[time-nuts] Time Dilation tinkering
Bob Camp
kb8tq at n1k.org
Tue Mar 21 07:28:25 EDT 2017
H
> On Mar 21, 2017, at 4:58 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>
> Hi Hugh,
>
>> If I do the math correctly that's about 14ns difference per 24h the
>> clocks are separated by that altitude. [1]
>
> That's correct. For your 1500m elevation gain, the gravitational redshift, the df/f frequency change, will be about 1.6e-13. To be able to measure with any confidence you'll want your clocks to be stable to about 2e-14, at tau 1 or 2 days.
>
>> This got me to wondering if a Rubidium based standard might do the trick
>> - the Efratom SLCR-101s seem readily available for ~USD$200 mark.
>
> There are tricks and technical factors, but the main one is how stable these are at tau 1 day. Buy or borrow a few of them and see if they are mutually stable to the level required. I suspect not. But perhaps other time nuts could comment on how stable their surplus Rb are out to tau 1 day.
>
> There are lots of other details; some to your advantage, some not. But if the surplus Rb can't perform down in the -14's at a day, even in laboratory conditions, then the rest of the discussion doesn't matter.
An ex-telecom Rb will run a bit below 1x10^-12, but not below 1x10^-13 at tau = 1 day. Roughly speaking it’s about 10X less stable than you need. That’s in a carefully controlled temperature and pressure environment. One fake out with Rb’s is that they are pressure sensitive. You *will* see an impact simply from the lower pressure on the top of the mountain. How much impact varies from unit to unit. It’s a good bet you need to compensate for it well before you get to 1x10^-13 on a mountain trip.
To put this all in context, a 5071 in good condition can (barely) do this experiment by running up to a mountain top. There’s a lot of fiddly details you need to take care of even with a 5071. AFIK, Tom is the first to do it this way (Cs in the back of the family car). All previous work with older Cs standards had to go to much greater lengths to observe the effect. If you take a standard up to satellite orbit sort of altitude, you get a shift of ~1.5x10^-10. That would be fine for a number of frequency standards. It is not very practical on the transport side of the experiment.
Many options, but none of them easy.
Bob
>
> Yes, the TAPR TICC counter would work well for this experiment. But to be honest, any old nanosecond-level counter is good enough. I say this not to discourage you from a good excuse to buy a TICC, but to encourage you to do the ADEV math to see how clocks and counters and tau can interact in your favor.
>
> /tvb
>
> ----- Original Message -----
> From: "Hugh Blemings" <hugh at blemings.org>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Monday, March 20, 2017 7:38 PM
> Subject: [time-nuts] Time Dilation tinkering
>
>
>> Hi,
>>
>> I've been mostly lurking on the list for some time now and follow with
>> interest the many discussions. Very much at the early stages of my
>> time-nut journey, but enjoying it so far :)
>>
>> I'd like to have a go at re-creating the efforts of Tom (and I gather
>> others) in taking a clock up a mountain for a while and seeing if I can
>> measure the relativistic changes.
>>
>> Being based in Australia gives me a couple of challenges, for one we
>> don't really do mountains in the same sense as much of the rest of the
>> world - so the highest peak I can readily get to from Melbourne is about
>> 1,600m ASL. I live at 80m ASL - so a delta of around 1,500m altitude
>> and several hundred km drive.
>>
>> If I do the math correctly that's about 14ns difference per 24h the
>> clocks are separated by that altitude. [1]
>>
>> We also lack quite the same surplus market here as the US, so purchasing
>> a Caesium based standard is well beyond my means.
>>
>> This got me to wondering if a Rubidium based standard might do the trick
>> - the Efratom SLCR-101s seem readily available for ~USD$200 mark.
>>
>> Clearly there'd need to be a bunch of extra gubbins [2] added to the
>> 10MHz standard to turn it into an actual clock/counter including battery
>> backup and so forth. And would need a pair of everything.
>>
>> Before I delve too far into the planning, I'd be interested in feedback
>> as to whether this style of Rb standard is likely to be up to the task
>> of being the core of such an endeavour or not ?
>>
>> Oh I should add - my plan was to build the systems such that they
>> function as nice standalone time/frequency references once this
>> experiment is concluded :)
>>
>> Thanks in advance,
>>
>> Kind Regards/73,
>> Hugh
>> VK3YYZ/AD5RV
>>
>>
>>
>> [1] gh/c^2 x 3600 x 24 Where h is 1500, g and c the usual values :)
>>
>> [2] I presume at a minimum a counter running at a 5ns or less "tick" fed
>> from a frequency source locked to the 10MHz of the Rb standard. This
>> counter would need to be latched for reading from an external signal so
>> that it can be compared to the second clock. Not sure but seems the
>> TAPR TICC might have role here :)
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