Energy of the 229 Th nuclear clock transition

You need to narrow their emission wavelength to something ~ the atomic transition (MHz) and then carefully tune it as you say.

Typically diffraction grating is used to carefully feedback some of the laser's output (<10%) back into it, and this can be used to to narrow the emission from several nm to MHz, and to coarsely tune the frequency to few 10-100 Ghz.

Fine tuning is done by changing the current flowing through the laser or the temperature of the junction. These both cause a frequency shift on the MHz scale scale.

You might fine it interesting, you can also use VCOs for fine tuning, but you take the generated RF and send it into a crystal called an Austo-optic modulator. The laser light refracts from the RF phonons propagating through the crystal and this can be used to shift the laser light by the RF frequency.

The most philosophical thing I have seen is formal verification software. Like metamath.

Philosophers could easily gain a lot of my respect if they switched from natural language philosophy, to slowly formalizing physics, law, norms and values, natural language dictionaries into actual formal concepts, in a collaborative way, so that all philosophy can actually be integrated into a theory.

Back to the clocks, what you claim is patently false. It is perfectly possible given 2 types of clock A and B to assess which type of clock is more precise:

Let's model an imprecise clock as one that reports as time passed: the actual time passed plus an error term. The error term undergoes a random walk, or diffusion.

This means all you have to do is make an ensemble of 2 (or more) clocks of type A: A1 and A2, and similarily 2 (or more) clocks of type B: B1 and B2, reset all clocks and then observe for which type of clock X we have a smaller difference between X1 and X2, as time passes.

if the difference in reported time between A1 and A2 wanders away from 0 slower than the difference in reported time betwween B1 and B2 then you know clock type A is more precise.

For any given definition, there are a handful of possible "defining experiments" being carried out at a handful of national labs. Their deviations with respect to each other are constantly being monitored and metrologists are constantly chasing down error terms. Yes, they choose one standard to "bless" as the official definition, but it's their job to live below that abstraction and to maintain it. If their "blessed" definition started to drift with respect to the other candidate definitions they would notice very quickly and react appropriately. The fact that most of us entertain a single definition isn't a consequence of philosophical confusion as to whether or not one exists, it's a consequence of delegating the ongoing experiments backing our simplifying assumptions to a group of people who are very good at them.

For instance, the disagreement between solar time and atomic time is monitored so closely that the slowing of Earth's rotation due to tidal forces is a gigantic signal compared to measurement noise:

https://en.wikipedia.org/wiki/Leap_second#/media/File:Deviat...

If, say, Earth flew through a cloud of dark matter that somehow messed with Cs absorption lines, we would see Cs clocks drift with respect to Rb clocks, unlocked quartz clocks, Earth rotation, Optical Lattice Clocks, etc, etc. The event would not go unnoticed. Cs clocks would be demoted from their position as primary time standards and the next best candidate would be promoted.

It feels intuitively obvious/ redundant that a prototype metre is one metre long, but a statement that light in a vacuum travels a fixed distance in one second is not so obviously redundant. It doesn't matter what colour the light is? Nope. It doesn't matter when I measure? Apparently not.