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## Main Question or Discussion Point

While searching the net I've come across numerous derivations for the equation of [tex]E=mc^2[/tex], the most understandable of which I found to be the photon-in-the-box momentum derivation. However, this derivation and other ones don't necessarily seem to be based on Special Relativity. So my question is this: is [tex]E=mc^2[/tex] a relativistic concept per se?