# B What is a clock?

1. Nov 7, 2017

### Torog

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

2. Nov 7, 2017

### Ibix

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.

Last edited: Nov 7, 2017
3. Nov 7, 2017

### Staff: 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.

4. Nov 7, 2017

### DrStupid

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.

5. Nov 7, 2017

### Staff: Mentor

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.

6. Nov 7, 2017

### hilbert2

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.

7. Nov 7, 2017

### Paul Colby

A clock is just like a meter stick but for time intervals instead of distances.

8. Nov 7, 2017

### DrStupid

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

9. Nov 7, 2017

### Staff: Mentor

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.

Last edited: Nov 8, 2017
10. Nov 7, 2017

### MarekKuzmicki

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.

11. Nov 7, 2017

### Torog

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).

12. Nov 7, 2017

### Staff: Mentor

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

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

13. Nov 7, 2017

### Staff: Mentor

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.

Last edited: Nov 8, 2017
14. Nov 8, 2017

### Torog

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?

15. Nov 8, 2017

### Staff: Mentor

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

16. Nov 8, 2017

### Torog

Thanks Russ,

I like this:

17. Nov 13, 2017

### Torog

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.

18. Nov 13, 2017

### Staff: Mentor

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

19. Nov 13, 2017

### Paul Colby

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.

20. Nov 13, 2017

### Torog

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?