# What is a clock?

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How do we decide what is a clock and how do clocks that rely on different technologies stay synchronized?

## Answers and Replies

Ibix
Science Advisor
2020 Award
Something that repeats regularly is a clock. You may wish to add a counter. They stay synched (leaving aside relativistic effects and noise) because that's what "repeats regularly" means.

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russ_watters
Mentor
Physicists seem to use the circular definitions:
-Clocks are devices that tell time.
-Time is what a clock measures.

I add the fact that "time" works similarly to other dimensions, so it is treated as a dimension...and describe that: Time is a non-spatial dimension that forms a continuum, separating events.
https://www.merriam-webster.com/dictionary/time

Regarding how we synchronize clocks of different technologies: At this point, the scale of time is defined to fit the caesium clock. The actual value of how many shakes equals 1 second is of course arbitrary.

How do we decide what is a clock

If the output of two or more devices comply with Einstein's clock synchronisation, the devices are a clocks and their output is a measure of time.

PeterDonis
Mentor
2020 Award
If the output of two or more devices comply with Einstein's clock synchronisation, the devices are a clocks and their output is a measure of time.

This is certainly a sufficient condition, but it's much too strong to be a necessary condition. Two clocks in relative motion in flat spacetime don't meet the condition, but that does not mean they can't be considered clocks.

hilbert2
Science Advisor
Gold Member
How do we decide what is a clock and how do clocks that rely on different technologies stay synchronized?

If you have two processes that are based on completely different mechanisms, and seem to be periodic with ##n## periods of process 1 taking the same time as ##m## periods of process 2, then it's an even stronger proof that both processes are actually periodic than two similar mechanisms behaving in that way. It's unlikely that the period of two different kinds of processes would happen to be changing at the same rate.

PeroK and Ibix
Paul Colby
Gold Member
A clock is just like a meter stick but for time intervals instead of distances.

Two clocks in relative motion in flat spacetime don't meet the condition, but that does not mean they can't be considered clocks.

Yes, they can be clocks. But how do you check that?

Nugatory
Mentor
Yes, they can be clocks. But how do you check that?
First you use each clock to establish a length standard usable when that clock is at rest (for example, the meter is the distance travelled by light in 1/299792458 second, using the cesium clock definition of the second). Then you calculate the proper time between two consecutive readings of one clock using that clock and the other clock. If these calculations agree to the limits of your measurement accuracy then they are good clocks.

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Ibix
Something that counts any events is clock. Of course you want clock to be precise. You want to control what you count. For example: You need to construct "tick" - which is event to count. Then end of your event is starting new event in loop. Or, another example - you know atomic decay law. "Tick" is not too reliable here, but statistick over atomic decay is reliable, and you can compute timeframe from that.

PeroK
I have a lovely pendulum clock on my wall. It is accurate to within a minute a week. The speed of the clock is regulated by the equivalence of inertial mass and gravitational mass. What about the other clocks? What do they depend on to be stable (lets assume we are not traveling at enormous speeds and we are all in the same gravitational field).

russ_watters
Mentor
I have a lovely pendulum clock on my wall. It is accurate to within a minute a week. The speed of the clock is regulated by the equivalence of inertial mass and gravitational mass. What about the other clocks? What do they depend on to be stable (lets assume we are not traveling at enormous speeds and we are all in the same gravitational field).
https://en.m.wikipedia.org/wiki/Caesium_standard

Why do I feel like I'm providing research for a paper?

Nugatory
Mentor
What do they depend on to be stable (lets assume we are not traveling at enormous speeds and we are all in the same gravitational field).
The "travelling at enormous speeds" part of this question is a red herring that you would do well to let go of. Everything is travelling at enormous speeds relative to something somewhere. For example, your pendulum clock works just fine (in the sense that I described in #9 above and to the limits of its accuracy) whether you or someone on Mars watching through a telescope and moving at a few kilometers per second relative to it is depending on it to measure time.

Where the stability comes from depends on the construction of the clock. Use an electronic LC circuit oscillator and you're depending on the laws of electricity and magnetism which govern the behavior of electronic circuits.... Use the slowly increasing length of your hair and fingernails, and you're depending on the biological processes that control their growth.... Use an ordinary bedside alarm clock plugged into a wall outlet and you're depending on your electrical utility's ability to provide 50 or 60 (depending on where you live) cycles per second, which in turn depends on classical physics applies to elextrical power generators.... And long ago we used sandglasses based on some faiurly complex Newtonian mechanics, sundials based on the Earth's motion through space, candles that burned at a more or less constant rate because the laws of chemistry are stable.

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russ_watters
https://en.m.wikipedia.org/wiki/Caesium_standard

Why do I feel like I'm providing research for a paper?

