Is There a Rigorous Proof That E_0 = mc^2?

[quZz]

Hello everyone
Can some one please provide (a link to) the most rigorous proof of $E_0 = mc^2$?
Actually, I got stuck on this question: why doesn't $E_0 = mc^2 + \rm{const}$? How to prove that $\rm{const} = 0$?

 

[DrStupid]

Can some one please provide (a link to) the most rigorous proof of $E_0 = mc^2$?

I can provide you with the original:

http://www.fourmilab.ch/etexts/einstein/E_mc2/e_mc2.pdf

 

[quZz]

Hi, thanks for a quick reply.
In this paper it only proves that if energy of a body at rest is changed by $dE$, its mass also changes by $dm=dE/c^2$. It doesn't prove, however, a much stronger statement that all its energy equals its mass times c squared...