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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Sat, 19 Apr 2025 17:15:42 +0200
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On 19/04/2025 09:46, Janis Papanagnou wrote:
> On 17.04.2025 17:56, David Brown wrote:
>> On 16/04/2025 02:53, James Kuyper wrote:
>>> On 4/15/25 18:56, Keith Thompson wrote:
>>> ...
>>>> The uncertainty in the timing of January 1, 1970, where 1970 is a
>>>> year number in the current almost universally accepted Gregorian
>>>> calendar, is essentially zero.
>>>
>>> Modern Cesium clock are accurate to about 1 ns/day.That's an effect
>>> large enough that we can measure it, but cannot correct for it. We know
>>> that the clocks disagree with each other, but the closest we can do to
>>> correcting for that instability is to average over 450 different clock;
>>> the average is 10 times more stable than the individual clocks.
>>>
>>> Note: the precision of cesium clocks has improved log-linearly since the
>>> 1950s. They're 6 orders of magnitude better in 2008 than they were in
>>> 1950. Who knows how much longer that will continue to be true?
>>>
>>
>> I don't think cesium is still the current standard for the highest
>> precision atomic clocks.
> 
> Well, the "Cesium _fountain_" atomic clocks are still amongst
> the most precise and they are in use in the world wide net of
> atomic clocks that are interconnected to measure TAI.[*]  And
> the standard second is _defined_ on Caesium based transitions.
> 

Caesium fountain clocks are old school, but still used.  Rubidium is 
popular because it is cheaper, and very high stability atomic clocks use 
aluminium or strontium.  Caesium is still the basis for the current 
definition of the second, but that will change in the next decade or so 
as accuracy of timekeeping has moved well beyond the original caesium 
standard.

>> But anyway, the newest breakthrough is thorium
>> nuclear clocks, which IIRC are 5 orders of magnitude more stable than
>> cesium clocks.  (And probably 5 orders of magnitude more expensive...)
> 
> I've not heard of Thorium based clocks. But I've heard of
> "optical clocks" that are developed to get more precise and
> more stable versions of atomic clock times.
> 

It was only last year that a good measurement of the resonant 
frequencies of the Thorium 229 nucleus was achieved - the science bit is 
done, now the engineering bit needs to be finished to get a practical 
nuclear clock.