The Maser: First Step to the Laser
Already in the 1930s scientists could have built a laser. They had the optical techniques and theoretical knowledge — but nothing pushed these together. The push came around 1950 from an unexpected direction. Short-wavelength radio waves, called microwaves, could make a cluster of atoms vibrate in revealing ways (a technique called microwave spectroscopy). Radar equipment left over from World War II was reworked to provide the radiation. Many of the world’s top physicists were thinking about ways to study systems of molecules by bathing them with radiation.
Charles Townes of Columbia University had studied molecules as a physicist in the 1930s, and during the war he had worked on radar as an electronics engineer. The Office of Naval Research pressed him and other physicists to put their heads together and invent a way to make powerful beams of radiation at ever shorter wavelengths. In 1951 he found a solution. Under the right conditions — say, inside a resonating cavity like the ones used to generate radar waves — the right kind of collection of molecules might generate radiation all on its own. He was applying an engineer’s insights to a physicist’s atomic systems. Townes gave the problem to Herbert Zeiger, a postdoctoral student, and James P. Gordon, a graduate student. By 1954 they had the device working. Townes called it a MASER, for "Microwave Amplification by Stimulated Emission of Radiation".
Townes had predicted a remarkable and useful property for the radiation from the device: it would be at a single frequency, as pure as a note from a tuning fork. And so it was. The high degree of order in such radiation would give the maser, and later the laser, important practical uses.
Townes was not alone in his line of thought. Joseph Weber of the University of Maryland expressed similar ideas independently in 1952. And Robert H. Dicke of Princeton worked toward the same goal along a different path. Neither tried to build a device. In Moscow, A.M. Prokhorov and N.G. Basov were thinking in the same direction, and they built a maser in 1955.
How do masers and lasers work? For an explanation of the science see this page from the University of Colorado.