The laser and its parent, the maser, can be traced back half a century to its theoretical beginnings. The great physicist Albert Einstein is most widely known for his work in relativity. But he did early and important work on that other gigantic 20th century scientific achievement, the quantum theory.[10] In one of his papers, published first in Zurich, Switzerland, in 1916, Einstein showed that controlled emission of light energy could be obtained from an atom under certain conditions. When an atom or molecule has somehow had its energy level raised, the release of this stored energy could be stimulated by subjecting the atom or molecule to a small “shot” of electromagnetic radiation of the proper frequency. Einstein wrote that when such a photon of energy caused an electron to drop from a higher to a lower orbit, the electron would emit another photon of the same frequency and in the same direction as the one that hit it.[11] In other words, the energy of the emitted photon would be added to that of the photon that stimulated the emission in the first place. Here, _potentially_, was light amplification. The three major factors, absorption of energy, spontaneous emission, and stimulated emission are diagrammed in Figure 14. There the matter lay for more than 30 years. In 1951 Charles H. Townes, then on the Columbia University faculty, was interested in ways of extending to still higher frequencies the range of microwaves available for use in communications and in other scientific applications. Townes and other scientists who were interested in the problem were to meet in Washington, D. C., on the 26th of April. The night before the meeting he slept in a small Washington hotel; but he awoke at 5:30—pondering, pondering the high frequency problem. He dressed and took a walk, then sat on a park bench and savored the beauty of azaleas in bloom. But all the while his mind was running over the various aspects of the problem. [Illustration: Figure 14 _An atom can release absorbed energy spontaneously or it can be stimulated to do so._] Before After Excited state —–— —•— Absorption ~~→ Relaxed state —•— —–— Excited state —•— —–— Spontaneous emission Relaxed state —–— —•— ~~→ Excited state —•— —–— Stimulated emission ~~→ Relaxed state —–— —•— ~~→ ~~→ Suddenly the answer came to him. Normally more of the molecules in any substance are in low-energy states than in high ones. He would change the natural balance and create a situation with an abnormally large number of high-energy molecules. Then he would stimulate them to emit their energy by nudging them with microwaves. Here was amplification.