Its secret was light—a very special kind of radiance produced by devices called lasers and channeled along threads of ultrapure glass called optical fibers. Today, millions of miles of the hair-thin strands stretch across continents and beneath oceans, knitting the world together with digital streams of voice, video, and computer data, all encoded in laser light.

When the basic ideas behind lasers occurred to Columbia University physicist Charles Townes in 1951, he wasn't thinking about communications, much less the many other roles the devices would someday play in such fields as manufacturing, health care, consumer electronics, merchandising, and construction. He wasn't even thinking about light. Townes was an expert in spectroscopy—the study of matter's interactions with electromagnetic energy—and what he wanted was a way to generate extremely short-wavelength radio waves or long-wavelength infrared waves that could be used to probe the structure and behavior of molecules. No existing instrument was suitable for the job, but early one spring morning as he sat on a park bench wrestling with the problem, he suddenly recognized that molecules themselves might be enlisted as a source.

All atoms and molecules exist only at certain characteristic energy levels. When an atom or molecule shifts from one level to another, its electrons emit or absorb photons—packets of electromagnetic energy with a tell-tale wavelength (or frequency) that may range from very long radio waves to ultrashort gamma rays, depending on the size of the energy shift. Normally the leaps up and down the energy ladder don't yield a surplus of photons, but Townes saw possibilities in a distinctive type of emission described by Albert Einstein back in 1917.

If an atom or molecule in a high-energy state is "stimulated" by an impinging photon of exactly the right wavelength, Einstein noted, it will create an identical twin—a second photon that perfectly matches the triggering photon in wavelength, in the alignment of wave crests and troughs, and in the direction of travel. Normally, there are more molecules in lower-energy states than in higher ones, and the lower-energy molecules absorb photons, thus limiting the radiation intensity. Townes surmised that under the right conditions the situation might be reversed, allowing the twinning to create amplification on a grand scale. The trick would be to pump energy into a substance from the outside to create a general state of excitement, then keep the self-duplicating photons bouncing back and forth in a confined space to maximize their numbers.