Comment by pclowes
10 hours ago
How do they know the accuracy of the clock? Eg: 2s of drift in 65M years.
Naively, measuring that seems more difficult than the clock. Wouldn’t you need a perfect clock as a baseline? There must be something really clever here.
They measured the stability by comparing the nuclear clocks with more stable conventional atomic clocks. In Vienna, they used a commercial single-ion optical clock [1], and in Beijing they used a hydrogen maser (a type of microwave atomic clocks) that was calibrated to the International Atomic Time scale via GNSS satellites.
[1] https://www.toptica.com/products/optical-quantum-clocks/topt...
By adding up the uncertainties of all intermediate measurements they made.
As a parallel, for a regular clock, you'd measure and compute the uncertainties of:
- The distance between the axis of rotation and the center of mass of the pendulum,
- The local acceleration of gravity.
And deduce the pendulum period and its uncertainty by calculus.
In that case I'm not sure you can build (and measure) a pendulum that's more precise than a previous realization of the second (because the meter is defined from the speed of light and the second, and gravimeters probably depend on the definition of the second too).
That's why the definition of base units is carefully chosen to not depend on previous realization. IIRC time is the most fundamental of base units, because other base units depend on it.
You measure precision with frequency combs and relative beat notes against an optical lattice clock