 ## Typed Letter Signed

EINSTEIN DISCUSSES ONE OF HIS GREATEST THEORIES: THE RELATIONSHIP BETWEEN MASS, ENERGY, AND LIGHT.

In 1905, while a young patent clerk and physicist in Bern, Switzerland, Albert Einstein obtained his doctorate and published a paper that explained his newly developed Special Theory of Relativity. This unlocked many mysteries of the universe, and introduced the world to the idea equating mass and the speed of light with energy, which we know today as e=mc2. Einstein's insight, and one element that distanced his work from that of Newton, was to recognize that mc2 was the proper energy of mass, the energy associated with that mass, and was independent of its motion. Mass must be measured in this way as resting mass. This was a foundation of e=mc2.

Part of Einstein's relativistic mass theory was that as an object gets faster and approaches the speed of light, in relation to its observer, its mass becomes infinite because the energy is associated with mass. Because an object in motion gains kinetic energy, and e/c2 = m, mass grows as the energy grows. In the case of a photon, which has a rest mass of 0, the energy is determined by Planck's constant, which states that a photon's energy is governed mainly by its frequency. Since there is an equivalence between mass and energy, and its mass remains at or near 0, then its energy must also be limited.

"Dear Sir, One should speak of mass only in the sense of something characteristic for the body and independent of its motion [rest mass]. Now it is true that the energy of a finite mass becomes infinite [in motion], provided the mass is finite. In the case of the photon the mass has to be assumed zero or infinitely small in such a way that in spite of its having the light velocity its energy is finite.”

Sincerely yours,

[Signed] A. Einstein.
Albert Einstein.

One page (one side) on Institute For Advanced Study letterhead, Princeton, January 5, 1954, to Dale B. Swanson. Usual folds, otherwise fine.

EINSTEIN LETTERS DISCUSSING HIS LEGENDARY MASS-ENERGY RELATIONSHIP ARE EXTREMELY RARE.

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