Exceptionally rare Albert Einstein letter in which he wrote down his famous 'E = mc²' formula has been sold for a whopping $1.2 million at auction There are only four examples of the famous equation and this is the only one in private collection It sold for more than three times what was expected It was a five-party bidding war, but once bids topped $700,000 just two bidders remained The winning bidder of the letter has preferred to remain anonymous The letter was written to fellow physicist Ludwik Silberstein in October 1946 E = mc², the mass–energy equivalence formula, arose from special relativity