Firstly, I'm seeking conventional answers only. I'm aware of the alternate theories on this site and would visit those relevant threads should I be interested to know more.

Secondly, I'm aware that many knowledgable members are probably bored to death and suffering nose-bleed from having to repeatedly answers questions about SR, specifically twin paradox. So, while answers from these members would be greatly appreciated, it wouldn't be reasonable of me to expect them to do so.

Therefore, if there are other members with conventional science wisdom, I hope you can enlighten me.

My questions concern just one component of the Twin Paradox:

a) How is a change in direction in the travelling twin's worldline construed as "acceleration"?

b) How is this "acceleration" dealt with - with GR or still within SR's scope?

Well, hopefully someone responds. Thanks in advance.

I will completely avoid what I do not know. So, I cannot answer the question about how GR and SR deal with acceleration.

But, a change in direction is viewed as acceleration. In a car, going around a turn causes our weight to shift tangent to the circular motion. We are slowed by the added friction and the change in momentum, but this is a form of acceleration under a change in momentum. If we turn left, we accelerate left. Our speed may be reduced, but now you can see that any change in direction requires us to accelerate in that direction.

Originally posted by 1119
a) How is a change in direction in the travelling twin's worldline construed as "acceleration"?
An unaccelerated frame is one in which there are no ficticious forces. If you're in a spaceship in uniform linear motion, and you release a tennis ball, the tennis ball will appear to hover motionless inside the cabin. This is called an inertial frane.

If the spacecraft were speeding up, slowing down, ot turning, the ball act as if it had a force pushing it back, forward, or sideways. This is an accelerated frame.
b) How is this "acceleration" dealt with - with GR or still within SR's scope?

GR deals with the geometry of curved spacetime. GR asserts that an inertial frame is one moving along a geodesic ("straightest line") through curved spacetime. An accelerated frame is one moving along any other curve. SR can deal with accelerations by performing integrals -- but when you get into gravitational fields (which produce their own form of accelerations), you have to use GR.

- Warren

1119 - This site deals with acceleration in SR: The Relativistic Rocket (http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html).