Speed of light defines the meter (1/299,792,458)
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Our limit on length used to be a standard meter stick in Sèvres, France. What is it now? Light. What they want to do in trying to measure things precisely, so that everyone can agree on everything, is come up with something — and in fact they defined it for us in March — that, if you have enough money, anyone in the world could do the experiment to measure the property. With the right equipment, people can measure the speed of light. So now the meter is defined in terms of the speed of light.