NPR Story I heard this morning
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.
Cheers,
Magnus
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@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)?
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
On Mon, 3 Nov 2014 11:54:41 -0800
Peter Monta pmonta@gmail.com wrote:
Sorry if this is a bit off-topic. I'd like a simple, clear explanation for
the layman that drills down on exactly how the current definitional scheme
can be realized to arbitrary precision. For example, assume that we must
go off-earth at some point to get a better timescale. How fuzzy is the
solar potential ("soloid")?
It will be done as usual: As soon as they can reliably measure an systematic
effect that is impossible to cancel out, they will redefine or ammend the
definition of the second to account for this issue.
And going by the presentations given at EFTF this year, there is quite
some interest in precision gravity measurements in the time/frequency
community. And yes, they use the same basic phyiscs as their atomic clocks :-)
(one apporach is to let Cs atoms fall down a tube and measure their
acceleration using doppler shift of the hyperfine transitions line)
Attila Kinali
--
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
-- Sophie Scholl
On Tue, 4 Nov 2014 11:04:58 -0800
Peter Monta pmonta@gmail.com wrote:
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".)
Actually, currently the limit of clock comparison is not the geoid
uncertainties, but the comparison itself. Common view GPS does not
cut it at all. TWSTFT might be enough, if more second/third order
effects are compensated for and the orbit is measured with a higher
precision [1,2]. For "short" distances (up to 1000km or so), temperature
and delay-variation compensated fibers seem to be the way to go [3,4].
Also, frequency transfer down to very low numbers seems to be easier
(depending on system and distance in the range of 1e-12 to 1e-16)
than accurate time transfer (around a couple 100ps seems to be the limit
for any non-trival distance, at the moment)
[1] "Two-way Satellite Time and Frequency Transfer:Overview, Recent
Developments and Application", by Wu et al, 2014
[2] "Practical Evaluation of Relativistic Effects in Two-Way
Satellite Time and Frequency Transfer", by Shemar, Delva, Lamb, 2014
[3] "Optical Frequency Transfer with a 1284 km Coherent Fiber Link",
by Calonico et al, 2014
[4] "Novel Techniques for Optical Fiber Links beyond Current Practice",
by Calosso et al, 2014
And these are just a few of the presentation given at EFTF this year
on that topic.
Attila Kinali
--
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
-- Sophie Scholl
But won't the doppler effect change as the Cs atoms fall down the gravity
well? :)
On Tue, Nov 4, 2014 at 3:14 PM, Attila Kinali attila@kinali.ch wrote:
On Mon, 3 Nov 2014 11:54:41 -0800
Peter Monta pmonta@gmail.com wrote:
Sorry if this is a bit off-topic. I'd like a simple, clear explanation
for
the layman that drills down on exactly how the current definitional
scheme
can be realized to arbitrary precision. For example, assume that we must
go off-earth at some point to get a better timescale. How fuzzy is the
solar potential ("soloid")?
It will be done as usual: As soon as they can reliably measure an
systematic
effect that is impossible to cancel out, they will redefine or ammend the
definition of the second to account for this issue.
And going by the presentations given at EFTF this year, there is quite
some interest in precision gravity measurements in the time/frequency
community. And yes, they use the same basic phyiscs as their atomic clocks
:-)
(one apporach is to let Cs atoms fall down a tube and measure their
acceleration using doppler shift of the hyperfine transitions line)
Attila Kinali
--
I pity people who can't find laughter or at least some bit of amusement in
the little doings of the day. I believe I could find something ridiculous
even in the saddest moment, if necessary. It has nothing to do with being
superficial. It's a matter of joy in life.
-- Sophie Scholl
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.