[time-nuts] nubie querie

J. Forster jfor at quik.com
Wed Mar 10 22:15:03 UTC 2010


This thread mentioned puilsars and the best clocks. Here are some comments
from those really in the know:

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Latest Al+ clock comparision at NIST is at better than 10^-17 level see.
http://arxiv.org/abs/0911.4527

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> >> richard at karlquist.com said:
> >>> I vaguely remember reading that pulsars have some fantastic stability
> >>> like 1E-20. [snip]

> > Pulsars decelerate due to
> > electrodynamic drag -- the reaction torque from the pulsar's external
> > magnetic field trying to spin the plasma surrounding the pulsar, and
> > generating MHD waves in this plasma. Because the plasma density
> > varies randomly, the drag torque varies randomly. Some pulsars are
> > subject to other torques, for example because they have close binary
> > companions.
> >
> > Even in the absence of external torque, some pulsars change angular
> > velocity abruptly because their moments of inertia change when
> > "starquakes" (like earthquakes, but in the neutron crystalline solid
> > body of the star) occur.
>
> > In early 1969, shortly after the Crab Nebula pulsar (the first so-
> > called "millisecond" pulsar, with a rotation rate of of about 30 revs
> > per second) had been discovered, and (IIRC) before a starquake had
> > been observed, we began observing this pulsar at Arecibo Observatory
> in Puerto Rico.
>
> > In one daily observation that
> > involved about an hour of averaging, we could determine the pulsar's
> > rotation phase angle with precision of about 50 microradians. (This
> > was after removing the time-varying propagation group-delay due to
> > plasma between us and the pulsar. To distinguish the plasma delay, we
> > observed the pulsar concurrently at radio frequencies ranging from 40
> > MHz to 430 or 611 MHz.) Within a few months, we found that the pulsar
> > was a rotten clock relative to the Observatory's H-P Cesium-beam-
> > referenced clock, which we checked daily against the Loran-C ground-
> > wave (over sea water, actually) signal from Jupiter, FL.
>
> > Since then, other pulsars have turned out to be more stable (and
> > others less so). AFAIK, none has yet beaten a good atomic clock.
> >
> > Pulsar PSR1937+21, discovered in 1982, attracted attention because its
> > rotation rate was about 600 revs per second. (So it was more
> > deserving of the title, "millisecond pulsar.) In its first two years
> > of being observed, it did not have a "glitch" or starquake. In two
> > years, about 2 x 10^11 rotations were observed, and the rotation phase
> > had been determined within about about 1/160th of a revolution, so the
> > average spin rate was known within about a part in 10^13. However, in
> > the next twenty-five years, its spin rate on a time-scale of about one
> > year turned out to be no more stable than about 1 in 10^13. AFAIK.
> > It's been a few years since I read anything about this pulsar.
> >
> > Millisecond pulsars are more stable than slower pulsars, and hope has
> > been expressed that, when thousands or tens of thousands of these
> > pulsars have been discovered and can be observed nearly continuously,
> > the ensemble of this many pulsars will allow establishment of a pulsar-
> > based standard of time having stability, on a time-scale of about one
> > year, of a few parts in 10^15. AFAIK, such a standard remains a
> > dream, not a reality.

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