Light is ubiquitous and is something many people take for granted. But it has the paradoxical quality of behaving in two forms--as waves and as streams of particles known as photons--which can make harnessing it for technological purposes a challenge. Until the development of lasers, scientists relied on 19th-century theories to describe how excited atoms and molecules emit photons of light. But the descriptions were inadequate and could not explain, for example, the fundamental differences between light radiating from a lightbulb (which contains a mixture of light wave frequencies and phases) and light beaming from a laser (which has a specific frequency and phase).
In 1963 Glauber published two breakthrough papers on the topic. He showed how conventional theories were inadequate and used the emerging field of quantum optics to reveal the difference between lightbulbs and lasers. Decades later, Hall and Hnsch, working separately, used Glauber's theories to build a high-precision device called an optical frequency comb, which can measure the color emitted or absorbed by atoms and molecules. Cosmologists use the technology to determine the strength of the interaction between light and matter. Changes in this so-called "fine-structure constant" could redefine the latter. And although that might not seem like much, recognizing such changes could help improve navigation signals in communication technologies that are becoming as pervasive as light itself.