When did exactly the Langevin twins occur in relativity?
My first idea was to simply look up Langevin's original paper (in 1911), but he never mentions them to be twins :
our traveller would need only to agree to being shut inside a projectile that the Earth would launch at a velocity sufficiently close to that of light, but still less than it, which is physically possible, arranging for an encounter with say, a star to take place at the end of one year in the lifetime of the traveller and to send him back towards the Earth at the same velocity. Having returned to Earth two years older, provided that his velocity has remained within the range of only one part in twenty thousands less than the velocity of light. The most reliably established experimental facts of physics enable us to state that this is indeed what would happen.
It does not seem to be mentioned anywhere in there that this is some experiment involving two twin brothers.
As far as I can tell, the earliest mention I can find of this would be in Schmidt's popular science book "Relativity and the universe" (1921), where quoting Hermann Weyl, he gives this example :
The life process of a human being may well be compared with the movement of a clock. Its course,therefore,will depend on the state of motion of the system of coordinates in which the particular man is spending his life. Now imagine twin brothers who one day take leave of each other. Let one of them stay at home, i.e., let him be permanently at rest in a suitable system of coordinates. But the other is to go on a journey and to travel with velocities as great as possible relatively to his home. If, many years afterwards,this traveller returned home he would find himself noticeably younger than his brother who remained at home.
Is this the actual earliest appearance of the Langevin twins?