According to Wikipedia thorium is probably created from neutron stars or supernovae making it one of the rarest elements in the galaxy. But, it’s quite abundant on Earth, maybe 3x as abundant as Uranium on earths crust. Because of this abundance it is, along with Radon, the primary constituent of earths natural background radiation.
Iron is the last element that can form through a sustaining fusion process. All elements above iron are formed in super nova or neutron star collisions. Specifically gold and platinum are thought to be formed primarily in neutron star collisions
Standard atomic clocks probe outer-shell electrons sensitive to stray EM noise; locking directly to the nucleus is how you actually kill environment drift.
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.
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.
Reality is amazing!!
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.
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.