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Atomic Clock

Atomic clocks are the world's most accurate time keepers—more accurate than astronomical time or quartz clocks. Originally, a second was defined as 1/86,400 of a mean solar day. Today it is defined as 9,192,631,770 periods or wavelengths of the radiation absorbed by the cesium-133 atom as it changes between two hyperfine energy levels. The change in definition was the result of the atomic clocks' ability to accurately measure these very short periods. Atomic clocks do not resemble ordinary clocks or watches. The atoms serve the same purpose as did pendulums or quartz crystals in earlier clocks. They have no "hands" to turn or liquid crystal displays for number read-outs. They simply produce electrical pulses that serve as a standard for calibrating other less accurate clocks.

In 1945, Isidor Rabi, a physics professor at Columbia University, first suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. Using Rabi's technique, the Commerce Department's National Institute of Standards and Technology (NIST) (then the National Bureau of Standards), developed the world's first atomic clock in 1949 using the ammonia molecule as the source of vibrations. In 1952, NIST revised its design using cesium atoms as the vibration source. This clock was named NBS-1.

Like all atoms, the cesium atoms used in an atomic clock are quantized; that is, they can absorb or give up only discrete quantities of energy. It is the quantum nature of the atom that is the underlying principle of atomic clocks. An atom of cesium can exist at a minimum energy level of, for example, E1, which is called its ground Repairs completed to an atomic clock. This clock is timed to the resonance frequency of cesium. Royal Greenwhich Observatory, National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. state. It may absorb a certain amount of energy and reach a somewhat greater energy level—E2, E3, E4, and so on. Thus, an atom in its ground state can accept a quantity of energy equal to E2 − E1, E3 − E1, E4 − E1, and so on, but it cannot exist at an energy level that lies between these values. It cannot, for example, absorb a quantity of energy equal to 1/2(E2 − E1). Once an atom is at an energy level greater than its ground state, it can only release energy quantities equal to the difference in energy levels: E2 − E1, E3 − E1, E4 − E1, E4 − E3, E3 − E2, and so on. When energy is emitted by an atom, it is in the form of electromagnetic radiation, such as light. Only radiation with frequencies between 4.3 × 1014 and 7.5 × 1014 Hz can be seen because those frequencies mark the ends of the range visible to the human eye. The greater the frequency of the radiation, the greater its energy. In fact, the energy, E, of the radiation is given by the equation E = hf, where f is the frequency and h is Planck's constant (6.626 × 10-34 J•s).

The energy of the radiation absorbed by the cesium atoms used in most atomic clocks is very small. It is absorbed (or released) when cesium atoms pass between two so-called hyperfine energy levels. These energy levels, which are very close together, are the result of magnetic forces that arise because of the spin of the atom's nucleus and the electrons that surround it. The frequency of the radiation absorbed or released as atoms oscillate between two hyperfine energy states can be used as a standard for time. Such frequencies make ideal standards because they are very stable—they are not affected by temperature, air pressure, light, or other common factors that often affect ordinary chemical reactions.

On December 30, 1999, NIST started a new atomic clock, the NIST F-1, which is estimated to neither gain nor lose a second for 20 million years. Located at NIST's Boulder, Colorado, laboratories, the NIST F-1, currently the United States's primary frequency standard, shares the distinction of being the world's most accurate clock with a similar device in Paris.

The NIST F-1 uses a fountain-like movement of atoms to keep time. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. The lasers gently push the cesium atoms together into a ball, thus slowing the movement of the atoms and cooling them to near absolute zero. The fountain action is created when two vertical lasers gently toss the ball upward. This little push is just enough to propel the ball through a microwave-filled cavity for about one meter. Gravity pulls the ball downward again toward the lasers.

While inside the cavity, the atoms interact with the microwave signal and their atomic states are altered in relation to the frequency of the signal. The entire round trip takes about a second. When finished, another laser, directed at the ball, causes the altered cesium atoms to emit light, or fluoresce. Photons emitted during fluorescence are measured by a detector. This procedure is repeated many times while the microwave energy in the cavity is tuned to different frequencies. Eventually, a microwave frequency is achieved that alters the states of most of the cesium atoms and maximizes their fluorescence. This frequency is the natural resonance frequency for the cesium atom; the characteristic that defines the second and, in turn, makes ultraprecise timekeeping possible.

The NIST F-1's predecessor, the NIST-7, as well as many versions before it, fired heated cesium atoms horizontally through a microwave cavity at a high speed. NIST F-1's cooler and slower atoms allow more time for the microwaves to "interrogate" the atoms and determine their characteristic frequency, thus providing a more sharply defined signal. NIST F-1, along with an international pool of atomic clocks, define the official world time called Coordinated Universal Time.

Atomic clocks have been used on jet planes and satellites to verify Einstein's theory of relativity, which states that time slows down as the velocity of one object relative to another increases. Until the advent of atomic clocks that can measure time to within one second in a million years, there was no direct way of accurately measuring the time dilation predicted by Einstein, even at the velocities of space probes. Although these space vehicles reach speeds of 25,000 MPH (40,000 km/h), such a speed is only 0.004% of the speed of light, and it is only at velocities close to the speed of light that time dilation becomes significant. Atomic clocks on satellites are used in navigation. The signals sent by the atomic clocks in satellites travel at the speed of light (186,000 mi/s or 300,000 km/s). Signals from different satellites reach a ship or a plane at slightly different times because their distances from the plane or vessel are not the same. For example, by the time simultaneous signals sent from two satellites at distances of 3,100 mi (5,000 km) and 4,970 mi (8,000 km) from a ship reach the vessel, they will be separated by a time interval of 0.01 second. By knowing the position of several such satellites and the time delay between their signals, the longitude and latitude of a ship or plane can be established to within several feet.

International Atomic Time, based on cesium clocks, is periodically compared with mean solar time. Because the Earth's rate of rotation is slowly decreasing, the length of a solar day is increasing. Today's day is about three milliseconds longer than it was in 1900. The change is too small for us to notice; however, it is readily detected by atomic clocks. Whenever the difference between International Atomic Time and astronomical time is more than 0.9 seconds, a "leap second" is added to the mean solar time. To keep these two time systems synchronized, leap seconds have been added about every year and a half.



Gribbin, John. Q is for Quantum: An Encyclopedia of Particle Physics. New York: The Free Press, 1998.

Itano, Wayne M., and Norman F. Ramsey. "Accurate Measurement of Time." Scientific American 269 (July 1993): 56-65.

Morrison, Leslie. "The Day Time Stands Still." New Scientist (27 June 1985).


NIST. "NIST F-1 Cesium Fountain Clock." NIST (29 December 1999).

Wineland, D. J. "Trapped Ions, Laser Cooling, and Better Clocks." Science (26 October 1984).

Robert Gardner


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—Number of oscillations or waves emitted per second.

Ionized atoms

—Atoms that have acquired a charge by gaining or losing an electron.

Mean solar day

—The average solar day; that is, the average time for the earth to make one complete rotation relative to the sun.


—The smallest units or bundles of light energy. The energy of a photon is equal to the frequency (f) of the light times Planck's constant (h); thus, E = hf.

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