We've answered the photon question several times before. Use our search for 'photon mass gravity'.
The pauli exclusion principle is exactly the same as the statement that no two fermions can be in exactly the same quantum state. It was first found as a simple observation. It was later understood via the 'spin-statistics theorem' to be a necessary consequence of Special Relativity combined with quantum theory. I think that this site is not the right place to try to reproduce that argument, but you can easily find it in print or online.
I'm not sure quite what you mean by 'significance'. If there were no exclusion principle everything would collapse. I think it's significant that that doesn't happen.
Mike W.
(published on 09/04/2008)
