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Ben
At the risk of causing a riot, a problem with that Smolin/Magueijo paper is that it presumes SR includes gravitation in the first place. Which it does not. So in that sense, there is no issue to resolve. Einstein defined SR several times, and it is not 1905.
In 1905, the two postulates are "apparently irreconcilable". Which is a bizarre statement when juxtaposed against the assertion that "These two postulates suffice for the attainment...theory...electrodynamics of moving bodies". The irreconcilabilty is because light is presumed to be in vacuo, whilst everything else is not, because they are subject to dimension and momentum variance. And the cause of that is subsequently revealed to be gravitational forces. In other words, light and matter cannot be co-existent and so 1905 is not a singular, cohesive, theory. So in expounding GR, where everything (ie including light) is subject to gravitation, Einstein refers to a special/entirely theoretical circumstance where there was no gravitation, ie SR. This is why SR only involves uniform rectilinear and non-rotary motion, light that travels in straight lines at a constant speed, and rigid bodies.
Einstein: Relativity 1916 section 18: "...the special principle of relativity, i.e. the principle of the physical relativity of all uniform motion..."
Einstein: Foundation of GR 1916 section 3: "...the case of special relativity appearing as a limiting case when there is no gravitation"
Einstein: Relativity 1916 section 28: "The special theory of relativity has reference to Galileian domains, ie to those in which no gravitational field exists...In gravitational fields there are no such things as rigid bodies with Euclidean properties; thus the fictitious rigid body of reference is of no avail in the general theory of relativity"
Einstein: Relativity 1916, section 22: "Let us further investigate the path of light-rays in a statical gravitational field... Let us find out the curvature which a light-ray suffers. [equation (74) refers] ...a curvature of rays of light can only take place when the velocity of propagation of light varies with position...
Paul