Hi, I have a really elementary question on the theory of special relativity.
Here is an excerpt from the website einstein-online.info that confused me:

The question is: How do we know that the light signal moves at the absolute speed? What proof is there that the person and the spaceship are seeing light travelling at the same speed and not a relative one? I'm really confused when I think of the cars and how they move at relative speeds

We only know that the roundtrip speed, or average speed, of light is constant, the one way speed of light depends on how you synchronize the two clocks, so there is no one correct answer. Of course in order to measure a speed you need some time and some distance.

How do we know the roundtrip speed is c, well we measured it many times and also with relative movement. Take a look at the article on Wikipedia on the speed of light here to get some info: http://en.wikipedia.org/wiki/Speed_of_light