bvr

IMHO if many people do not understand it correctly, it's because it is not explained correctly.

I see, on many forums and other places, many questions asked by many intelligent people, struggling with the concepts of time dilation, space contraction, "light" speed constancy etc. No symmetry is indeed a common argument, but it's almost always given as a superficial explanation: of course there is no problem, since the two situations are different. But thinking further, it's not that simple, especially when trying to explain what is happening, concretely and physically, in the traveller's reality.

I think they are. A "light clock" (a system that counts the rebounds of a light signal between two mirrors) will evidently slow when it is in movement. But all clocks are sorts of light clocks. A "light ruler" (a system than determines the distance between two points by mean of a light signal) will evidently shrink when it is in movement. But all objects are sorts of light rulers. The "twin paradox" is easily solved when one realizes that the symmetry of the situation is merely theoretical. The two frames are not physically equal, although all laws of physics are the same and light speed = c in both frames, and no "proper" movement can be determined. I wonder why these explanations, which are truly compatible with Einstein's theory, are so rarely found. I wrote an article about some years ago: link deleted I hope it can help understanding the relativistic effects.