Erratum

I was looking through a previous post and I realized I made a mistake. In The day the world stopped making sense, I wrote that the faster you go, the more time contracts. Actually, at high speeds, time dilates and it is space which contracts. However, this incident reminded me of the famous twin paradox where one brother stays on Earth and the other embarks on a spaceship traveling close to the speed of light. When the astronaut came back, he realized that his brother on Earth aged much more than he had because time passed much slower on the spaceship than on Earth. The paradox consists in the fact that for the brother who stayed on Earth (let's call him Stan) it was his twin (who we shall name Bob) who was moving and therefore Stan saw the clock on the spaceship as counting time at a slower pace. However, Bob was at rest in the reference frame which was the ship and for him, it was Stan who was moving and therefore, he saw clocks on Earth as being slower (kind of like when two metros are stopped in a station and when one begins to move, you're not quite sure which one is moving and which one is at rest; all you know is that they're moving relative to each other). So that constitutes a paradox because, for Stan, Bob should be younger and, for Bob, Stan should be younger. But the way that physicists have explained it is that when Bob made a U-turn in order to return towards Earth he accelerated (you need to accelerate to change directions) and at that moment he changed reference frames and that requires a clock adjustment (relativity only considers inertial reference frames therefore by accelerating, you leave your initial reference frame). Therefore the paradox does not exist because you are now comparing apples and oranges. What I don't get though, when Bob left the Earth, didn't he have to accelerate as well? Why are we only talking about the U-turn as being an acceleration.... I need to discuss this with someone.