Can the concept of c be validly expressed as "the rate at which an event propagates through space"?
There was a television program last year featuring Prof Brian Cox. The presenter asked him "Is it true that when we look at this particular star, we are seeing it as it was 18 million years ago?" And Professor Cox replied "Or 'now'." This was a turning point in my comprehension of relativity and I want to make sure I understood it correctly. Was his point that with simultaneity and time not being absolute, the idea of "now" is in effect meaningless?
This method of expression makes sense to me in that it explains length contraction and thought experiments such as the barn / ladder paradox: from the POV of the front of the barn, the event of the rear of the ladder crossing the threshold of the rear barn door hasn't reached it yet, while from the rear of the barn the same is true for the front of the ladder.
Is this a valid analogy, or is it over-simplistic and will lead to failure of comprehension elsewhere?