I figure the answer is somewhere in the sticky FAQ above but I’m having trouble finding it. I’m hoping y’all can help. I understand the Michelson–Morley experiment shows that the speed of light is the same in all directions and that that is a postulate in SR. But just like every other experiment, it has its accuracy limits. I figure there must be some other experiment that shows they’re exactly the same or it might be that the universal speed limit is slightly faster than light speed. I don’t think the universal speed limit is less than light speed because that truly would mess up the theory of SR. So I guess I’m looking for the upper limit. To assume they’re exactly the same seems unnecessary. Such an assumption is not required by the theory of SR. All the formulas would stay the same except “C” would be slightly faster than light speed. In other words light speed would not be a constant but the universal speed limit would be that constant used in SR.