vec_set( player, vector(x,y,z) )?
Since "inertial system" is just a fancy word for "we take this point as the origin of space", I could argue that space is just a made-up concept and our entire universe is defined by accelerations, thereby acceleration is the effect. But you could just as easily argue back your point of view .

What interests me is when you take the cosmic background radiation as inertial system. Then the space in our universe shrinks inwards instead of extending outwards!

Well, inertial systems have a velocity too, not only an origin. So it's the change from one velocity to another which causes the dilation.
http://en.wikipedia.org/wiki/Twin_paradox

Yes I know that, but isn't "the change from one velocity to another" defined as "accelerating"? I think this is just a matter of definitions, a conceptual thing, but physically we mean to describe the same. That's what I try to say.

Then again maybe teleporting isn't describable by the true definition of acceleration, unless teleporting without acceleration is not physically possible.

Yes I agree change in velocity is a more general description than acceleration, and thus be the preferred term to use.

Joey's question hasn't been answered yet, has it? Cause I always wondered the same thing... My Physics teacher tried to explain it to me, and I never got it...

All Movement is relative, as far as we know. So if I move away at the speed of light, I could also say YOU'RE moving away at the speed of light, so how does the universe "know" which one will have to age slower?

I was under the belief that it was the "relative speed" that caused the dilation, not "change of velocity".

Someone mentioned that the space-shuttle clocks ned adjusting after a flight...
I have also 'heard' that the clocks in the GPS-satellites also need adjusting occasionally,
and they are not 'changing velocity' to my eye, but they are 'travelling' faster
than us due to their greater distance from the center of the earth's axis.

That's exactly the problem. Relativity itself does only tell you that if you're moving, your clock is slower. Then you tell me that I am moving, so my clock is slower. Of course both are right.
The thing which causes a permanent "shift" of time, if you so will, is changing the inertial system (however that is done). That effect is hard to explain in few words but in principle that's the answer. That question is called "twin paradox" (see my link above).

edit: @evilsob: the problem with the gps satellites is not their greater speed but their constant acceleration; this is not a question of special relativity but of general relativity and much much harder.

The simplest and shortcoming answer is that the one who undergoes accelerating forces move slower through time. It's the acceleration (or as stated above, change in velocity) that causes time dilation, not the velocity itself.

Joozey, I Don't get it. In my eyes, what you said doesn't answer my problem.
If I drive away in a car, you'll say I'm accelerating. But then I tell you that relative to me, you're accelerating in the opposite direction. And, apart from the direction, your acceleration is exactly the same as mine. So we should both age slower, and both age "the same amount" slower.

Joey, I might find some time to look at the link, right now I'm in a rush...

space shuttle clocks need adjusting not because of the movement but because of the fact that time moves slower in space. and while this has something todo with movement, its not the movement of the clock, but of the earth. one ages slower in space. in fact, if you COULD travel at near light speed for ten years away from earth and then return for another ten years, you aged 20 years while on earth roughly 400 years passed (or something).

HOWEVER if you love on a planet bigger than earth, you age faster...

The standard reply you'll get when you ask people is that since you had to return to get back to earth, at that point you had to accelerate, and thus special relativity isn't valid anymore, and the problem has to be solved with general relativity.
This, however, is not correct. The truth is that the equation for time dilation you are using is only valid for the above, simplified case of constant movement with no acceleration. So here's the short reply: When the twin is returning home, the inertial system you've been using CHANGES. It suddenly moves in a different direction - that's not something inertial systems generally do ;), and is the cause of the problem: In your calculation, you've been assuming that you're calculating stuff in a inertial system, but you're not. That false assumption leads to the wrong result.


EDIT:



Nebolous statements like that really don't help much, don't ya think?
Relativity predicts TWO effects that change the "flow of time", if you will. One is described by special relativity - what is usually referred to as time dilation. It can be summarized as "Moving clocks go slower".

The second effect is due to general relativity. Masses curve spacetime, in other words, it changes the geometry of it. The time you're measuring (the "proper time") is nothing but the length of your path through spacetime (disregarding units). Now, obviously, if masses curve spacetime, it only makes sense that the lengths of your paths change, too (and thus your measured proper time).

Whats fascinating is that for satellites, bothe effects actually work in different ways: They're moving quiet fast relative to the earth, so that makes their clocks go SLOWER, yet at the same time, the mass of earth makes their clocks go FASTER compared to the one of someone on earth (the closer you are to masses, the slower your clocks go).