I will give you the short form answer

These reactions have to be relativistically invariant otherwise they would vary ... the chances of two unrelated things sharing the EXACT same relationship to light is incredibly unlikely.

So again we see it is almost requirement for gravity mass and inertia mass to co-exist with exactly the same relationship to light we require relativity.

So I stayed with almost there because you can't exclude that there was some incredible fluke and the two happen to be the same weird value but lets just say it is unlikely.

Edited by Orac (11/05/13 03:32 PM)
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.