A handwritten letter by Albert Einstein containing the famous E=mc2 equation has been sold at an auction for $1.2 billion. RR Auction, which sold the most recent letter, identified the buyer as an anonymous document collector.

The letter was sold at about 3 times more than the expected bid, which was $400,000. According to archivists at the Einstein Papers Project at the California Institute of Technology and the Hebrew University of Jerusalem, there are only 3 other known documents consisting of the world-changing equation in Einstein’s own handwriting. RR Auction states that this letter is the only one in a private collection and only became public recently. The letter was in possession of Silberstein, a well-known critic, and challenger to some of Einstein’s theories. It was written as a response to Silberstein, stating “Your question can be answered from the E=mc2 formula, without any erudition,” according to a translation provided by RR Auction. The letter was sold by Silberstein’s descendants.

At first, there were 5 bidders, due to the rarity of the letter, but once the price reached around $700,000, only 2 bidders continued.