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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There are 7 major units that everything is based on.
Meter, second, kilogram, Ampere, Kelvin, candela, mole
The original definition of the meter was 1/10,000,000 the distsnce around the Earth through the poles and through Paris. The modern definition is the distance it takes light to travel in 1/29972458 seconds.
The second was originally defined as 1/86400 the length of a day (in mean solar time). The modern definition is the amount of time it takes for a cesium-133 atom to make 9192631770 oscillations between energy states
The original definition of the kilogram was the mass of 1000cm^3 (or 1L) of water. It was later changed to be the weight of a specific chunk of metal that was kept in France, but it had some radioactive Iridium in it and its mass actually changed over time. It wasn’t until 2018 we got a new definition for it based on Planck’s constant.
I won’t go into all of them, but the idea was originally to base them off of things in the real world that other people can verify. However they proved to not be as good as they thought, so over time we changed them to be very specific things that can be verified anywhere in the universe with things that can’t change according to the laws of physics.
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