[time-nuts] Re: Project Great

[Lux, Jim]

On 11/27/21 11:08 PM, Tom Van Baak wrote:
> Hi Thomas,
>
> Good to hear the experiment was contagious for you. If you have 
> additional questions let me know.
>
> Your suggestion about Mount Evans and Pikes Peak are excellent. You 
> will enjoy this 2017 paper:
>
> "An Undergraduate Test of Gravitational Time Dilation"
> https://arxiv.org/abs/1710.07381
> https://arxiv.org/pdf/1710.07381.pdf
>
> ---
>
> As for CSAC, the news is not so good. I've worked with several groups 
> to explore CSAC for gravitational time dilation experiments. Those 
> clocks are so cute and small, it's irresistible; but the numbers just 
> don't add up. Over a day their stability is in the low e-12's vs. a 
> "real" cesium clock like a 5071A in the low e-14's. So when you are 
> doing a relativity experiment trying to detect a frequency shift 
> that's on the order of e-13's you reach for a 5071A instead of a CSAC. 
> The performance is nearly 100 to 1.
>
> One solution is a taller mountain. The best on the planet is Mauna Kea 
> (Big Island, Hawaii) where you can literally drive from sea level to 
> the summit (13,800 ft, 4200 m) in a few hours. The frequency shift up 
> there is 4.5e-13, which is 40 ns per day. But still, to have even the 
> slightest chance of success you'd want your clocks to be good to 1e-13 
> or better. CSAC aren't even close, and probably neither are telecom Rb.
>
> I'm currently involved with another solution -- a HAB (High Altitude 
> Balloon) CSAC flight. Getting to 100,000 ft altitude is quite common. 
> Up there, clocks run a whopping 3.3e-12 faster, which is 280 ns/day, 
> or 12 ns/hour. This is a clear case where the amazing low mass and low 
> power of a CSAC is a  critical advantage. However, the numbers still 
> aren't working out and the logistic and environmental conditions are 
> brutal. I won't say it's impossible, but it may take years and a huge 
> bag of tricks before it works or it's proved too impractical.
>
> ---
>
> Jim, I'd be interested in any Cubesat / CSAC results. They don't 
> exactly land in one piece so the typical round-trip clock comparison 
> method wouldn't work. A direct frequency comparison might. In that 
> case the drift and re-trace specs of a CSAC are probably more 
> important than the stability.
>
> /tvb


The CHOMPTT folks were trying to do time transfer using optical, but 
they also flew a CSAC (maybe even two).   One problem is that "what do 
you compare to", as you noted.  One could compare to on board GPS 1pps 
or to an onboard OCXO.  Both the CSAC and the OCXO would speed up 
relative to surface. But you also have the velocity problem (7 km/s) so 
they "apparently" run slow.  I don't know that CSAC vs GPS would 
actually be able to do the measurement - the uncertainty in the GPS is 
perhaps too high.  Maybe with post processed GPS - GIPSYX/RTGx should 
give you position and time to <1ns.