[time-nuts] Nature: Hyper-precise atomic clocks face off to redefine time
Tom Van Baak
tvb at LeapSecond.com
Sat Jun 6 07:36:20 UTC 2015
> 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
Any lab with more than one clock will compare their clocks on a periodic basis. The results of those comparisons are just numbers, which can be, and usually are, adjusted in order to create a mean or paper clock. The adjustments consist of phase offset and frequency offset and, if quartz or rubidium or hydrogen, a frequency drift factor also.
Relativistic (elevation) corrections are just a simple frequency offset correction. Every UTC(k) lab does this. So don't see that gravitational fields will be a problem. It's just a systematic calibration constant.
Given the e-13 level retrace spec of the 5071A it does not surprise me that you were just barely able to detect this between Santa Clara and Boulder. Note that noise floor of 5071A (~5e-15) is far better than the retrace so time dilation is much easier to detect if you first compare the clocks at the identical elevation and keep them powered on; to ascertain their frequency offsets. That's what I did in Project GREAT.
The relativistic correction is about 1e-16/meter, or 1e-13/km. The accumulated phase difference due to different clock elevation, aka time dilation, is about 10 ns/day/km (a nice rule-of-thumb).
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