Comments about "Atomic clock" in Wikipedia

This document contains comments about the article Atomic clock in Wikipedia
In the last paragraph I explain my own opinion.

Contents

Reflection


Introduction

The article starts with the following sentence.
Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS
The issue is how accurate an Atomic clock is as a function of its speed.
Currently, the most accurate atomic clocks first cool the atoms to near absolute zero temperature by slowing them with lasers and probing them in atomic fountains in a microwave-filled cavity.
That is a technical highstand. The issue is what happens which such atomic clocks when you move them at 0.5*c.
The accuracy of an atomic clock depends on two factors. The first factor is temperature of the sample atomsócolder atoms move much more slowly, allowing longer probe times. The second factor is the frequency and intrinsic width of the electronic transition. Higher frequencies and narrow lines increase the precision.
See

1. History

2 Mechanism

3. Power consumption

4 Evaluated accuracy

5 Research

5.1 Secondary representations of the second

5.2 Quantum clocks

In March 2008, physicists at NIST described a quantum logic clock based on individual ions of beryllium and aluminium.
It's exact workings is important.
The accuracy of experimental quantum clocks has since been superseded by experimental optical lattice clocks based on strontium-87 and ytterbium-171.
The same comment as above.

5.3 Optical clocks

These clocks are based on optical rather than microwave transitions.
That means frequencies in the range of vissible wavelengths.

5.4 Clock comparison techniques

These 4 European labs are developing and host a variety of experimental optical clocks that harness different elements in different experimental set-ups and want to compare their optical clocks against each other and check whether they agree.
In some sense all these 4 clocks are at rest.
It is much more interesting that they do not agree and then study why they don't agree

6 Applications

The development of atomic clocks has led to many scientific and technological advances such as a system of precise global and regional navigation satellite systems, and applications in the Internet, which depend critically on frequency and time standards.
No Comment.

6.1 Global Navigation Satellite Systems

GPS Time (GPST) is a continuous time scale and theoretically accurate to about 14 ns.
See: Reflection 1 - 'The Science of Time keeping' by HP
However, most receivers lose accuracy in the interpretation of the signals and are only accurate to 100 ns.

6.1.1 System under construction

6.2 Time signal radio transmitters

7. See also

Following is a list with "Comments in Wikipedia" about related subjects


Reflection 1 - Can atomic clocks be used to test time dilation (SR).

Inorder to explain SR and time, the book "SpaceTime Physics" by E.F. Taylor and J.A. Wheeler (First or Second Edition), they use a clock based on light flashes. These light flashes bounce back and forward between two mirrors. That means the inner working of the clock is an oscillator which has a base frequency.
The problem starts when you move such a clock. The light path between each tick (or count) becomes longer, the number of ticks decreases. The frequency of the decreases accordingly.

My understanding is that an atomic clock has the same problems.
The importance of an atomic clock is a hugh improvement for time keeping on earth, but has the same mechanical problems as the rather artificial clock in the book "SpaceTime physics". Both are subject of the complications introduced by the Earth, that gravitation is involved and that the clock is not at rest when used continuously.


Reflection 2 - The Science of Time keeping by HP

To get a copy of this document select http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
At page 31 we read:
Since USNO has been successful in predicting UTC to within about 10 ns, combining these two independent error sources yields a real-time potential uncertainty for UTC available from GPS at about the 14-ns level.
At page 37 (at bottom) we read:
Ideal clocks at rest anywhere on the rotating geoid will tick at the same rate.
At page 55 we read:
Appendix A
Time and Frequency Measures Accuracy, Error, Precision, Predictability, Stability, and Uncertainty
This is the most interesting section.
The most important question to answer for each is the cause.
At page 66 (Appendix B) we read:
A pendulum clock will not work at sea because its time uniformity is dependent upon the regular motion of a swinging bob.
This explains why a pendulun clock has it's limitations. At the same time it underscores my opion that you have to study the innerworkings of a clock in order to understands its behaviour under different circumstances, like speed.


Reflection 3

http://www.exphy.uni-duesseldorf.de/optical_clock/Motivation.php Why testing General Relativity (GR) and Special Relativity (SR)?
Since 1983 the definition of the meter is based on the velocity of light.
In some sense this is strange because the length of a meter has nothing to do with the speed of light.
Of course, all other derived units are influenced by this definition.
Correct.
The uniqueness of this definition rests on the constancy of the speed of light and the universality of the influence of the gravitational field (which is always present) on the ticking rate of clocks.
When you declaire the speed of photons physical constant it means that nothing can influence this speed. In fact you take the easy route because this is very difficult to test.
A gravitational field can influence both: the movement/speed of the photons and the movement of the clock.
It is clear that experiments should confirm these facts in order to extend the foundations in metrology in view of future progress.
Correct.


Reflection 4


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Created: 4 July 2018

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