One of the "good" things about the "metric" or SI system is if we go by "the width of a man's thumb" it becomes "who's thumb" because there are slight variations from person to person. So the idea was to try and tie these measurements to physical constants. SO that my inch and you inch are the same because we have some reference. THe original meter is literally a bar in a museum in France (calculated--incorrectly I might add--to be like 1/10 millioth of the curve of the earths circumference).
But then some idiot (probably related to me) comes by and says the earth isn't s sphere, its an oblate ellipsoid!
ANd that idiot Einsitein came along and said "length changes if you are in a gravity well" so it becomes this running battle to find some measurement that anyone anywhere can do to get the same answer without the need for a reference.
As you can imagine it gets kinda stupid real quick.
I believe mass still has to use the original "weight" which of course decays over time! They were trying to replace that recently.
For instance the "second": taking the fixed numerical value of the caesium frequency, Δ
νCs, the unperturbed
ground-state hyperfine transition frequency of the
caesium 133 atom, to be 9192631770 when expressed in the unit
Hz, which is equal to s−1.
Yup that seem really odd and specific because it has to be--everyone who measures it must get the EXACT same answer.
Of course that number (9192631770) is based on the old definition which was 1 second in the year 1900 as per the Earth's Orbit around the sun. However, the earths orbit changes (fucking juptiter!) so we can't go back in time and measure it.
Then for length we have a meter: the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a
second. Of coures THAT number comes from the old bar of platinum in france based on the wrong number calculated a couple hundred years ago.
As we get better and better measurement systems we keep having to refine the standard--for the second with quartz wristwatches, we have a deivce more accurate that the previous (observation of the earth's orbit) and then forward to atomic clocks
And it looks like they just settled on the kg in 2019: The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and Δ
νCs.
All of these of course are related to measurements throughout history, the OLDEST is actually the angle in degrees which dates from around 2000BC or before and we don't actually know why...but we have guesses. Of course angles are easy as you are just dividing a circle into "parts" and so radians vs degrees is just what ratios you use for 1 full turn. is it 360 units in a circle or 2pi units in a circle or 402568 units in a circle. We all agree on the circle.
I mean you can use kiloinch, kilofoot, centifoot, picopound etc if you want. In the end its just a ruler.
but I agree.
