Comment by perlgeek
Our models use single atoms (or single molecules), and for those we have pretty good models that we can solve numerically, at least.
In a gas, the atoms or molecules only interact weakly, so you just get some known effects like a line broadening due to thermal motion of the particles
But you really still want experimental validation before you declare any of these as a new standard, for a whole variety of reasons:
* it's often complicated to calculate multiple excitations
* you might forget something in the models, like isotope ratios
* the models don't really give you a good sense of how impurities in your materials will affect the clocks
* there might be some practical issues, like glass (used in the optical fibers) not being a very good medium for some frequencies of light that would otherwise look promising as a time standard
... and so on.
> Our models use single atoms (or single molecules), and for those we have pretty good models that we can solve numerically, at least.
We can only solve these with assumptions, like assuming that protons or neutrons are indivisible particles with experimentally determined sizes and perfectly spherical shapes - even though we know very well that they are in fact collections of quarks and gluons whose size and shape is fully determined by more fundamental intercations. We are nowhere near a point where we could compute anything about a whole hydrogen atom using only the standard model and no other assumptions. Quantum chromodynamics is far to complex to allow for a perfect simulation like this.