How do companies that make measuring tools like rulers make sure the product is accurate? Is there a universally ruler that is used to check? How do they make sure the measurements are exact?
In: Engineering
When a ruler is made, you check it against a ruler that you know is more accurate than the ruler you’re making.
In turn, when that ruler was made it was checked against another ruler that was known to be more accurate.
… repeat a bunch of times …
So, where does this end? Well, there are a handful of laboratories around the world that have the ability to make a very accurate ruler. This is done with a science experiment. The official definition of a meter is:
“The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”
(Note that other units of length, such as inches, are now defined based on the length of a meter, so it always ends up with that definition of a meter).
But, to do that the laboratory needs to be able to time a fraction of a second very accurately. Just like rulers, clocks are compared against more and more accurate clocks. And, just like the meter, the official definition of a second is based on a science experiment:
The second is “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom” (at a temperature of 0 K)
Translating that into ELI5: If you take this material and do complicated stuff to it, it gives off microwaves. You can measure the frequency of those microwaves – i.e. how many waves there are per second. This is exactly 9,192,631,770, so if you get a different number then your clock is wrong and you need to adjust your clock until you get the right number.
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