Back on my hobby horse.

Originally Posted By: Paul
because we know that the speed of light is not constant.


That's right. The speed of light varies according to the medium through which it is traveling. But C is a constant. C is not the speed of light. It just happens that light travels at a speed equal to C when it is in a vacuum.

C is the universal, and positively enforced, speed limit. Nothing is allowed to go faster than C under any conditions.

Bill Gill

C is not the speed of light in a vacuum.
C is the universal speed limit.