No not a research paper. Trying to get my own thoughts in some sort of a shape.

So in my clock one of the stabilizers is the inertial weight of the pendulum.

Does the inertial weight of the bits of the cesium atom provide stability to the atomic clock?

russ_watters
Mentor
Does the inertial weight of the bits of the cesium atom provide stability to the atomic clock?
No, weight/mass have nothing to do with the operation of an atomic clock.

Thanks Russ,

I like this:

Physicists seem to use the circular definitions:
-Clocks are devices that tell time.
-Time is what a clock measures.

No, weight/mass have nothing to do with the operation of an atomic clock.

From: http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/nspin.html#c1
"Nuclear Spin
It is common practice to represent the total angular momentum of a nucleus by the symbol I and to call it "nuclear spin". For electrons in atoms we make a clear distinction between electron spin and electron orbital angular momentum, and then combine them to give the total angular momentum. But nuclei often act as if they are a single entity with intrinsic angular momentum I. Associated with each nuclear spin is a nuclear magnetic moment which produces magnetic interactions with its environment. "

Surely the Spin or angular momentum - inertia - is an integral part of the mechanism to the hyperfine structure of the nucleus.

russ_watters
Mentor
Surely the Spin or angular momentum - inertia - is an integral part of the mechanism to the hyperfine structure of the nucleus.
I don't know. I'm pretty thin on QM. What does this have to do with the topic?

Paul Colby
Gold Member
Surely the Spin or angular momentum - inertia - is an integral part of the mechanism to the hyperfine structure of the nucleus.
I think what @russ_watters was getting at is the ratio of weight to mass (weight/mass) has nothing to do with atomic clocks, which is correct. Certainly the mass of the particles involved will effect the transition frequencies in an atomic clock. Current theory is these masses are fixed.

I don't know. I'm pretty thin on QM. What does this have to do with the topic?

The question is: What is it that stabilizes the clock rate? And does the inertial weight/mass of the bits form part of the stabilizing mechanism?

Grinkle
Gold Member
The question is: What is it that stabilizes the clock rate? And does the inertial weight/mass of the bits form part of the stabilizing mechanism?

Its not fundamentally a clock rate, it takes a human to interpret it as a clock. Its a stable periodic thing that can be counted, and that is why it us useful as a clock. Why it is stable, and whether the mechanism for its stability has anything to do with why your pendulum clock is stable is not part of what makes it a handy clock. Some people may say "that is not a good clock" and have their own subjective value judgements on what makes a good clock. They may value ease of observing over accuracy, for instance.

I think that is what @russ_watters means by saying you are now asking a different question.

jbriggs444
Science Advisor
Homework Helper
The question is: What is it that stabilizes the clock rate? And does the inertial weight/mass of the bits form part of the stabilizing mechanism?
As I understand the mechanism, the energy requirement to make the hyperfine transition is quite precise. One tunes a microwave beam accurately to maximize the rate at which the hyperfine transition is made and uses the associated frequency as the basis for a clock.

The stabilising "mechanism" to the extent that there is one is the fact the the hyperfine transition has a well defined energy level.

Empirically, the fact that atomic clocks agree with one another is a demonstration that the stability exists regardless of why it does.

russ_watters and Paul Colby
Ibix
Science Advisor
2020 Award
The question is: What is it that stabilizes the clock rate?
What would you consider to be an acceptable answer? You say earlier that the equivalence of gravitational and inertial mass stabilises your pendulum clock, but you don't justify that statementand I'm not at all sure it's correct. For example, I would think that I could take the clock into zero g, give the pendulum bob an electric charge, place it in a uniform electric field and expect it to work (or at least, I could design a pendulum clock for which that was true - too much metal in an off-the-shelf model). I don't immediately see a reason why it would be any more or less stable operating in an electric or gravitational field.

russ_watters
A layman's answer to the second part of the very first question - 'how do clocks that rely on different technologies stay synchronized?' It depends on the technology utilised to make the clock i.e. a mechanical clock, an electrical clock or an atomic clock. Each device has a built in regulator i.e. centrifugal weights, pendulums for mechanical clocks, pulses or crystals for electrical clocks and the movement of atoms for atomic clocks. Each can be adjusted until synchronised to a similar device. Devices of a dissimilar technology will not synchronise.

Ibix
Science Advisor
2020 Award
Devices of a dissimilar technology will not synchronise.
The Earth is a sort of clock - it rotates at a steady rate. A quartz watch is a sort of clock which vibrates a crystal at a steady rate. These are dissimilar technologies. Are you telling me you can't tell the time of day with a quartz watch?

russ_watters