 Regardless of whether Ohanian (2009, 2011) is correct that Einstein’s own derivations do not constitute a “proof” of mass-energy equivalence because they consider physical configurations that are too specific, a new concern arises when one considers “simplified” versions of derivations of \(E_o = mc^2\) such as the one we have just reviewed. Baierlein concludes his derivation by praising, as one of its merits, that it “requires only the ratio of momentum to energy for electromagnetic radiation” (Baierlein 1991, p. 172). We have already used this aspect of the derivation in our presentation above. 