The initial price for the letter stood at about $400,000.
The sold letter was written by Einstein to Polish American physicist Ludwik Silberstein, one of his critics, on October 26, 1946.
“Your question can be answered from the E=mc^2 formula, without any erudition,” the opening phrase of the letter says.
The formula defines the energy (E) as the product of mass (m) with the speed of light squared (c^2).
