Pulsar Pumped

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Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 15
DOI: 10.1126/science.309.5731.15b

Stimulated emission, in which a photon interacts with an excited molecule and causes a second identical photon to be emitted, forms the basis for coherent light generation and amplification in lasers. The same effect was discovered in interstellar molecular clouds in the 1960s in the form of unusually bright and narrow microwave spectral lines. Weisberg et al. (p. 106; see the Perspective by Elitzur) report their observation of stimulated microwave emission in an OH cloud caused by photons from a distant pulsar. These results not only yield insights into maser action in interstellar clouds, but also into molecular-cloud density and distribution.

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