Atomic Clock

Atomic Clock, device that measures time by the frequency of radiation emitted by an atom or molecule when it makes a transition between two energy states. Atomic clocks are extremely precise and are used to keep universal time—the international basis for establishing legal and scientific times and for setting all public and private clocks worldwide. See also Clocks and Watches.

An atomic clock measures the frequency of electromagnetic radiation emitted by an atom or molecule. When an atom or molecule moves from one energy level to another, a specific amount, or quantum, of energy is absorbed when jumping to a higher energy level or emitted when falling to a lower energy level. The amount of energy absorbed or emitted depends on the difference between the energy levels. The atom or molecule emits energy in the form of electromagnetic radiation and the wavelength and frequency of the radiation depends on the amount of energy released by each transition between energy levels. Because the atom or molecule can only emit or absorb a specific amount of energy, the radiation emitted or absorbed has a regular frequency. A counter in the atomic clock keeps track of each cycle.

Atomic clocks determine the frequency of an atom or molecule’s transition in one of two ways. An active atomic clock induces a group of atoms in an elevated energy state to drop to a lower energy state, measuring the frequency of radiation emitted by the atoms. A passive atomic clock exposes a group of atoms in a lower energy state to electromagnetic radiation with changing frequency. When a majority of atoms jump to the next energy level, it signals that the correct frequency has been achieved.

Most atomic clocks in use today are passive cesium clocks. The United States National Bureau of Standards (NBS, now National Institute of Standards and Technology, or NIST) established the second as the time radiation would take to go through 9,192,631,770 cycles at the frequency emitted by cesium atoms making the transition from one state to another. Cesium clocks are so accurate that they will be off by only one second after running for 300 million years.

The atomic clock has led to new and more precise techniques for measuring time and distance. Satellite navigation and positioning systems such as the Global Positioning System rely on atomic clocks. Astronomers use atomic clocks to measure the amazingly regular cycles of spinning astronomical objects called millisecond pulsars. Atomic clocks helped support German American physicist Albert Einstein’s theory of relativity by showing that the passage of time appeared to change with speed. The U.S. National Aeronautics and Space Administration (NASA) uses atomic clocks to time its transmissions to space probes.

American physicist Isidor Rabi and his associates at Columbia University built the first apparatus to measure radiation frequencies. The NBS built the first molecular clock, using ammonia gas, in 1949. The first model of the cesium clock in use today was built by American physicist Norman Ramsey in 1957.