your head is older than your feet, and now it's even older

[eub]

http://www.sciencenews.org/articles/20060422/bob8.asp

Every decade since the mid-1950s, the accuracy of atomic clocks has improved tenfold, notes Kleppner. The clocks are approaching an accuracy of 1 part in 10¹⁶, and newer systems, based on the vibrations of laser-cooled atoms and ions, are expected to eventually attain 1 part in 10¹⁸.
[...]
Given the current accuracy of clocks, this gravitational effect requires that researchers know the altitude of timekeeping laboratories to within a few meters. Ultimately, altitudes would have to be measured to within a centimeter.

That becomes tricky because gravitational theory dictates that the altitude isn't measured relative to average sea level, but to the geoid, a hypothetical surface that approximates the shape and size of Earth. The geoid's size fluctuates in response to, for example, ocean tides and the redistribution of water due to climate changes.

This is impressive.

Comments

[hattifattener]

That is impressive.

When reading up on surveying a while ago I was impressed to learn that surveyors have also had to take into account the lumpiness of the geoid for quite a while (or whatever gravitational equipotential surface they're standing on at the moment). If you're near a large mountain, your plumb-bob will be out of true, and you need to estimate the mountain's mass and distance so you can correct for it... it's not just Cavendish who sees these effects...

[eub.livejournal.com]

This is wild.

http://www.cage.curtin.edu.au/~will/gra68_05.pdf

ABSTRACT
The geoid is the equipotential surface to which orthometric heights are referred, whereas the quasigeoid is the non-equipotential surface to which normal heights are referred. The Australian Height Datum is a hybrid of these vertical datum surfaces, being called a normal orthometric height system. It is therefore appropriate to determine the separation between these reference surfaces with a view to future gravimetric determinations of the geoid or quasigeoid of Australia. Using Bouguer gravity anomalies and a digital elevation model, the maximum separation between these surfaces has been estimated to be ~150 mm, with a standard deviation of +/- 18 mm, in Australia.
[...]
The geoid undulation (N) refers to the separation between the reference ellipsoid and the geoid measured along the ellipsoidal normal, whereas the height anomaly () refers to the separation hetween the reference ellipsoid and the quasigeoid, also measured along the ellipsoidal normal. Correspondingly, the heights that refer to the geoid are orthogonal heights (II) measured along the plumbline, whereas the heights that refer to the quasigeoid are normal heights (II’) measured along the ellipsoidal normal. These two reference surfaces and their corresponding height systems are shown schematically in Figure 1.
[...]
As Molodensky et al. (1962) have shown, a slightly different geodetic boundary value problem may be formulated and solved at the Earth’s surface without the Stokesian hypothesis. Molodensky introduced two new surfaces, called the telluroid and the quasigeoid, in which the concept of the geoid undulation is replaced by the height anomaly (see Figure 1). Plotting the height anomalies above the reference ellipsoid results in the quasigeoid surface that is identical to the geoid over the oceans, and a reasonably close approximation to the geoid over most land areas. However, exceptions do occur in areas of large Bouguer gravity anomalies and high topography, such as the Himalayas

[bhudson.livejournal.com]

So clock people have a Moore's Law of their own? Freaky.