[time-nuts] NPR Story I heard this morning

Bob Bownes bownes at gmail.com
Tue Nov 4 14:27:26 EST 2014

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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