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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Historically there were volumes of metal held in places, like Paris for metric measurements, for length, volume and weight. Can’t remember if that’s still the case, but I remember hearing about how they calculate for atomic reduction via electron loss so pretty accurate and still contemporary I guess.
As pointed out, time is measured by atomic clocks using radioactive decay.
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