[time-nuts] Nature: Hyper-precise atomic clocks face off to redefine time

[Magnus Danielson]

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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