> In the Earth frame of reference, time on the spaceship will be observed to pass more slowly than on the Earth due to time dilation. It may seem as if only a few years have past on the ship while decades pass on the Earth. So the twin of the astronaut waiting on the Earth expects the astronaut to be the much younger of the two upon return. > > In the spaceship frame of reference, it is the Earth that is moving at a very high speed so time on Earth will be observed to pass more slowly than on the spaceship. Decades will pass on the ship while only a few years pass on Earth. So the astronaut expects that the twin sibling waiting on the Earth to be the much younger of the two upon return. > > For the astronaut to return to the Earth, he/she must change direction and thereby switch from one inertial frame to another, and that breaks the symmetry between the two observers.