I think the answer we want now is a non-mathematical one. It gets tiresome how most of the &quot;answers&quot; are just too afraid( or maybe naive?) to just say &quot;we don&#x27;t know&quot; why the speed of light is the constant of proportionality in the mass-energy conversion equation.<p>You can answer the &quot;how&quot; very well which is what the mathematical derivation is, and which is what most people are answering in this thread but you see that none of those answers start with &quot;Because...&quot; but rather just keep on spitting out more math. And what we want is an explanation for why c^2 is there and what it means.<p>We don&#x27;t know what it means, we haven&#x27;t answered that question and we don&#x27;t know how to answer it. It&#x27;s similar to how pi keeps coming up in a lot of equations and we struggle to understand why.