Did I understand this correctly - time slows down due to acceleration? Well I need to know if I understood the textbook correctly: Times goes slower in a gravitational field, but according to the equivalence principle you can't distinguish between a system being accelerated by a gravitational field with an acceleration g or the entire system being accelerated in the opposite direction by an acceleration g-. OK, so this means that a basketball being accelerated in empty space - time slows down for it in comparison with an observer who stands still, because it doesn't matter if the acceleration comes from rockets or from black holes as long as the directions are opposite and the absolute value equal? Another question: you guys know the twins-paradox? The explanation of why it isn't a paradox has to do with one of the twins being accelerated and the other not. I would like some explanation behind this because I'm a bit fuzzy. And try to keep it simple, please!