I have been reading this thread from the beginning, and I have followed all the links, and have finally caught up to the present - I must be mad
and despite the acceleration necessary to get here in less than real-time, I actually feel a lot older rather than younger. SR doesn't apply to forum spacetime.
The journey has been aggravating, frustrating, amusing, surprising and very educational, so thanks to all involved - especially MacM, for starting it off and continually stirring it up with that spiky stick.
The one characteristic that sums it up so far for me is misunderstanding - apart from any misunderstanding of the actual physics, there seems to have been two other major problems of understanding that have dominated most of the thread, a metaphysical one, and a linguistic one. The linguistic one is that in terms of physics, MacM has been talking a different language from everyone else - using common words but with different sematics and idioms. In terms of the specialist usage and idioms of physics, they have been inconsistent, imprecise, inaccurate, and often plain incorrect, which has made it extremely difficult to follow his arguments. These difficulties have been compounded on both sides by the abuse of this semantic gap to make the other party appear in error. Accidental misunderstanding is damaging enough to the discussion, but when (for example) you know MacM doesn't mean 'absolute' in the commonly used physical sense, and seems determined not to change, it's pointless and unnecessary to continually pretend that he does and criticise his arguments for that. Equally, a clear definition of what he does mean by it would probably avoid a lot of faffing about in the first place.
The other metaphysical misunderstanding I noticed hasn't really been commented on, and I may have it wrong. I was taught that theories like SR and GR don't and cannot in themselves directly tell us anything about physical reality. They are just attempts to model our
observations (measurements) of physical reality. Most theories are limited to a particular context, outside which they are known to be inapplicable, and in general their utility is in how well they model and can predict the observations we make. They are limited tools. As soon as our observations conflict with the theory we must either constrain the context or discard the theory. In that sense they are pragmatic.
Anyone can decide what they believe the model tells them about physical reality, and such speculation is fertile ground for new hypotheses, but the model is just a model. If SR predicts results that are confirmed by observation, and (within its context) is simpler to use and/or more accurate than other models, it will continue to be used. If not, it will become history.
Following on from that, it seems to me that when using these models in Gedanken Experiments, 'reality' for an observer is what that observer can observe (i.e. measure), and the essence of relativity is that this will vary according to the observer's frame of reference. When I hear the pitch of a passing police siren change, it's not an illusion in the sense that I misinterpret my perception - the sound waves reaching me really do measurably change wavelength, so for me the change in pitch is real, even though for the driver, the pitch is constant (for the sake of argument). What happens when we meet up and both hear a constant pitch is that my perception of reality now matches his. It may be comforting to suggest that because the siren makes the sound it's the siren's reference frame that counts as physical reality, but that's totally irrelevant to all those people who hear it higher or lower pitched or changing pitch and will never see the police car or join its frame. To them the pitch is really what they perceive it to be. And so it is, I feel, with SR and time dilation due to relative motion.
