http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ says the original, translated from German, was the text "I

http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ says the original, translated from German, was the text "If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²", that is, m = L / c². L was used for energy at that time, instead of E. There is no square root. If you include the momentum term it's E²/c² - p² = m²c², which solving for E gives a square root, leading to http://en.wikipedia.org/wiki/Dirac_equation http://en.wikipedia.org/wiki/Dirac_equation .