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Indeed. "The speed of light is constant" is a slightly abused phrase. The speed of light is constant in a vacuum locally in curved space-time (of globally in flat space-time). In your black hole example, the speed of light is constant when measured over a tiny enough patch of space, but it isn't constant over large distances because of the curvature of space-time.And of course, if light hits matter, it slows down because of interactions with the matter.

By the way, different observers measure time differently (time dilation) but they also measure lengths differently (length contraction). Is there a reason you think time dilation is more important than length contraction?

Quote from: JP on 06/05/2011 21:43:32By the way, different observers measure time differently (time dilation) but they also measure lengths differently (length contraction). Is there a reason you think time dilation is more important than length contraction?I just cannot understand what does gravitational delay of light? Length contraction or time dilation?

Remember, the reciprocal views of clocks in relative motion has, to my knowledge, never been experimentally verified.

SR also predicts the observation of a slow clock in the stationary frame when viewed from the moving frame.

Imatfaal,I agree there is no paradox, but what are you saying about the final age difference ? It is not clear

Twinstayathome and his clock,his grey hairs, his infeasibly long beard etc all show that 14ish years have passed, Twinspaceman and his clock and all other measures show that two years have passed. there are complications because the time dilation factor isn't constant during speed ups and slow downs but the errors brought in by those are minimal.

What do you think will be observed from each position?

Lengths contraction does not reduce amount of rotations.Time dilation makes.

I think we're stuck at this point with all of us maintaining our positions, except to say, that I fully understand the mainstream views tabled so far, but I still see a massive contradiction between the assertion that both clocks appear red shifted and the fact that there is an inevitable blueshift one way, however "invisible" this may be from any observation.

I think this abdication of interest was a feint.

net overall effect is a relative redshift of the moving clock, but a relative blueshift of the stationary one.

but until someone can answer the above concerns without simply throwing the party line at me, then I will continue to have these ideas.

Imatfaal,The statement that intrigues me most is;- "The redshifting/slow ticking is not solely an artifact of the time dilation."I am interested in why you believe this to be so?

Simplified,Do I take it you mean that length contraction cannot explain the difference in clock hand rotations, whereas time dilation can?If so, then I agree. The implication from this idea is that time dilation must be real as well as relative, because it has a real and permanent effect from one frame to another, whereas length contraction cannot be real because this does not have a permanent effect from one frame to another. Length contraction is therefore purely relative, an illusion.Are we beginning to think the same?