moving optical clocks to test Einstein's general relativity
On Mon, 13 Nov 2023 14:24:03 -0000
"john.haine--- via time-nuts" time-nuts@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@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@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
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@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