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

[Attila Kinali]

On Mon, 13 Nov 2023 14:24:03 -0000
"john.haine--- via time-nuts" <time-nuts at lists.febo.com> wrote:

> 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?

These are different effects. For the pendulum, gravity enters directly
into the formula for the period. While for atomic clocks we are talking
about gravitational redshift. The latter has an effect size that is
roughly 1e16 smaller (unless I messed up the calculation).

On Mon, 13 Nov 2023 16:42:58 -0500
Bob kb8tq via time-nuts <time-nuts at lists.febo.com> wrote:

> It looks like few dm of height change is plenty to compensate for
> the typical changes in gravity. We already put clocks in fancy enclosures to 
> compensate for temperature (and possibly humidity). Maybe we simply add an 
> adjustable height table to that enclosure. Servo it using a fancy G meter to 
> keep gravity a constant ….. 

That would be one way, but changing the height of a clock disturbs it.
It is nearly impossible to lift a clock without tilting it slightly.
Not to mention the vibrations that this causes. Especially for something
as sensitive as an optical clocks, which need to be placed in an acoustically
quiet environment because the noise of even people talking is visible on
the output. It is by far easier to just use frequency stepping of the output to correct for the gravitional shift, or do it in post-processing of the numbers.

On Mon, 13 Nov 2023 18:24:42 -0800
Hal Murray via time-nuts <time-nuts at lists.febo.com> wrote:

> Do I need a fancy G meter?
> Do they publish tide charts for solid land?

There are maps for that, yes. IIRC one of the most accurate to date are
those of the GRACE mission. The only disadvantage of those is that they
have a very coarse resolution. There is an ongoing effort to do more detailed
maps of major cities, especially those with optical clocks. But this
will take some time, especially as we do not have any reliable and easily
transportable absolute gravimeters. There are some in development (especially
in France), but these are still mostly lab instruments.

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