When I wrote my paper on Special Relativity in my final year of high school, I explained the Twin Paradox as follows: Twin A stays on earth, twin B takes off to some star S. (I'm making this a very short version because I'm assuming we all know the structure of the paradox) Twin B measures less time passes on the way S and on the way back. But, using the Lorentz transformations, we find that even less time is supposed to have passed on earth (due to the equality of the reference frames). However, this is fixed because during the turnaround, suddenly a lot of time has passed on earth (this is as it was explained to me here). This all works out perfectly if you calculate things by putting B in a new frame of reference after the turnaround. My SR teacher explained it like this last week: Since B doesn't have a constant velocity, he doesn't have a valid view in Special Relativity, so we toss his observations out. He used a third observer to show that twin A was right. Now my question is, who has a better point of view, I or my teacher. Of course, I fully agree that he is right in saying B doesn't have a true inertial frame of reference, but then I wonder why it works out so perfectly if you do what's not allowed. Also, though Special Relativity is only valid for constant velocities, it would appear from these results that it is able to say something about differences of relativistic effects between different constant velocities, i.e. if the velocity changes, the effects of the Lorentz transformations change accordingly. To illustrate, my teacher said that if the velocity changed from v to -v, that earth's location would suddenly be shifted (where I reasoned that earth's time would suddenly jump forward, I'm not really sure why he arrived at position and I at time), which he claimed indicates the invalidity of B's point of view. However, doesn't it make perfect sense for the time to have shifted so? (I'm not really sure about the position, I don't really know how he got to that, it's puzzling me a little, but I'm assuming it's rather the same thing as the shift in time) For example, if B slows to a halt, then what he formerly perceived as so many lightyears will now seem like a bigger stretch of space, because the effect of length contraction is gone, so the spatial axes of A and B are synchronized (suddenly moving Earth back, now I see. The position part is easier to understand physically than the time part, though I don't see how one should use it to explain the paradox). It would seem most logical to me (in fact, I don't really see any other possibility) if the effect of length contraction changes continually when the velocity changes continually, thus there isn't really the problem of "it only works if you assume the acceleration in instantaneous". Alright I'm getting more confused as I type all this, so for the sake of keeping my post as clear and understandable as possible, I'll stop here and see what you all want to say about this. I'm afraid that my post isn't as clearly set up as I intended it to be, because my thoughts kept wandering around the subject as I was typing, but I hope you will be able to understand what I'm saying.