the history of science.

Pentcho Valev