In this undated photograph, provided by Boston-based RR Auction, shows a letter written by Albert Einstein, in which he wrote out his famous “E = mc2” equation, that sold at auction for more than $1.2 million. Archivists at the Einstein Papers Project at the California Institute of Technology and the Hebrew University of Jerusalem say there are only three other known examples of Einstein writing the world-changing equation in his own hand. (Nikki Brickett/RR Auction via AP)