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

[john.haine at haine-online.net]

Fascinating that the most accurate pendulum clock, the Fedchenko, was in effect a gravimeter so moon and sun gravitational effects were the ultimate limit on its stability.  So will our ability to measure gravity be the ultimate limit on optical clock stability, unless we can put them in free fall?

-----Original Message-----
From: Azelio Boriani via time-nuts <time-nuts at lists.febo.com> 
Sent: Monday, November 13, 2023 10:25 AM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Azelio Boriani <azelio.boriani at gmail.com>
Subject: [time-nuts] Re: moving optical clocks to test Einstein's general relativity

Is a gravimeter with 1 uGal stability able to support the performance of optical clocks? Maybe we must first relate the performance of gravimeters with the performance of optical clocks.
Gravity measurements below 10e−9 g...  <https://arxiv.org/pdf/1809.04908.pdf>
The PTB has made a transportable optical clock that fits in a truck:
<https://www.ptb.de/cms/fileadmin/internet/presse_aktuelles/Presseinfos/2017/D1694_015_komprimierter.jpg>
Geodesy and metrology with a transportable optical clock <https://arxiv.org/ftp/arxiv/papers/1705/1705.04089.pdf>

On Mon, Nov 13, 2023 at 4:36 AM Attila Kinali via time-nuts <time-nuts at lists.febo.com> wrote:
>
> 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 
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