BOSTON, MASS.- A rare letter in which Albert Einstein writes out his famous equation, "E = mc2," the only known example in private hands, sold for $1,243,707, according to Boston-based RR Auction.

In German, the one-page letter was written by Einstein to Polish-American physicist Ludwik Silberstein, dated October 26, 1946. In full (translated): "Your question can be answered from the E = mc2 formula, without any erudition. If E is the energy of your system consisting of the two masses, E0 the energy of the masses when they approach infinite distance, then the system's mass defect is E0 - E / c2.

In your case, (E0 - E)pot = k m2/r. On account of the kinetic energy, however, the total energy deficit is only half as large, in accordance with the virial theorem. Therefore, if is the mass of the total system, 2m - M = 1/2 k/c2 m2/r on the first approximation, that is, if the influence of the finiteness of the radius of the masses is ignored. I am convinced that this (or a formula corrected with respect to the radius of the masses) cannot help in shedding light on atomic constants. For that one must first have a theory that contains the correct unification of gravitation and electricity." Silberstein notes the dates of receipt and reply at the top, also writing "E = mc2."

According to archivists at the Einstein Papers Project at Caltech, and the Hebrew University of Jerusalem, only three holograph examples are known—and none of those are in private hands. This example—the fourth—was revealed to the public for the first time.

"It's an important letter from both a holographic and a physics point of view," said Bobby Livingston, Executive VP at RR Auction. "Einstein says to use the formula "without any erudition," and he's correct. The world knows E = mc2 as the most famous equation in the world."

Additional highlights from the sale include, but are not limited by:

• Albert Einstein letter with Gravitational Field Equations sold for $152,920.

• Albert Einstein typed letter with handwritten equations on Spacetime Metric sold for $66,898.

Other featured items included letters by Max Planck—often defending Einstein's theory of relativity —and other Nobel Prize winners, including Thomson, Bragg, Michelson, and Lorentz.

The Einstein Archives of Ludwik Silberstein auction by RR Auction began May 13 and concluded May 20.