Einstein letter with world's most famous equation up for auction The sale is highlighted by a rare letter in which Einstein writes his famous equation, "E = mc2," the only known example in private hands. BOSTON, MASS .-RR Auction is presenting the Einstein Archives of Ludwik Silbersteina remarkable collection of correspondence focused on the theory of relativity. The sale is highlighted by a rare letter in which Einstein writes his famous equation, "E = mc2," the only known example in private hands. Everyone has seen the famous equationit is part of the background of our lives. But h...