Re: Why is acceleration absolute in relativity theory?

Looks like you're asking about the infamous twin paradox of relativity. Two twins are moving apart at relativistic speeds, and when they come back together one has aged less than the other.

It is true that relativity works both ways: each twin sees the other twin as aging slower, while they are moving apart. It is only when one twin accelerates -- turns around -- that the symmetry is broken, and it is determined which twin will be younger when they come back together again.

So why, you ask, isn't acceleration relative as well? The short answer is that you can tell if you're accelerating or not -- the inertial forces you feel in a turning bus are REAL -- they're not just a matter of perception. You can't claim that it's really the rest of the world that's accelerating around the bus, and you're not accelerating at all: you can feel the force, so it must be real.

Constant motion in one direction, though, IS relative. Suppose the bus was just cruising along some constant speed. Einstein postulated that you can't tell if the bus is moving forward or if the rest of the world is moving backwards -- that there's no difference between those two cases. And for this non-accelerating example, Einstein was right: you CAN'T tell which is moving. But you CAN tell which is accelerating. So that's the difference.

Now, in general relativity there is an equivalence principle, stating that all KINDS of acceleration are equivalent -- gravity is the same as the swerving bus -- but in both cases the acceleration is clearly different from no acceleration at all.