[LEAPSECS] the big artillery

[Warner Losh]

On Nov 6, 2014, at 5:55 AM, Zefram <zefram at fysh.org> wrote:

> Warner Losh wrote:
>> On Nov 5, 2014, at 10:54 AM, Zefram <zefram at fysh.org> wrote:
>>>   TAI(k) = TAI + (UTC(k)-UTC) = UTC(k) + (TAI-UTC)
>> 
>> Except that's not how others define it.
> 
> Michael Deckers has now pointed at ITU Rec TF.536-2 which defines "TAI(k)"
> in the same way as I do.  What conflicting definitions do you see?

The recommendation has been withdrawn. The conflicting definitions I’ve seen
have been from one of the time scientists that helped to setup TAI when he was
at NBS(later NIST) who strenuously instructed me that they weren’t equivalent and
was quite patient with my stupid questions about “why not". I did this research
about 10 years ago now, so I don’t have pointers to the original sources. Most likely
it was on the English pages of the BIPM’s web site in the 2003 or 2004 time
frame. Not ideal, I know.

> Anyway, this isn't about the notation, it's about the concept.

The concept I’d agree with you. But without a realization of the time scale, it
doesn’t actually exist. I know that I’m being picky here about the difference
between realizing TAI directly and deriving it from some realization of UTC.
And for most uses, I’d totally agree that the answers you get from doing this
will be the same. But if you zoom in far enough, you’ll see there’s a lot more
chaos than that going on, and that TAI and UTC(k) aren’t quite the same thing
when you get to the nanosecond level or beyond. TAI and UTC are, but nobody
has UTC, they only have UTC(x). The folks from BIPM are worried that if people
are talking about TAI time, and measurements against it, confusion will result in
the atomic time clock time scale community about what the measurements really
mean.

As a practical matter, I don’t see the problem with people using the TAI numbering
of seconds. Or even using the informal definitions of “The TAI time I got from UTC.”
But to the folks that set up this system, such a suggestion is rings in their ears like
‘you could just pound the screws in with a hammer’. Most of the time that works
well enough, for a while, but it isn’t quite the right thing to do. But since they control
the recommendations, to a degree, they get to make the rules.

Warner
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