eli5 how they define common measurement units

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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.

In: 2

The exact definition of what a “second” is has been refined over and over throughout history.

Right now, our definition of a second is measured using the time that elapses between 9.192631770 x 109 cycles of radiation produced by the transition between two levels of Caesium-133

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.

Time because of [Ancient Egyptians and Babylonians](https://www.nist.gov/si-redefinition/second-introduction). 12 hours in a day, 12 hours in a night, then base 60 beyond that for minutes and seconds because the Babylonians were really into base 60. Now it’s based on the number of “ticks” recorded at a specific frequency when dealing with energized cesium.

[Meter](https://www.nist.gov/si-redefinition/meter) was originally based on the distance from the equator to the North Pole but more recently has to do with the distance light travels in a vacuum in an incredibly short period of time.

Edit: sorry, I didn’t pay attention to the sub.

Eli5: meter is based on how far light travels in a specific amount of time, seconds are based on a specific measurement recorded from a specific material.

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

It used to be that units were defined based on some physical examples.

Somewhere in the city hall there would be an object that weighed a pound and a rod that was foot long etc and authorities would make copies that weighed as much or where exactly that long and would check what merchants were selling against that.

That led to every kingdom and sometimes every large town having their own units. If you traveled around Europe you would find people something called pound in many places but it would always be a different value and they would divide it differently into different subunits. since trade meant traveling that was bit of a problem.

When they created the metric system they tried to get rid of most of the trouble.

One system that was the same everywhere.

Instead of some pound having 16 ounces and other 12 and others again 15 ounces all smaller and bigger units were power of 10.

They were all 10,100, 1000 or 1000 to some power as large as or a fraction of the main unit and you would use the same prefixes for all units.

Next they tried to define units as much as possible by the main units. A Watt is just a kilogram times a square meter divided by a second cubed for example.

You could even define some helper units like a liter being a decimeter cubed and a hectare a square 100 to the side, if you didn’t want to use cubic and square meters for that

That however left a small number of core units that you couldn’t define by reference to something else.

They tried to define those by reference to the planet itself. a second is defined by the length of the solar day and a meter was defined by reference to the circumference of the planet.

A day has 24 hours and each hour has 60 minute and each minute 60 seconds. That seems easy enough if you can look at the sun.

A meter was defined by taking the distance from equator to pole and dividing it into 10,000,000 meters. You can figure out the circumference of earth quite accurately even with limited tech.

Kilogram and kelvin were defined by the mass and the melting and boiling point of water.

1 kg of water weighs 1 liter and we divide the difference between melting and boiling point of water in 100 degrees and count kelvin from absolute zero.

Ampere was also defined with reference to tother metric units of distance and force.

None of those were really very good.

The world turns out to be not quite as uniformly spherical as thought and initial measurements were of and the length of the day varies a bit more than you would think.

Later these original definitions were refined in reference to natural constants.

We can define and measure time extremely accurately and the speed fo light is fixed, so we can define a meter by the speed of light and the amount of time it needs to travel a meter.

The kilogram was the hardest to pin down with natural constants and we used a physical object until quite recently. It is now defined with reference to the speed of light and the Planck constant.

Someone who had gone to the travel of memorizing all the definitions and the numbers involved could by transported to a different planet and recreate the metric system exactly as it is from ground up.

Since all other system still in use are now defined in reference to the metric units you could recreate them too if you memorized the legally defined conversion numbers.