[bobdsmith]

Even .99c isn't going to do it for you. Time has slowed down on your ship to only 14% of earth time, which will seem like you're living a long span (to the folks back home), and you'll live long enough for an otherwise impossible journey; but to you, subjectively, everything will seem quite normal.

Here is where I get confused. The spaceship is travelling .99c relative to the earth. But that means also the Earth is travelling .99c relative to the spaceship. So why doesn't that mean time on the earth slows down to only 14% of spaceship time, rather than the otherway round? What decides which body gets the "time bonus" when both are actually moving the same speed relative to one another? Where is the object of reference?

[RightWhale]

The ones who stay home also are seen as living slow, as seen from the ship.

[PatrickHenry]

We've had threads on the "twin paradox," and why the universe "knows" which twin stays young. Basically (the limit of my recollection) it's because the traveling twin is the one who's experiencing all the acceleration, both in leaving earth and then in returning. I'll leave it to others to explain it better.

[grey_whiskers]

What decides which body gets the "time bonus" when both are actually moving the same speed relative to one another? Where is the object of reference?

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!