Question

Consider a situation where immediately after birth, twins are separated. One continues to live on earth, while the other is whisked away in a space ship at 90% of the speed of light. After 65 years they are reunited. Will they look the same?

I know that the twin that was whisked away will look older

Answer 1

Your answer is not correct. The twin who is "whisked away" at high speed and then returns (presumably at the same speed) will have aged less, by a factor of sqrt[1-(0.9)^2] = 0.436

He will have aged 28.3 Earth years.