Ok say you had two galaxies moving away from each other at exactely half the speed of light. If each galaxie had a clock assigned to it after 1 year would both the clocks stay synched? As both the galaxies are moving at the same speed but away from each other. But im confused because if this is the case how do you know one galaxy isnt moving at the speed of light and the other galaxie is staionairy in this case time would come to a stop for the moving galaxy. Soemone help me out hre cuz im confused????????????????????

Also, stoner, you shouldn't think of the galaxies moving away from each other---the space in between them is expanding.  There is no reason to expect that space can't expand faster than the speed of light.

Also, I'd like to clarify something you said: "But im confused because if this is the case how do you know one galaxy isnt moving at the speed of light and the other galaxie is staionairy"

They are each stationary. That's the idea behind relativity; that's the reason it's CALLED "relativity." You're thinking of the way things work in everyday life, like two cars moving away from each other at 60 mph but relative to each other they're moving 120 mph. The difference is that here when we say 60 mph we're talking about speed relative to the road, using the road as a point of reference and 120 mph relative to the cars, where they use each other as a reference. But galaxies aren't on any kind of surface like a road, we can only say how fast they're moving relative to other objects, so when you say they're each moving at half light speed, what you're saying is that each sees the other moving away at half light speed. That's the idea behind relativity. Each galaxy can say that it is perfectly stationary and the other is moving away from it, and there is absolutely no way to tell any kind of difference. They would both be right, because the only motion they're experiencing is relative to one another.
To make this easy to understand, imagine not one galaxy A but a thousand of them, all moving in the same direction relative to lonely old galaxy B. Were all the galaxy A's to talk to each other, they'd say "Where's galaxy B going? He's running away!" Relative to one another all the galaxy A's are stationary, with B moving away. But now imagine the opposite; a thousand B's wondering where that oddball galaxy A is going. They don't think about the fact that they're opinion has no basis; the only reason they think B is moving away is because they are using one another as points of reference. B is equally valid in saying that he isn't going anywhere and all the galaxy A's are ditching him. All motion in space is RELATIVE.
I hope this helps.

Last edited by Gungnir (2007-03-14 21:50:44)

Galaxy A and B would know if they are moving or not because they would be moving away from one another and toward some other reference point.  They could tell if they are stationary or not by looking at all available reference points.

Steve