[dr_lew]

GPS depends on extremely accurate time measurements by atomic clocks.

... which are much more accurate than distance measurements by direct length comparison. In practice, length is measured most accurately optically. For this reason, they abandoned the length standard and defined the meter by a fixed value of the speed of light, in vacuo. That's why the kilogram is the only standard artifact left.

Of course, you could make some atomic isotope the mass standard, but then you would have to count trillions OF trillions of them to high accuracy, so this would amount to an artifact standard anyway.

[omega4412]

Of course, you could make some atomic isotope the mass standard, but then you would have to count trillions OF trillions of them...

You just hook up a counter to your molecular beam!

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