I am a time-nut and have a couple highly accurate clocks - no atomic ones as yet but mine are still accurate to a second or two every year or two. Here is an interesting development in Europe, from Physics World:
Optical link connects atomic clocks over 1400 km of fibre
The time kept by atomic clocks in France and Germany has been compared for the first time using a new 1400 km optical-fibre link between labs in Paris and Braunschweig. Hailed as the first comparison of its kind made across an international border, the link has already shown that two of the most precise optical atomic clocks in Europe agree to within 5 × 10–17.
The clock I have at home is accurate to 6.8 x 10-13 so this is four orders of magnitude better - quite the jump. When you get this accurate, other things begin to rear their heads:
If they were side by side, the clocks would tick at exactly the same frequency. However, there is a 25 m difference in the elevation between the two locations, which means that the Earth's gravitational field is not the same for both clocks – causing them to tick at slightly different frequencies. This gravitational redshift was confirmed by the link, which can detect differences in elevation as small as 5 m.
And this net is being expanded:
The optical clock at PTB Braunschweig is already linked to the Max Planck Institute for Quantum Optics (MPQ) in Garching near Munich. This is done via a 920 km pair of optical fibres, and researchers at the MPQ plan to use the clock signal to make extremely precise spectroscopy measurements. A further expansion of this network would provide researchers in other labs in Europe with access to high-precision clock signals.
Very cool - not only useful for in-lab applications where high precision timing is needed, it can also be used to synchronize interferometers for astronomical and terrestrial observation.
As for timing at different elevation, one of the core things Einstein came up with. Local scientist Tom VanBaak did a wonderful demonstration of this effect in 2005 driving up to Paradise Lodge at Mt. Ranier - the website is here: Project GREAT: General Relativity Einstein/Essen Anniversary Test. The television producers for GENIUS with Stephen Hawking asked him to repeat this experiment in 2016 - here is the website from that expedition to the 9100 foot summit of Mt Lemmon, near Tucson, Arizona: GENIUS by Stephen Hawking, Relativity by Einstein, Time dilation by cesium clocks.
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