h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think it is a great theme but the article is not very informative. There are two types of relativity. Special and General.
There was no mention of proof, suggestion or mathematics.
All in al, it was a very good guide entry and I would rate it 3.5/5.

The speed of light is a 'constant' in the sense that it mediates the equivalency relationship between matter and energy, as well as many other important physical relationships.

It is not, though, a universal invariant and varies for example with mass density of the medium it travels through and the direction of the motion of that medium, a point alluded to by Einstein in his book on Special Relativity in the passage relating to the Fizeau effect.

One of the great advances of General Relativity was a proof that the speed of light varies with local gravitational flux densities as well, actually the point that won Einstein his second Nobel prize.

Just one of the many aspects of 'stretchiness' characteristic of Einstein's elastic universe.

Post 3 >< http://www.bbc.co.uk/dna/h2g2/brunel/F36067?thread=52573 Time, like distance, is a component of motion and hasn't any existence independent of motion.