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

[Magnus Danielson]

Attila,

In a conversation with SYRTE they judge that they are still just out of 
reach of seeing that in their data from the previous measurement 
campaigns, but I am sure they will be looking harder for it. They where 
aware that it would be there and acknowledge the analysis that it would 
show up as comparing clocks between the labs. This is done in campaigns 
and not an ongoing process. As they have a campaign, they do not do 
hands on research work on the optical clocks, but just let them operate. 
Inbetween, they try to improve them.

Recall, the spread of the clocks isn't all that good for the 
observability of this phenomena, but it is better than having the clocks 
in the same lab. This is why you get around 1/10th of your sensitivity 
to the effect at best for the moment. This is why links and clocks need 
to be so good.

I sent a follow-up question to the SYRTE-team, and they seem to have 
enjoyed the discussion.

Cheers,
Magnus

On 2023-11-12 23:57, Attila Kinali via time-nuts 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