FOUNDING LASER PHYSICS.

EINSTEIN, ALBERT.

Zur Quantentheorie der Strahlung.

Leipzig, S. Hirzel, 1917. Royal8vo. Bound in contemporary half calf with gilt lettering to spine and 5 raised bands with ornaments in gilt. In "Physikalische Zeitschrift", Bd. 18, 1917. Spine and hinges with wear, otherwise a fine and clean copy. Pp. 121-128. [Entire volume: XI, (1), 604 pp. + 14 plates.

The paper was first published in 1916 in Mitteilungen der Physikalischen Gesellschaft in Zürich, but here for the first time in Physikalische Zeitschrift. All subsequent research on absorption and emission of radiation and the entire discovery of the maser, later the laser, was based on the research presented in the present paper. The paper is also notable for introducing the concept (but not the name) of the photon; Einstein argues that in the interaction of matter and radiation there must be, in addition to the processes of absorption and spontaneous emission, a third process of stimulated emission. If stimulated emission exists then he can derive the Planck distribution for blackbody radiation and without it the same argument implies the invalid Wien-distribution theory.

"In this paper he derived Planck's original quantum law from a different starting point, he suggested that as well as spontaneous emission and absorption, there could also take place the process of stimulated emission. In 1917 this seemed mainly of theoretical interest; forty years later it was utilized to provide the maser and laser of modern technology.
In 1916, "Einstein came back once more to blackbody radiation and made further progress. In November 1916 he wrote to Besso: 'A splendid light has fallen on me about the absorption and emission of radiation'. His reasoning is divided into three papers, two of which appeared in 1916 and the third one early in 1917 [the two papers above - note that these are the two papers of Einstein on radiation theory cited by Weil as "principal works"; a third paper from 1916 is not.] In these papers, Einstein proposed a statistical theory of the interaction between atoms and photons, gave a new demonstration of Planck's radiation theory and introduced the concept of 'stimulated emission', providing the basis for the discovery of masers and lasers " (Bertolotti, The History of the Laser).

"When Einstein returned to the radiation problem in 1916, the quantum theory had undergone a major change. Niels Bohr's papers had opened a new and fertile domain for the application of quantum concepts-the explanation of atomic structure and atomic spectra. In addition Bohr's work and its generalizations by Arnold Sommerfeld and others constituted a fresh approach to the foundations of the quantum theory of matter. Einstein's new work showed the influence of these ideas . He had found still another derivation of Planck's black-body radiation law, an "astonishingly simple and general" one which, he thought, might
properly be called "the derivation" 12 of this important law. It was based on statistical assumptions about the processes of absorption and emission of radiation and on Bohr's basic quantum hypothesis that atomic systems have a discrete set of possible stationary states. The proof turned on the requirement that absorption and emission of radiation, both spontaneous and stimulated, suffice to keep a gas of atoms in thermodynamic equilibrium. (This paper introduced the concept of stimulated emission into the quantum theory and is therefore often described as the basis of laser physics.) Einstein himself considered the most important contribution of this work to be not the new derivation of the distribution law but rather the arguments he presented for the directional character of energy quanta. (DSB)

Weil No 91 (with an asterix denoting major paper).

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