[time-nuts] Re: moving optical clocks to test Einstein's general relativity

[Attila Kinali]

On Thu, 9 Nov 2023 13:39:10 -0800
Tom Van Baak via time-nuts <time-nuts at lists.febo.com> wrote:

> The magnitude of gravitational effects on earth are about 1e-13/km, or 
> 1e-16/m, or 1e-19/mm. Solid earth tides are somewhere around 20 to 50 cm 
> so with optical clocks getting into -18 and -19 levels of precision this 
> starts to be a real effect. I'm pretty sure the experimenters simply use 
> a tide-free geoid model like EGM2008 to make it go away. Note if the 
> clocks are in a similar geographical area earth tides are common mode 
> and so you won't see them. For maximum effect you would want them 90 
> degrees latitude apart (10 000 km at the equator).

To add to this:
Current optical clock comparisons in Europe are done at the 1e-18 level.
I.e. where a height difference of 1cm is already significant. At these
comparisons not only solid earth tides show up, but also the gravitational
pull of sun and moon, as well as any change in the groundwater level of
more than 10-20cm. As optical clocks are inching towards 1e-19 stability 
and higher uptimes where comparisons at this level become possible, it is
very likely that those contributing to TAI will, at some point, install
gravimeters next to the clocks to constantly account for the shift in
frequencies. Currently, our comparison capabilities are just not good enough
to justify this, but it will come. Probably in the near future even.

			Attila Kinali
-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes