many different theories, characterized by different values of such parameters, are equivalent to SR in that they are experimentally indistinguishable from SR (though they differ from SR in other aspects). In addition there is the "standard model extension (SME)" of Colladay and Kosteleck? that extends the standard model of particle physics with various plausible Lorentz-violating terms.  This is in the context of quantum field theory, and is well beyond the scope of this article (which is limited to SR and its immediate consequences). The goal of many of the recent tests is to put limits on the many parameters of this test theory. Colladay and Kosteleck?, Phys. Rev. D55 (1997) pg 6760 ( arxiv:hep-ph/9703464 ) , and Phys. Rev. D 58, 116002 (1998) ( arxiv:hep-ph/9809521 ). Optical Extinction Many measurements of the speed of light involve the passage of the light through some material medium.  This can invalidate some conclusions of the measurement due to the extinction theorem of Ewald and Oseen.  This theorem states that the speed of light will approach the speed c / n relative to the medium ( n is its index of refraction), and it also determines how long a path length is required for that approach.  The distance required depends strongly on the index of refraction of the medium and the wavelength: for visible light and optical glass it is less than a micron, for air it is about a millimeter, and for the intergalactic medium it is a few parsecs.  So even astronomical