# Special Relativity and time [duplicate]

Possible Duplicate:
Question about Time Dilation..

I have a question about special relativity which was bothering me for a while now. I know that as one approaches the speed of light, time moves slower for him. So, if I start moving as fast as 99% of the speed of light and travel away from Earth for 1 day and come back, I'll see that (suppose) about 1 year has passed on Earth.

But my question is, if everything is relative, then how can we say that I was the one moving and Earth was the one staying? I mean if I consider myself as the center o the coordinate system, then I can say that Earth went on a travel with the speed of light and came back. So why is that the time slows down for me and not for the Earth? How do we distinguish between the moving and the static object according to relativity?

• Possibly related: physics.stackexchange.com/q/9354/2451 May 20, 2011 at 15:56
• In short: Acceleration. By turning around, you had to slow down and speed up again. This means that you have changed inertial frames. May 20, 2011 at 15:58
• ok, that clears the issue to some degree. What about the far far galaxies which are traveling relatively with a very high speed (I guess in some cases with half the speed of light). How can we determine if we are experiencing time slower or them? May 20, 2011 at 17:15
• Both experience the same thing, and this is no problem because someone would have to accelerate to meet to compare clocks. May 20, 2011 at 17:18
• That's the thing, distant galaxies aren't necessarily traveling any faster than we are. That's just how it appears because space itself is expanding between us. May 20, 2011 at 17:21