The atomic clock is a timekeeping device of unparalleled precision that uses the vibration frequencies within certain atoms and molecules. The time kept by atomic clocks is now accepted as the international standard. American physicist William Frank Libby (1908-1980) formulated the atomic clock theory in 1946. He envisioned a clock whose timing was controlled by an oscillator, but one whose frequency did not drift. This was accomplished by comparing the oscillator's frequency to that of an electron moving from one energy level to another. Since the electron's frequency is unfaltering for any particular element, Libby's clock was very accurate--the model built at the National Bureau of Measures in Washington, D.C., was precise to about 21 picoseconds per year. The name most commonly associated with the invention of the atomic clock is that of Charles Hard Townes. The research Townes was conducting on microwave oscillation closely paralleled that of Libby. Townes, however, took the invention a step further: by using an ammonia molecule--the electrons of which vibrate within the microwave range--he constructed an atomic clock whose timing deviated by less than one second for every 30,000 years of operation--far more accurate than Libby's device. The work Townes did on the atomic clock led directly to his invention of the maser and, eventually, the laser. The atomic clock most often found in laboratories was designed by Norman F. Ramsey, Jr.
(1915-) of Harvard University and is driven by the atomic frequency of the element cesium's electrons. There are two basic types of atomic clocks: active and passive. Active clocks generate an oscillation signal directly from their atoms using the process called stimulated emission. Passive clocks use atoms that have been irradiated with electromagnetic energy to provide a signal. In addition to the cesium and ammonia varieties, clocks using rubidium, hydrogen, thallium, or ions of mercury, barium, and magnesium have been developed. Clocks that produce optical and infrared oscillation frequencies are being developed. In a world of speed-of-light communications and hyperdistant exploration, the need for a device that accurately and reliably measures very small increments of time is great. For example, atomic clocks are often used in navigation systems: the distance and position of a vessel can be precisely defined by measuring the difference between the time it takes for two signals to reach it; this difference is usually very small, and so a very accurate timing device is essential. The ability to measure small deviations in time has made the atomic clock a useful tool for testing Albert Einstein's (1879-1955) theory of relativity, which claims that time is slowed at speeds approaching that of light. Since manmade technology can achieve only a fraction of the speed of light, atomic clocks are vital to recording these very small shifts in time. Atomic clocks are also valued for their extremely stable frequencies, which are used in communicating with deep-space probes. The most accurate atomic clock yet constructed was put into operation in April, 1993, at the National Institute of Standards and Technology in Boulder, Colorado. It is designed neither to gain nor lose a second in the next one million years. As of the late 1990s, research was under way to shift the standard of timekeeping from atomic clocks to devices that measure time by millisecond pulsars, the extremely fast speed of neutron stars.
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