Cesium Atoms at Work
"...till like a clock worn out with eating time."
John Dryden (1631-1701)
The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK. It kept time to a second in 300 years.
A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency,9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time, which may thus be measured by cesium clocks.
Today, cesium clocks measure frequency with an accuracy of from 2 to 3 parts in 10 to the 14th, i.e. 0.00000000000002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years. It is the most accurate realization of a unit that mankind has yet achieved. A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time. The frequency involved is that of the energy absorbed from the incident photons when they excite the outermost electron in a cesium atom to jump ("transition") from a lower to a higher orbit.
According to quantum theory, atoms can only exist in certain discrete ("quantized") energy states depending on what orbits about their nuclei are occupied by their electrons. Different transitions are possible; those in question refer to a change in the electron and nuclear spin ("hyperfine") energy level of the lowest set of orbits called the "ground state." Cesium is the best choice of atom for such a measurement because all of its 55 electrons but the outermost are confined to orbits in stable shells of electromagnetic force. Thus, the outermost electron is not disturbed much by the others. The cesium atoms are kept in a very good vacuum of about 10 trillionths of an atmosphere so that the cesium atoms are little affected by other particles. All this means that they radiate in a narrow spectral line whose wavelength or frequency can be accurately determined.
John Dryden (1631-1701)
The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK. It kept time to a second in 300 years.A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency,9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time, which may thus be measured by cesium clocks.
Today, cesium clocks measure frequency with an accuracy of from 2 to 3 parts in 10 to the 14th, i.e. 0.00000000000002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years. It is the most accurate realization of a unit that mankind has yet achieved. A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time. The frequency involved is that of the energy absorbed from the incident photons when they excite the outermost electron in a cesium atom to jump ("transition") from a lower to a higher orbit.
According to quantum theory, atoms can only exist in certain discrete ("quantized") energy states depending on what orbits about their nuclei are occupied by their electrons. Different transitions are possible; those in question refer to a change in the electron and nuclear spin ("hyperfine") energy level of the lowest set of orbits called the "ground state." Cesium is the best choice of atom for such a measurement because all of its 55 electrons but the outermost are confined to orbits in stable shells of electromagnetic force. Thus, the outermost electron is not disturbed much by the others. The cesium atoms are kept in a very good vacuum of about 10 trillionths of an atmosphere so that the cesium atoms are little affected by other particles. All this means that they radiate in a narrow spectral line whose wavelength or frequency can be accurately determined.
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