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.
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