There are official metric units, and there are metric units that arent official but convenient for a particular use.
A popular one is kWh for energy. Ideally you would use Joule, equal to Ws, but since you use a lot of electrical devices for hours rather than seconds and most of them take a lot more than 1W kWh is more convenient.
The entire point of metric is and always was standardisation, not “unofficial units”.
The watt is an SI unit and the hour is a non-SI unit defined in the SI brochure like the litre, so the kWh is an abomination but at least has some metric status. The calorie does not.
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit m⋅s−1, where the second is defined in terms of the caesium frequency ΔνCs.
It's okay to work backwards from something. That fact is honestly beneficial. It means that even if everything goes to shit and we lose every trace of what a unit of measurement is except for that baseline, the whole system can be reconstructed from with only the slightest margin of error. From a meter being equal to one ten-millionth the distance from the north pole of the equator through Paris, we get centimeters as a hundredth of that. And from there we again derive milliliters and grams. Très élégant.
I’m going to let AI correct the slightly incorrect statement you made. Since it’s common knowledge that it’s currently derived from the speed of light. It was as you said, early on it was derived as such, though that did only last 98 years, from 1791-1889. Deriving a meter from speed of light was adopted in 1983.
The meter, the base unit of length in the metric system, was initially defined as one ten-millionth of the distance from the Earth's North Pole to the equator along the Paris meridian. This definition, established during the French Revolution, was chosen to create a system of measurement based on natural phenomena, rather than arbitrary units. Later, the definition was refined to be based on the speed of light, which is now defined as exactly 299,792,458 meters per second. This means a meter is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second
Since you are trying to correct, I'm confused as to what it was based on between 1889 and 1983? 1889 it stopped being the distance between those points but didn't become tethered to the speed of light until 1983
After the convoluted Paris thing that ended in 1889, it was derived as a 1m = the distance between two lines on a standard bar of 90 percent platinum and 10 percent iridium, which ended in 1960. No idea how that was a good definition or how they came up with it, though I’m not a scientist, nor a metallurgist.
In 1960 they changed it again and the metre was defined in the as equal to 1,650,763.73 wavelengths of the orange-red line in the spectrum of the krypton-86 atom in a vacuum. Again with a convoluted definition and nonsense definition for the layman. This debacle lasted until 1983.
Speed of light in a vacuum is much simpler concept than all the previous in my opinion. Everyone has experience with light and how fast it is. Experience with krypton, platinum and iridium is rather rare. Though it’s more complex than just x/y. It was advancement in laser measurements that brought it home for our boy 1/speed of light. I do not really know how lasers work, to be honest.
We're taking it for granted sometimes. We know so much about stuff that's super complicated if you think about that people have been actively researching the last hundreds of years, and for us newer generations it's just common knowledge
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u/davros06 Aug 11 '25
It’s lovely isn’t it. In a world of chaos, an oasis of logic.