I realize this should be in the Theory forum, but I don't seem to have "permission" to post there, so I am posting here. Moderators, please move this thread there if you get the chance. Thanks. Though I find VSL sort of troubling, I have a hard time understanding why it is so easily dismissed by some. So I just have a few questions for both VSL haters and lovers (I do not know that much about the theory). According to VSL, the speed of light nowadays is decreasing pretty slowly, so doesn't that imply that the constancy of light is an excellent approximation to VSL nowadays? Similarly, wouldn't VSL reduce to GR in any local spatial slice of spacetime? With this in mind, how does the fact that experiments have shown c to be constant during the past couple of centuries prove that it is a universal constant? Does anyone know if, according to VSL, c is actually approaching some constant value (besides 0)? Thanks a lot for any replies.