[time-nuts] NPR Story I heard this morning

Magnus Danielson magnus at rubidium.dyndns.org
Tue Nov 4 14:57:34 EST 2014

I wish I could take the credit for being evil here, but no.

What the natural consequence is that every atomic clock of this type 
should have a gravitational sensor that compensates for gravitational 
shift, as it now has become a frequency shift component. The first 
degree compensation should not be too shabby.


On 11/04/2014 08:27 PM, Bob Bownes wrote:
> You people are evil. Now you have me wondering where I can get a microgram
> level accurate scale. Simply tracking the weight of a 'constant' (anyone
> got a silicon sphere with exactly 1 mole of Si atoms in it? :)) over time
> would be an interesting experiment.
> As a geologist, I also have to say, that while we know the geoid to ~1cm,
> it is ~1cm at the time it was measured, which is constantly changing. The
> obvious tidal effects, as well as internal heating effects (and I suspect
> external heating effects), continental drift (both long term events and
> short term events like earthquakes), currents in the molten layers,
> probably magnetic effects all are going to contribute to geoid uncertainty.
> I really do need to spin the seismograph back up.
> On Tue, Nov 4, 2014 at 2:04 PM, Peter Monta <pmonta at gmail.com> wrote:
>> Hi Tom,
>>> Based on mass and radius, a clock here on Earth ticks about 6.969e-10
>>> slower than it would at infinity. The correction drops roughly as 1/R
>> below
>>> sea level and 1/R² above sea level. For practical and historical reasons
>> we
>>> define the SI second at sea level.
>> Yes, the change in clock rate at sea level is about 1e-18 per centimeter,
>> and the geoid is known only to about 1 centimeter uncertainty at best.
>>> The non-local gravity perturbations you speak of are 2nd or 3rd order and
>>> so you probably don't need to worry about them. Then again, if you want
>> to
>>> get picky, it's easy to compute how much the earth recoils when you stand
>>> up vs. sit down. So it's best to avoid the notion of "arbitrary"
>> precision;
>>> that's for mathematicians. For normal people, including scientists, we
>> know
>>> that precision and accuracy have practical limits.
>> Let me rephrase what I'm after.  The geoidal uncertainty sets a hard limit
>> on clock comparison performance on the Earth's surface (for widely-spaced
>> clocks).  At some point, as Chris Albertson noted, the clocks will measure
>> the potential and not the other way around.  (It should be possible to
>> express this geoidal uncertainty as an Allan variance and include it in
>> graphs with the legend "Earth surface performance limit".)
>> What I'm curious about is this:  what are the limits on clocks in more
>> benign environments?  How predictable is the potential in LEO, GEO,
>> Earth-Sun L2, solar orbit at 1.5 AU, solar orbit at 100 AU, etc.?  I
>> imagine the latter few are probably very, very good, because the tidal
>> terms get extremely small, but how good?
>> Suppose a clock dropped into our laps with 1e-21 performance, just to pick
>> a number.  Where would we put it to fully realize its quality (and permit
>> comparisons with its friends)?  And is the current IAU framework adequate
>> to define things at this level (or any other arbitrarily-picked level)?
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