Is there like a master ruler? Who decided what a length a cm, mm and inch would be and why?
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The traditional definitions of the units in the International System (S.I., from its French initials) are interesting and occasionally useful. A meter was supposed to be the distance from a pole to the equator along a meridian divided by 10,000,000. A second was a day divided by 24*60*60. A kilogram was the mass of 0.001 cubic meters (a.k.a. 1 liter) of pure water at a temperature of 4°C (the pressure matters very little, but the definition probably specified it as 760 mm of mercury or something; I didn’t look it up but you can).
For a while there was a master kilogram and a master meter, so people wouldn’t have to go measure Earth every time they wanted to calibrate an instrument. Unfortunately those references weren’t completely constant over time and new definitions were needed. The rotation of the Earth is also not constant enough to serve as the basis for defining the second.
These days the second is defined by fixing the frequency of the photons emitted by some transition in a cesium 133 atom to a particular value, the meter is defined by fixing the speed of light to a particular value and the kilogram is defined by fixing Planck’s constant to a particular value.
Many other units are derived from these three: A newton is the force that would accelerate 1 kilogram by 1 meter per second per second; A joule is the energy spent when displacing an object by 1 meter by applying a force of 1 newton; etc.
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