Einstein letter with E=mc2 equation written in his own hand sells at auction for £850,000

An anonymous collector snapped up the letter, penned by Einstein to a physicist who had challenged some of his theories

A handwritten letter by Albert Einstein containing his famous E=mc2 equation has sold at auction in the US for more than $1.2m (£850,000) – three times more than was expected.

According to archivists at the Einstein Papers Project at Caltech in the US and the Hebrew University of Jerusalem, where Einstein’s papers reside, there are only three other known examples of the equation in the physicist’s own hand.

RR Auction, which sold the letter, says it was the only one in a private collection and became public only recently.

The one-page letter which is signed off “A Einstein” had been expected to sell for about $400,000 (£283,000).

The equation – energy equals mass times the speed of light squared – was first published in a scientific paper by Einstein in 1905.

It explains the interchangeability of energy and mass when a body moves at the speed of light.

Bobby Livingston, the executive vice-president at Boston-based RR Auction said: “[It’s] an important letter from both a holographic and a physics point of view,” adding that it is “the most well-known equation ever set forth”.

The letter which is in German is dated 26 October 1946.

It has Einstein’s blind-stamped personal Princeton letterhead and is addressed to Polish-American physicist Ludwik Silberstein.

Silberstein was a well-known critic and had challenged some of Einstein’s theories.

The letter was part of Silberstein’s personal archives which were sold by his descendants.

RR Auction only identified the buyer as an anonymous document collector.

Mr. Livingston said the rarity of the Einstein letter set off a bidding war.

Initially, five parties were bidding aggressively, but once the price hit about $700,000 (£495,000) there were only two parties left, he said.

The auction which began on 13 May concluded on Thursday 20 May.