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I read recently that spacetime has a preferred speed, c, and that all observers would agree on the speed of anything moving at c. In particular, there is nothing unique about light when talking about relativity, light just happens to move at c.
1) Is this correct?
2) If so, how do we know this? I have a better than average math background and I would be interested to see how this is derived.
I am not asking about special relativity per se, or relativistic effects. I am quite down with the concept and mathematics of time dilation, length contraction and such. I am specifically asking about whether there is some feature of spacetime that makes c "special," what this feature is, and how we know this feature exists and/or model it mathematically.