NIST already measured the shift as they jacked one lab-bench 3 dm up.
Already for the EAL to TAI conversion, the altitude correction is done.
So, they are aware of it and already compensate for it when needed.
Gravity shifts is definitely on the map of comparison issues they need
to deal with.
Cheers,
Magnus
On 06/06/2015 02:19 AM, Richard (Rick) Karlquist wrote:
Can someone explain to me how this is going to work in
light of the fact that each clock is in a different
gravitational field? Or is accuracy not the measurement,
but rather stability? No, that can't be because any
lab that wants to measure stability merely needs to build
two or three copies of their favorite clock and insure
against synchronization. They in principle shouldn't
need to compare against a dissimilar type of clock.
Therefore, we are back to the gravity issue.
When we worked on the 5071A, we barely had enough sensitivity
to notice a few parts in 10^13 between Santa Clara and
Boulder (~5000 feet).
Rick Karlquist N6RK
On 6/3/2015 12:18 AM, Hal Murray wrote:
Nice picture: A strontium-ion optical clock housed at the National
Physical
Laboratory in Teddington, UK.
Over the past decade, various laboratories have created prototype optical
atomic clocks, which use different elements such as strontium and
ytterbium
that emit and absorb higher-frequency photons in the visible spectrum.
This
finer slicing of time should, in principle, make them more accurate:
it is
claimed that the best of these clocks gain or lose no more than one
second
every 15 billion years (1E18 seconds) -- longer than the current age
of the
Universe -- making them 100 times more precise than their caesium
counterparts. Optical clocks are claimed to be the best timekeepers in
existence, but the only way to verify this in practice is to compare
different models against each other and see whether they agree.
Starting on 4 June, four European laboratories will kick off this testing
process -- the National Physical Laboratory (NPL) in Teddington, UK; the
department of Time-Space Reference Systems at the Paris Observatory; the
German National Metrology Institute (PTB) in Braunschweig, Germany; and
Italy's National Institute of Metrology Research in Turin. Between
them, the
labs host a variety of optical clocks that harness different elements in
different experimental set-ups.
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