Any research about real measurements with objects moving faster than me - like GPS, for example - leads to a mix of different sideway phenomenons like different gravitational potentials on Earth and in orbit or accelerations during liftoff and landing. I'm curious; was Einstein had able to answer this thought experiment?
According to Wikipedia,
Langevin solved the paradox by alluding to the fact that one twin accelerates and changes direction
, so perhaps it is more precise to ask about when the twin paradox was simplified to "a travelling twin and a twin in rest". Perhaps the question more specific has to be: When the simplification of the twin thought experiment to the twin paradox took place? When did this happens?