only way to make measurements this accurate was to control a clock by the infinitesimal oscillations of the atom itself.
According to the laws of quantum physics, atoms absorb or emit electromagnetic energy in discrete amounts that correspond to the differences in energy between the different electronic configurations of the atoms, i.e., different configurations of the electrons surrounding their nuclei. When an atom undergoes a transition from one such “energy state” to a lower one—it emits an electromagnetic wave of a discrete characteristic frequency, known as the resonant frequency. These resonant frequencies are identical for every atom of a given type—cesium 133 atoms, for example, all have a resonant frequency of exactly 9,192,631,770 cycles per second. For this reason, the cesium atom can be used as a metronome with which to keep extraordinarily precise time.
The first substantial progress toward developing clocks based on such an atomic timekeeper was achieved in the 1930s at a Columbia University laboratory in which I.I. Rabi and his students studied the fundamental properties of atoms and nuclei. In the course of his research, Rabi invented the technique known as magnetic resonance, by which he could measure the natural resonant frequencies of atoms. Rabi won the 1944 Nobel Prize for his work. It was in that year that he first suggested—“tossed off the idea,” as his students put it—that the precision of these resonances are so great that they could be used to make a clock of extraordinary accuracy. In particular, he proposed using the frequencies of what are known as “hyperfine transitions” of the atoms—transitions between two states of slightly different energy corresponding to different magnetic interactions between the nucleus of an atom and its electrons.
In such a clock, a beam of atoms in one particular hyperfine state passes through an oscillating electromagnetic field. The closer the oscillation frequency of that field to the frequency of the hyperfine transition of the atom, the more atoms absorb energy from the field and thereby undergo a transition from the original hyperfine state to another one. A feedback loop adjusts the frequency of the oscillating field until virtually all the atoms make the transition. An atomic clock uses the frequency of the oscillating field—now perfectly in step with the precise resonant frequency of the atoms—as a metronome to generate time pulses.
Rabi himself never pursued the development of such a clock, but other researchers went on to improve on the idea and perfect the technology. In 1949, for instance, research by Rabi's student Norman Ramsey suggested that making the atoms pass twice through the oscillating electromagnetic field could result in a much more accurate clock. In 1989 Ramsey was awarded the Nobel Prize for his work.
After the war, the U.S. National Bureau of Standards and the British National Physical Laboratory both set out to create atomic-time standards based on the atomic-resonance work of Rabi and his students. The first atomic clock was established at the National Physical Laboratory by Louis
Essen and John V.L. Parry, but this clock required a roomful of equipment. Another of Rabi's former associates, Jerrold Zacharias of MIT, managed to turn the atomic clocks into practical devices. Zacharias had plans for building what he called an atomic fountain, a visionary type of atomic clock that would be accurate enough to study the effect of gravity on time that had been predicted by Einstein. In the process, he developed an atomic clock small enough to be wheeled from one laboratory to another. In 1954, Zacharias joined with the National Company in Malden, Massachusetts, to build a commercial atomic clock based on his portable device. The company produced the Atomichron, the first commercial atomic clock, 2 years later and sold 50 within 4 years. The cesium atomic clocks used in GPS today are all descendants of the Atomichron.
By 1967, research in atomic clocks had proved so fruitful that the second had been redefined in terms of an atomic standard relative to the 9 billion-plus oscillations of a cesium atom per second. Today 's atomic clocks are typically accurate to within 1 second in 100,000 years.
Physicists have continued to experiment with novel variations on the atomic-resonance ideas of Rabi and his students and to put them to work in atomic clocks. Rather than using magnets, one technique makes use of a phenomenon known as optical pumping to select out the energy levels of the atoms that will do the timekeeping and employs a beam of light to force all the atoms in the beam into the desired state. This work led to a Nobel Prize for Alfred Kastler of the École Normal Supérieure in Paris. Today, many atomic clocks use optically pumped rubidium atoms instead of cesium. The rubidium clocks are considerably less expensive and smaller than cesium clocks, but they are not quite as accurate.
Another type of atomic clock is known as the hydrogen maser. Masers originated in research on the structure of molecules by Charles Townes and his colleagues at Columbia University in 1954, work for which Townes shared the 1964 Nobel Prize in physics. The maser, which is the precursor of the laser, is a microwave device that generates its signal by direct emission of radiation from atoms or molecules. While Townes's original maser used ammonia, Ramsey and his colleagues at Harvard developed a maser in 1960 that operates with hydrogen and serves as an atomic clock of extreme precision.
By 1967, research in atomic clocks had proved so fruitful that the second was redefined in terms of the oscillations of a cesium atom. Today's atomic clocks are typically accurate to within 1 second in 100,000 years. Our nation's primary time standard is the recently inaugurated atomic clock at the National Institute of Standards and Technology, called NIST-7. Its estimated accuracy is to within 1 second in 3 million years.
Over the years, all three clocks—the cesium-beam clock, the hydrogen-maser clock, and the rubidium clock—have seen service in space, either in satellites or in ground control systems. GPS satellites ultimately rely on cesium clocks that resemble those conceptualized by Rabi 60 years ago.
In 1993, 2 decades after it was conceived in the Pentagon, GPS became fully functional with the launching of its 24th satellite. The satellites are operated by the U.S. Air Force, which monitors them from five ground stations around the world. The data gathered are analyzed at the Air Force Consolidated Space Operations Center in Colorado, which transmits daily updates to each satellite, correcting their clocks and their orbital data.
GPS and the Future
It is often forgotten that GPS is still a military device built by the Department of Defense at a cost of $12 billion and intended primarily for military use. That fact has led to one of the few controversies surrounding the remarkably successful system. As with any new technology, progress brings risk, and GPS potentially could be used to aid smugglers, terrorists, or hostile forces. The Pentagon made the GPS system available for commercial use only after being pressured by the companies that built the equipment and saw the enormous potential market for it. As a compromise, however, the Pentagon initiated a policy known as selective availability, whereby the most accurate signals broadcast by GPS satellites would be reserved strictly for military and other authorized users. GPS satellites now broadcast two signals: a civilian signal that is accurate to within 100 feet and a second signal that only the military can decode that is accurate to within 60 feet. The Pentagon has also reserved the