# Atomic clocks underground?

Have there been any experiments using atomic clocks underground as a comparison to those on the earth's surface?

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SteamKing
Staff Emeritus
Homework Helper
What sort of experiments?

Cthugha
Not really underground, but there have been experiments performed by Wineland's group checking relativity using optical clocks at different heights and therefore also different gravitational potential. However, the height difference was pretty small, 20 cm or so because the clocks are pretty sensitive. See:
C.W. Chou et al., "Optical Clocks and Relativity", Science 329, 1630-1633 (2010).
or have a look here: http://www.doylegroup.harvard.edu/wiki/images/e/e3/Optical_clocks_and_relativity.pdf

I know of experiments above the surface. I was wondering what the results of time dilation experiments would be underground. We assume that it would slow down but if it hasn't been proved experimentally how do we know? It could do 1 of 3 things. 1. Speed up. 2 Stay the same. 3 Slow down. If we only rely on the mathematics to tell us we may be missing something important about the mechanisms of gravitation.

Pretty well everything you can think of has been done.

SteamKing
Staff Emeritus
Homework Helper
I understand that experiments have been done on spacecraft orbiting the earth to check time dilation. That seems to be a better experiment, since the craft are traveling at high (but not relativistic) velocities.

I understand that experiments have been done on spacecraft orbiting the earth to check time dilation. That seems to be a better experiment, since the craft are traveling at high (but not relativistic) velocities.
Not if you are trying to establish the internal gravitational effects on a mass. Does the gradient of the inverse square law peak at the surface for time dilation and then decrease towards the centre of gravity. I don't think anyone can answer this without experimental evidence. This proposition may on the surface of it appear to be trivial. Is it?

Claude Bile
The best atomic clocks (found in standards laboratories) eject Cs atoms upward into a vacuum and measure the resonant frequency at the apex of their trajectory in order to negate time dilation due to general relativity.

So presumably, the effect of GR, even at these scales is well understood and routinely corrected for.

Claude.

Drakkith
Staff Emeritus
Not if you are trying to establish the internal gravitational effects on a mass. Does the gradient of the inverse square law peak at the surface for time dilation and then decrease towards the centre of gravity. I don't think anyone can answer this without experimental evidence. This proposition may on the surface of it appear to be trivial. Is it?
Are you suggesting that we test our theories in every conceivable way to make sure they are accurate?

Are you suggesting that we test our theories in every conceivable way to make sure they are accurate?
Well all the test I have come across have been above the surface. We assume we know how gravity acts internally but this is taken on faith in the mathematics alone.

The best atomic clocks (found in standards laboratories) eject Cs atoms upward into a vacuum and measure the resonant frequency at the apex of their trajectory in order to negate time dilation due to general relativity.

So presumably, the effect of GR, even at these scales is well understood and routinely corrected for.

Claude.
Interesting.

f95toli
Gold Member
Underground with respect to what?

Since earth is not spherical, the UTC used a geodesic sphere (similar to the GPS) as a reference "surface". For each clock that is part of the UTC one can then calculate a correction in their reported time due to GR.

Hence, some clocks are already "underground" with respect to this sphere.

One if the main problems with the new generation of optical clocks is that they are so accurate that it is very difficult to compare them (which you have to be able to do if they are going to be used in the UTC); one of the biggest sources of uncertainty is their position with respect to the geodesic sphere, even the GPS system is not accurate enough since the positions change over time because of tidal motion and seismic activity (with don't actully know exactly where NIST, NPL, PTB and the other NMIs are located).

why does it need to be an atomic clock? just because it's more accurate?

Nugatory
Mentor
why does it need to be an atomic clock? just because it's more accurate?
The accuracy is the major reason; we're dealing with extraordinarily small effects that can only be detected with extraordinarily sensitive and accurate equipment.

Of course we also need a clock whose mechanical components aren't affected by changes in the gravitational field. An old-fashioned pendulum clock, for example, wouldn't be a good choice no matter how carefully it was designed.