An exceptionally rare letter written by Albert Einstein that includes the world’s most famous equation is up for auction.

The typed letter, dated April 21, 1946, showcases Einstein’s legendary scientific formula, energy equals mass times the speed of light squared, addressed to high school student Jack Monsted. In the letter, Einstein corrects Monsted’s understanding, stating: “In the formula E=mc², M is measured in gram, E in erg and c in cm/sec.”

Notably, the equals sign and comma are handwritten by Einstein in pencil, with his signature handwritten in pen.

Monsted, the original recipient, provided a letter of provenance explaining the context. He says that while studying physics during his senior year in high school he sent a letter asking Einstein for clarification on the units of measurements in the equation. Monsted’s letter also discussed the shrinking of a yardstick as it approaches the speed of light. Einstein’s response acknowledges the inaccuracies in Monsted’s information but indicates a lack of time for corrections.

The letter is also accompanied by the original mailing envelope addressed to Monsted’s home in New London, Wisconsin.

The estimated value of the letter, which is being auctioned by Boston-based RR Auction, is over $10,000 (£8,000) and after 35 bids is currently sitting at $28,204 (£22,000). The auction finishes on February 22 after which the “30-minute rule” will commence.

The auction house says letters from Einstein featuring his iconic equation are “exceptionally” rare. The sale of another letter written by Einstein featuring the equation and facilitated by RR Auction fetched $1.2 million at auction in May 2021.

Bobby Livingston, executive VP at RR Auction, told the JC: “There are very few examples in the world of Einstein bringing up his equation in writing. It was much more common for him to refer to the theory of relativity more broadly in correspondence, but the formula was for the chalkboard.

“This is only the second letter of Einstein’s we’ve seen with the E=mc2 equation in RR Auction’s 45-year history, and the first that is in English.”

Einstein, born in Germany to Jewish parents and the most well-known physicist in history, proposed the theory of relativity in 1905 with the accompanying equation, E=mc², which laid the foundation for modern physics. He explained the significance of the equation as following “from the special theory of relativity that mass and energy are but different manifestations of the same thing.” Einstein further clarified that the equation demonstrated how a small amount of mass could be converted into a large amount of energy and vice versa.

The letter is one of more than 50 historically significant items in RR Auction’s Remarkable Rarities series. Other items include a “Little Red Book” signed by Mao Zedong, an authentic cheque bearing the signature of George Washington, a pivotal letter by Karl Marx, a special edition of “The Hobbit” signed by J.R.R. Tolkien, and a wristwatch recovered from the ruins of Hiroshima, Japan. There is also never-before-seen footage capturing The Beatles behind the scenes while filming “Help!”.