He could even take some of the emitted radiation and feed it back into the device to stimulate additional molecules, thereby creating an oscillator. This _feedback_ arrangement, he knew, could be carried out in a cavity, which would resonate (just like an organ pipe) at the proper frequency. The resonator would be a box whose dimensions were comparable with the wavelength of the radiation, that is, a few centimeters on a side. On the back of an envelope he figured out some of the basic requirements. Three years, and many experiments, later the maser (_m_icrowave _a_mplification by _s_timulated _e_mission of _r_adiation) was a reality. The original maser was a small metal box into which excited ammonia molecules were added. When microwaves were beamed into the excited ammonia the box emitted a pure, strong beam of high frequency microwaves, far more temporally coherent than any that had ever been achieved before. The output of an ammonia maser is stable to one part in 100 billion, making it an extremely accurate atomic “clock”.[12] Moreover, the amplifying properties of masers have been found to be very useful for magnifying faint radio signals from space, and for satellite communications. Ammonia gas was chosen for the first maser because molecules of ammonia have two individual energy states that are separated by a gap corresponding in frequency to 23,870 megacycles (23,870 million cycles) per second. Ammonia molecules also react to a nonuniform electric field in ways that depend on their energy level: low-level molecules can be attracted and high-level ones repelled by the same field. Thus it is possible to separate the low-energy molecules from the high, and to get the excited molecules into the cavity without too much trouble. This procedure for getting the majority of atoms or molecules in a container into a higher energy state, is called _population inversion_ and is basic to the operation of both masers and lasers. It should be noted that two Russians, N. G. Basov and A. M. Prokhorov, were working along similar lines independently of Townes. In 1952 they presented a paper at an All-Union (U.S.S.R.) Conference, in which they discussed the possibility of constructing a “molecular generator”, that is, a maser. Their proposal, first published in 1954, was in many respects similar to Townes’s. In 1955, Basov and Prokhorov discussed, in a short note, a new way to obtain the active atomic systems for a maser, a method that turned out to be of great importance. Thus on October 29, 1964, the Nobel Prize in Physics was awarded, not only to Townes, but to Basov and Prokhorov as well. The award was for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the “aser” principle. A LASER IS BORN