Atomic Clock

Atomic Clock The mechanical clock served the needs of people for many centuries. Most people were satisfied with reckonings of time that gained or lost only a minute or two each day, and more accurate timepieces were available where greater precision was necessary, as with the operation of railroads or the determination of longitude at sea. In the course of the 20th century, however, the need for much greater accuracy arose. At the same time, recently discovered scientific principles made possible the building of clocks with almost unimaginable accuracy. In the 1920s, the rapidly expanding radio industry generated a need for the precise allocation of broadcast frequencies. Since frequency is by definition the number of cycles per second, the ability to broadcast on an exact frequency required an exact division of time. In the United States, the task of assigning radio frequencies was given to the National Bureau of Standards (NBS), an agency of the federal government's Department of Commerce. In response to the need for better time measurement, in the 1920s the NBS invented a clock based on a quartz crystal. When electrically stimulated, a quartz crystal vibrates at a fixed rate determined by its size and shape. This attribute allowed the construction of timepieces that were accurate to 1 second every 3 years. Quartz-crystal clocks were used to set radio frequency standards for 3 decades, but they suffered from a serious defect: Each clock ran at a slightly different rate. For most applications the differences were so small as to be irrelevant, but for others they were of crucial importance. Much more accurate clocks became possible in the late 1940s as a result of fundamental discoveries regarding the behavior of atoms. In particular, quantum physics had determined that atoms absorbed and emitted energy only at certain frequencies. In 1948, three physicists at the NBS—Harold Lyons, Benjamin Husten, and Emory Heberling—used this principle to build the first atomic clock. The clock used microwaves that were directed into ammonia gas. When the microwave frequency (about 24 billion hertz) was the same as the natural frequency of the hydrogen atoms in the ammonia, the atoms absorbed them. If the frequency was different, the microwaves hit a detector that triggered an electrical current. The current in turn adjusted the microwave frequency, which was then used to keep a crystal vibrating at a uniform rate. The clock was accurate to 1 second every 8 months. This was not as good as the best conventional quartz crystal clock, but there was ample room for further development. The next generation of atomic clocks was built around the element cesium. These clocks used vaporized cesium that had been magnetically divided so that one beam of atoms had identical energy states. When a microwave signal matched a cesium atom's natural frequency, it changed the atom's energy state. The atoms with a changed energy state were magnetically segregated and sent to a detector. As with the first atomic clock, the signal from the detector was used to stabilize the vibration frequency of a quartz crystal. In this way, it became possible to tell time that was accurate by 1 second every 300 hundred years. Development continued, and by 1970 the latest cesium clock operated with an accuracy of 1 second in 6,000 years. This made it more accurate than our natural timepiece, the Earth, which spins erratically due to axial wobbling caused by tides, the movement of the interior molten core, and even heavy snowfalls. Because atomic time slowly gets out of synch with Earth time, the Paris-based International Bureau of Time occasionally has to add or subtract a "leap second" to bring the two back together. Atomic clocks gain or lose only a few millionths of a second annually, yet the quest for even more accurate clocks continues. Increasingly accurate clocks are vital to a variety of scientific and technological ventures. Elementary-particle physicists are concerned with the detection of subatomic particles that may exist for a fraction of a nanosecond (a nanosecond is 10-9 seconds), while geologists study tiny movements of the Earth's tectonic plates in order to learn more about the cause of earthquakes. Television and radio broadcasting still require precise reckonings of time, especially as the airwaves get increasingly crowded and frequencies have to be kept within very narrow limits. Atomic clocks are also vital to the operation of sophisticated navigation technologies, where an error of a fraction of a second can throw an airplane several miles off course. References: 1.Margaret Coel, "Keeping Time by Atom," American Heritage of Invention and Technology, (1988)