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
If you have a laser, a detector and a good clock you can measure the distance of an object to a very precise degree. A meter is exactly defined as the distance light travels in 1/299,792,458 of a second.
There was at points in history actual physical objects which defined units like the meter, for example the International prototype metre bar but now they are defined in terms of universal constants.
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