The research that would eventually give rise to the laser had its origins in the branch of physics now known as quantum mechanics. In 1900 Max Planck hypothesized that excited atoms radiate energy in discrete packets, which he called quanta, and not as a continuous range of energies, as the prevailing wave theory of electromagnetic radiation would have it. Planck never pursued the implications of this notion, but 5 years later Albert Einstein did, suggesting that light itself was made up not of waves, but of packets of energy (later named photons); the higher the frequency of the light, the more energetic the photon. He then demonstrated how under some conditions, electrons could absorb and emit the energy of photons, and--in a breakthrough that would earn him the Nobel Prize--he used this demonstration to explain what was called the photoelectric effect (the discharge of electrons from matter by the impact of radiation, especially visible light).

Meanwhile, not everyone agreed with Einstein's theory of light-as-particle; the debate would continue for a couple of decades. But even before physicists accepted that light was somehow both wave and particle, Einstein discovered yet another phenomenon. According to Niels Bohr's model of the atom, set forth in a series of papers in 1913, electrons occupy specific orbits around the nucleus, determined by the electrons' energy levels. An electron can absorb only the exact amount of energy needed to kick it from one orbit up to a specific higher one, and it emits a specific amount of energy on dropping from an orbit to a lower one. That explained why atoms of a given gas, such as neon, emit a distinctive pattern of wavelengths, and why vapor discharge lamps such as those based on mercury or sodium have a characteristic color.

Atoms that are in an excited state--that is, when their electrons are in higher-energy orbits--will eventually, and spontaneously, fall back to their lowest, or ground state, giving off stored energy in the process. In a given system of atoms, this spontaneous emission occurs at random, with the emitted photons of energy heading off in random directions. Einstein recognized that if atoms in an excited state encounter photons of light with the right amount of energy (namely, an amount equal to the difference between the lower-energy and higher-energy states), the encounter can trigger a kind of chain reaction of emission that boosts the intensity of the light passing through--as though the electrons, greedy to capture the incoming photons, dropped the ones they already had stored up. Furthermore, the emitted photons all headed in the same direction as the incoming photons. The process is called "stimulated emission."

The catch was that amplification by stimulated emission would occur only if more atoms in the population of atoms were in an excited state than in the lower-energy state. That is the opposite of the normal situation, so stimulated emission required what is known as a population inversion--an entire population of atoms had to be artificially boosted into an excited state, usually by exposure to light.

Fast-forward now to 1951. Charles Townes was head of the Columbia University Radiation Laboratory, which was continuing research begun during World War II on microwave physics. Townes was working on microwave spectroscopy and was eager to use short wavelengths in the submillimeter range. But he needed to scale down the mechanical oscillators then being used to generate microwaves in the centimeter range, a problem that seemed insoluble--until he thought of using molecules.

During the next 2 years, Townes worked with James Gordon and Herbert Zeiger to build such a system. Finally, in late 1953, they demonstrated the results of their research. They sent a beam of ammonia through an electric field that deflected molecules that were in a low-energy state and sent high-energy molecules to another electric field; exposure to the second field caused all the high-energy ammonia molecules to drop almost simultaneously to the ground state, emitting microwave photons that were all at the same frequency and traveling in the same direction. Townes called his device a maser, for microwave amplification by stimulated emission of radiation. As Townes continued to experiment with masers, it became clear that stimulated emission could work with the much shorter wavelengths of infrared and even of visible light. The name "laser" was coined for the device, with the "l" standing for "light." Seeking to develop a more complete theory of laser action, Townes approached his brother-in-law, Arthur Schawlow, a physicist at Bell Laboratories, one of the nation's leading centers for research in physics and materials.

In late 1958, the Townes-Schawlow paper, "Infrared and Optical Masers," appeared in Physical Review, a leading physics journal. The paper inspired scientists to try to build a laser device, and in June 1960, physicist Theodore Maiman, at the research laboratory of Hughes Aircraft Company succeeded, using a synthetic ruby.

Lasers, which emit much more highly focused light beams than other light sources, attracted immediate interest. In one experiment performed in 1962, a laser beam 1 foot in diameter was aimed at the moon, 240,000 miles away, where it illuminated a surface area only two miles in diameter. A beam of ordinary light would spread so much in traveling the same distance that it would illuminate an area 25,000 miles in diameter. Journalists took to the new technology enthusiastically, writing of "light fantastic" and hailing lasers as harbingers of a new age. Film-makers featured lasers as weapons of doom--most notably in the James Bond movie Goldfinger. Scientists pointed to the enormous promise of lasers in communication and other fields.

In reality, early lasers had a long way to go to meet those expectations. Creating the population inversion necessary to generate laser action required so-called optical pumps, such as flash lamps, and these could only produce a pulse of energy rather than continuous laser light and were not efficient in the use of power. Another quite different version, developed later in 1960 by Ali Javan at Bell Laboratories, used a glass tube containing a mixture of the gases helium and neon. This laser had a lower energy threshold and did not overheat, but the glass tube was both bulky and fragile. The first lasers resembled the vacuum tubes that had earlier been used in radios, television sets, and the first computers. By 1960, vacuum tubes had given way to the amazingly small and highly reliable transistor. Could lasers make the same transition?