Here is an explanations due to Daniel F. Styer, Prof Physics at Oberlin Daniel's original is at https://mail.google.com/mail/?ui=2&view=bsp&ver=ohhl4rw8mbn4". He uses general relativity and the equivalence principle. The equivalence principle is not entirely true -- it IS possible to distinguish between gravity and acceleration -- but Daniel says that it is good enough for this purpose, that no one has ever succeeded in measuring the difference. Consider an accelerating space ship and two clocks. Clock T is in the tail and clock N is in the nose. Each clock sends out a signal once a second. The situation is not symmetric. Clock N measures that clock T's signals come more than one second apart, and clock T measures clock N's signals as closer than a second apart. Both clocks agree that T is slower: no paradox. What's neat about this is that this difference depends on the distance between T and N. The further apart, the greater the differential in speed. This is what you need to get agreement with the special relativity equations: it depends directly on the distance between the clocks. When the two clocks are reunited the T clock will be behind the N clock by the appropriate amount.