Can you say "Twins Paradox" ??
The problem is that there is NO preferred reference frame for things moving at constant velocity...
Supposedly it can be resolved by going to General (not Special) Relativity.
Cheers!
The correct solution to the twins "paradox" has been known all along. It's only ever been a source of mystery to those who don't know the math.
The answer is that the axis of simultaneity is frame dependent. The twin who remains in one inertial frame is the "object of reference". The one who changes frames (by turning around and coming back) is the one who experiences less time.
On each leg of the traveller's journey, time is passing more slowly on Earth than on the traveller's ship, as measured from the traveller's point of view. At the same time, time is passing more slowly on the ship as seen from the Earth's point of view.
Those statements seem irreconcilable until you understand one key fact: that on the ship, the time spent on the outbound leg is simultaneous with a (short and slowly passing) period of Earth time shortly after the traveller's departure, and on the inbound leg is simultaneous with a (short and slowly passing) period of Earth time shortly before the traveller's arrival. At the turn-around, there is a "simultaneity gap": a large stretch of the Earth time that was never (or rather, only very briefly) simultaneous with the ship time, during the turn-around.
Let's put it another way. Suppose the traveller isn't coming back, but zooms out past Betelgeuse, and passes an Earth-bound ship at very close range. At the moment the ships pass, the time on Earth is extremely different for both ships, even though they're at the same place at the same time.
Why? Because they're in different inertial frames.