Distance or time for example. I look at my watch and I can see how long 1 second takes. I can look at a ruler and see how long 1 centimeter is. But how do they make rulers and watches? How do you define what a centimeter or a second is without just saying “1/10 of a decimeter” or “1/60 of a minute” or just pointing at another ruler/watch?
I guess time is easier since you can just reference recurring events (like moon phases for example) and then go down in scale from there until you get hours, minutes, seconds. But distance just seems completely arbitrary.
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All Units on earth are based on the seven SI units, which are legally defined by the bureau of weights and meassures in paris.
The seven SI units are:
Meter
Second
Ampere
Candela
Kilogramm
Mole
Kelvin
Every unit on earth is either defined in relation to one of these (so for example an inch is legally defined as being 0.0254 meters) or is a combination of these, so for example a Newton, the unit of force, is equal to a kg × m / s².
The 7 SI units used to be mostly based on actual physical objects, so there was a lump of metal in a a vault in paris that was legally defined **as** being exactly 1 Kilogramm, same for a metal metre stick. A second used to be defined as a fraction the duration of a full day.
For manufacturing e.g. a measuring device companies occasionally have to take a reference measurement of the actual object on paris, and then use that reference measurement to calibrate their own products.
Nowadays 7 units are defined in relation to universal unchanging physical constants, but the principal is still the same. Physicists will use these definitions to generate exact references, that can be used to calibrate equipment for making measuring tools.
1 Meter is defined equal to the distance light travels in a vacuum in 1/299792458 seconds
1 second is defined equal to the duration of 9192631770 hyperfine transition frequency of Cs-133 (this is actually what nuclear clocks use)
1 Ampere is defined equal as the movement of 6.241509074×10¹⁸ electrons per second
1 Candela is defined such that the “luminous efficacy” (amount of light per energy in the radiaton) of a certain wavelength becomes exactly 683
1 Kilogramm is defined such that Plancks constant becomes 6.62607015×10^-34 when written in kg m²/s
1 Mole, since it’s an amount of stuff, is defined fairly simply as being equal to 6.02214076×10^23
1 Kelvin is defined such that Boltzmann’s constant becomes exactly 1.380649×10^−23 when expressed as J/K
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