Although the knowledge and use of lasers is now widespread, the laser is actually a light-emitting cousin to the maser, a device used to generate and amplify radio and light waves. The maser--an acronym for Microwave Amplification by the Stimulated Emission of Radiation --was originally designed to meet the need for an oscillator that could produce radiation shorter than one millimeter in wavelength. Previously, scientists had used microwave oscillators to study the basic structure of matter (the branch of physics known as microwave spectroscopy), but by the late 1940s it became increasingly clear that coherent oscillators at shorter wavelengths were essential.

Two concepts figured highly into the development of the maser: coherence and stimulated emission. Coherence describes a wave or particle whose vibratory pattern is "in step," the crests and troughs aligned perfectly. Stimulated emission was first studied by Albert Einstein (1879-1955) in relation to his theory of...