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I answered a question Why periodic motion as a clock? to which @SolomonSlow made two interesting comments one of which was,
Yes, it's nice that caesium-based clocks are extremely precise, but that's only half of the story. The other half of the story is that anybody can build one without the need to compare theirs against anybody else's. Every caesium atom behaves the same as every other caesium atom. If I build a caesium clock, I can feel confident that my oscillator beats at the same frequency as yours even though we live on opposite sides of an ocean, and we have never met each other..

And as a result @Steve commented as follows,
Caesium clocks vary like all others do. A simple pendulum clock offers the same guarantees as you're talking about (that they will be beat equally if built to the same specification, net of any ongoing environmental influence), albeit to a lower level of precision generally due to their size. It's for this reason that networks of clocks are used and the averages taken.

So asking one question but in two parts.

I would like to know, (a) If one made a Caesium clock following whatever recipe there is for manufacturing one but initially without recourse to comparison with an accepted "standard clock, how accurate would it be compared to the accepted "standard" clock"s"?
(b) Wikipedia has a List of atomic clocks. How and when are these clocks compared

Farcher
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2 Answers2

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The different institutions record the time that their clock reads at some pre-defined event, and then they compare those readings.

Per the Wikipedia page on the topic, a common technique for such synchronization is to use a satellite signal such as those from GPS. The atomic clocks cannot trust GPS for determining what time it is (that would be a chicken and the egg problem, as GPS relies on those clocks). However, they can rely on it for a globally agreed upon event in time.

Cort Ammon
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The instability of atomic clocks - in the sense that their progression is influenced by their environment - is well-known.

The Hafele-Keating experiment showed this, for example, where they flew atomic clocks in different directions around the Earth, and the clocks came back with different readings. The point of that experiment was to show relativistic influence on the atomic clocks - perhaps then more a showcase of 1970s atomic clock technology, rather than resolving a serious question of theoretical physics.

The issue is that people like me say "that's instability, that's environmental influence!", whereas the physicists say "oh no, that clock is perfectly stable and constant, it's just that the progression of time wasn't stable for each clock".

The issue there really is that simple.

I could have two atomic clocks on the floor, and then lift one with my own two arms above my head, and when they proceed at different rates a physicist will swear they are both proceeding at the same constant rate, the difference being entirely that time passes more quickly above my head than at my feet.

The average person might well conclude that it is a distinction without a difference, and that the physicist is merely preferring an obtuse explanation fitted around the bizarre commitment to a "stable" clock, rather than the everyday explanation of an unstable clock whose progression is subject to certain kinds of environmental influence (like every other clock we know).

Steve
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