Re: Why is the speed of light considered the highest speed ?

Your first statement is "Because it was a supposition and unproven...".  
Actually, there are MANY experimental confirmations that the theory of 
relativity is a good and correct theory, and, therefore, we can take the 
speed light, c, as the fastest possible speed.

Another of your statements is "Because of e=mc^2, light has mass."  
Unfortunately you have misapplied the equation.  It is intended to give the 
equivalence of energy to mass, but not to assign mass.  It has been shown 
very well that photons do NOT have mass.

Another of your statements is "Light will slow down when entering an 
optically dense medium."  Actually this is not quite true.  While it is 
true that we measure a lower AVERAGE speed of the light through a medium, 
the propagation of light through the medium, between atoms, is actually at 
the normal vacuum speed of light, c.  What happens is that the light moves 
at speed c between atoms, but photons are "absorbed" by the medium's atoms.  
By "absorption" I mean that the energy of the photon causes an electron of 
the atom to be kicked to a higher energy level, and the photon ceases to 
exist.  Then, after a very small time delay, the electron goes back to its 
original (usually ground state) energy and "emits" a photon of the same 
energy (and thus same frequency and thus same wavelength) as the original 
"absorbed" photon.  (In fluorescent materials the energy of the photon is 
downshifted, but I am talking here of "elastic", or non-energy shifting, 
absorptions.)  It is this very small time delay which makes us measure the 
average "speed of light" through the medium as slower than the vacuum speed 
of light.  But, again, between atoms the light does travel at the speed c.

Please look at the archives of the MadSci Network, searching on 
"relativity".  You will find many helpful questions and answers that are 
relevant to your question.