standards and units

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I need to know the context or concept as the case may be, of the SI standard of measurement

1: TIME

2: LENGTH

3: MASS

why is time measured as a 9 192,631,770 cycles of microwave radiation.

etc.

Please i need an obvious and less gobbledygook kind of explanation.

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Edited by StringJunky
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I need to know the context or concept as the case may be, of the SI standard of measurement

1: TIME

2: LENGTH

3: MASS

why is time measured as a 9 192,631,770 cycles of microwave radiation.

etc.

Please i need an obvious and less gobbledygook kind of explanation.

The standard of length is based on the discovery that the speed of light in a vacuum is a constant (this is from relativity). So the length of the path traveled by light in vacuum during the time interval of 1/299 792 458 of a second is just from inverting the speed of light and letting the time be 1 second.

The basis of time is from finding an oscillator that can be used. It used to be the earth, but atoms can be measured more precisely, so it's based on a transition in Cesium-133. That was chosen because it's the largest frequency of the simplest type of atom that could be measured in a straightforward way. Cesium is an alkali atom, so there's only one electron that defines its basic spectroscopy. The transitions in the other alkali atoms that are similar to Cs all have frequencies that are smaller, so Cs gives the opportunity for the most precise measurement.

Mass is defined in terms of a physical object — a platinum-iridium chunk. AFAIK it's the only standard left based on an artifact. People are working on a way to define this via a measurement recipe like the other standards.

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