On 01/12/2011 10:30, Steve Allen wrote:
On Wed 2011-01-12T16:36:35 +0000, Tony Finch hath writ:
Yes, but how accurately do you need clocks to track it? How frequently do
you need to make adjustments to correct for the atomic/angular rate error,
and what size of adjustment is acceptable?
It would appear that making adjustments every 10 days is not
often enough, at least in the US, viz:
http://www.nist.gov/pml/div688/grp50/NISTUTC.cfm
http://www.nist.gov/pml/div688/grp50/nistusno.cfm
Even if we abandon the leap second, we have issues at the nanosecond level.
These are the computed errors between the 'paper clock' that is UTC and
the various realizations of UTC. This data shows an error of about
.2ns/day (give or take) over the periods listed. This is an error of
about 2.5e-15. This likely corresponds to the accuracy of the cesium
standards used by NIST to realize time, since high precision 5071A's are
good to something less than a nanosecond per day.
This is several orders of magnitude smaller than the UT deviation from
86400 SI seconds, which is on the order of 1ms per day with variations
on the order of 1ms over time (The 10 day estimates are in the 10's of
microsecond range as far as accuracy).
For instance, what authority will historians or lawyers consult to learn
the applicable timezone offsets that were in force in some location(s)
during some epoch(s) in question?
That problem exists whether universal time is atomic or angular so it
makes no difference to the proposal.
When the leap second was invented there were countably few systems which
could count every second, so a second was not a problem. Now it is.
Right now there are countably few systems which can count every nanosecond.
Unless there is some sort of conceptual barrier which prevents a need
for nanoseconds, when such systems do become common the problem of
historic time zone offset reconciliation will be trivial by comparison
to the issues of systems which believe that nanosecond (picosecond)
synchronization is possible without table lookups and continuous
effort to track the table values.
Most phones need to be synchronized to the microsecondish level, so the
day is coming when that will be more of an issue.
But keep in mind that when you get down to the nano-second level, all
those crazy things you learned about in physics start to matter. You
have relativistic effects between different frames of reference. You
have gravitational effects if you change altitude from sea level, you
even have Sagnac effects due to the rotation of the earth, etc. All of
these effects can cause changes in time elapsing at the nanosecond
level. I think puts a practical limit on how closely clocks can and
will be synchronized and syntonized.
Of course, reading this now, I may be suffering from the 'Nobody will
ever need more than 640k of RAM in their personal computer' argument.
Warner
Abandoning leap seconds simply sweeps the need for good timekeeping
practices under a rug rather than giving ongoing incentive to design
systems which match the way chronometers actually work.
--
Steve Allen<[email protected]> WGS-84 (GPS)
UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855
University of California Voice: +1 831 459 3046 Lng -122.06015
Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
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