Seconds are measured by the frequency emission of cesium. Why is a frequency from the emission spectrum of cesium used as the standard in defining a second? Why particularly cesium?
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1From an improbable source: "Cesium has a single valence electron (unpaired) in the S-shell. In other words, it has an electron that's easy to observe (thus easy to use to tell time)." This HowStuffWorks article has a very lucid explanation of how a cesium clock actually functions. – Robert Harvey Sep 23 '14 at 18:19
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1See also http://www.thenakedscientists.com/forum/index.php?topic=12732.0 – Robert Harvey Sep 23 '14 at 18:27
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@RobertHarvey I had to check your first link -- for a moment I thought you were linking to AIR – Carl Witthoft Sep 24 '14 at 11:37
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@CarlWitthoft: It's the only useful answer I've ever seen on Yahoo Answers. – Robert Harvey Sep 24 '14 at 15:00
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Related: https://physics.stackexchange.com/questions/191871/why-do-atomic-clocks-only-use-caesium/191876#191876 – Watson Apr 22 '18 at 15:40
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Rather than write something unintelligible, I'll quote from a page on cesium clocks.
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 point is that we want an extremely spectrally "pure" source so there's mimimal uncertainty about the wavelength.
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