Since 1983, the meter is defined in terms of the distance traveled by light in a vacuum during 1 second, see https://en.wikipedia.org/wiki/Metre#Speed_of_light_

Since 1983, the meter is defined in terms of the distance traveled by light in a vacuum during 1 second, see https://en.wikipedia.org/wiki/Metre#Speed_of_light_definition https://en.wikipedia.org/wiki/Metre#Speed_of_light_definitio...