F-ckin Leap Seconds, how do they work?

Wed Jul 4 01:13:15 UTC 2012

On 7/3/2012 4:15 PM, Tony Finch wrote:
> Vadim Antonov <avg at kotovnik.com> wrote:
>>
>> But in theory, if you can get the technical wrinkles worked out, you can
>> derive the same frequency standard in your lab with a single instrument.
>>
>> (One more issue is that non-relativistic time is not only the frequency of
>> oscillators, but also a reference point).
>
> Your parenthetical point explains why TAI does not tick at the same rate
> as the SI second in your lab, expecially if your lab is (for example) in
> Colorado. You have to adjust the frequency depending on your difference in
> gravitational potential from the geoid.
>
> Tony.
>

I'm afraid I didn't express my thoughts clearly... I means besides 
agreement of what a second is there is also an agreement on when the 
zeroeth second was, a fixed reference point in time. *That* cannot be 
recreated in a lab. (You can correct for relativistic effects of local 
gravity and moving frame of reference, though, to match conditions on 
the Earth and thus the SI definition of second).

However, the whole concept of universal standard of _time_ (as opposed 
to standard of second) is thoroughly non-relativistic because it claims 
to have clocks at different locations ticking simultaneously.  The 
special relativity, of course, makes it clear than simultaniety is in 
the eye of the observer:)  In the end, you can only do limited 
Einstein-Poincare synchronization within a chosen reference frame.

An interesting factoid: the notion of synchronized time differs if you 
synchronize clocks from East-to-West and from West-to-East, due to 
Sagnac effect:)

--vadim

PS. I would vote for using TAI instead of UTC as the non-relativistic 
time base in computer systems. The idea of expressing UTC as a single 
number (instead of <minute, second within minute> tuple) is silly 
because it creates aliases or gaps.  You cannot do simple interval 
arithmetic over UTC, no more than you can do that over local daylight 
savings time; and doing accurate time computation for events in the 
future is impossible in both because they depend on unpredictable 
factors (Earth rotation rate, politics, etc).

TAI is also not a fixed given, because the standards are being refined, 
but at least the refinements tend to be predictably in the direction of 
improved accuracy, so they don't break things.