# How is one second measured?

1. Apr 4, 2015

### CAH

hey!
What if we got rid of all the clocks around the universe, how would be remesure a second?

2. Apr 4, 2015

### nuuskur

It was something about a Cesium atom flashing 9 billion something times and that period is defined to be a second.

3. Apr 4, 2015

### jbriggs444

The definition of a second is: "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom."

That is the definition because it is what an atomic clock measures. So the answer to the question is: You would measure a second by building a new atomic clock.

4. Apr 4, 2015

### Buckleymanor

Eventualy, first you would have to decide what to base your measurement on.So back to basic principles a year the time it takes for the Earth to travel once around the Sun then a month, a week followed by a day (one Earth rotation)..Then divide your your day up into usefull portions lets say 24 and then call them hours devide them into minutes and the minutes into seconds.You then find that fluctuations in the Earth's rotation make the measurement unreliable for certain applications so you then build a new atomic clock based as close as possible on one of Earth's rotation and it's sub-divisions.

5. Apr 4, 2015

### Blackberg

Sundials. Hourglasses. Stargazing. Divisions by 24, and 60. You'd get there.

6. Apr 4, 2015

### Staff: Mentor

We may be risking confusing people by bringing the movement of the heavenly bodies into this discussion. There was a time long ago when we derived our units of time from the earth's rotation, the sun rising and setting, the seasons coming and going, and the earth circling the sun, but those days are long gone. The modern definition of the second is, as jbriggs444 says above, "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom." That definition works anywhere that you can find a cesium atom; we could communicate it to an alien civilization in a distant galaxy that has never heard of the earth and knows nothing of its orbital and rotational periods.

7. Apr 5, 2015

### Drakkith

Staff Emeritus
Without some kind of clock, we wouldn't be able to measure a second. This doesn't change how a second is defined, as explained above, but without some device to refer to we couldn't accurately measure a second. (Unless you consider going "one one-thousand, two one-thousand..." accurate enough)

8. Apr 5, 2015

### Buckleymanor

Totally disagree, 9 192 631 770 might well the period of transition between two hyperfine levels of the ground state of the cesium 133 atom.That in itself means nothing though without putting it into context . So you communicate to an alien civilisation that 1 second is equal to 9 192 631 770 vibrations of a cesium atom and they reply why!

9. Apr 5, 2015

### sophiecentaur

Not sure I agree with looking at it that way. It would be more likely to communicate 'time' in any message we were to send in terms of cycles of a particular atomic transition. There would be little point in introducing the second into the message. It would be like talking about mass and length on this forum using pounds and feet.

10. Apr 5, 2015

### Buckleymanor

Can see your point but in the begining there was pounds and feet it wasn't till metres and kilo's became a more usefull tool that they were adopted.You could argue that if it were not for pounds and feet there would be no metres and kilo's and then again there might have been kilos and metres without the invention of pounds and feet .However it's all part of the rich tapestry of the history of physics and I suppose we would be all be that much poorer if we ignored it and came to the conclusion that it is what it is because it is.

11. Apr 5, 2015

### Staff: Mentor

We wouldn't even mention the second or that 9172631770 number in the conversation. They'd ask "What's your unit of time?" and we'd reply "cesium atoms, one cycle is one unit", and we'd be done (almost - there are other issues to work through, but they're a digression here).

Original poster's question, however, is a bit different. He was asking how we would reestablish the second if every existing clock were to be destroyed or otherwise gotten rid of. Jbriggs444 answered that question above: we'd find a cesium atom and start counting cycles.

After hearing that answer, the original poster might reasonably ask "but why 9192631770?". The answer is that that value was chosen to minimize the amount of retooling that would be required. A wristwatch that sometimes ticks after 9192620000 cycles of a cesium atom and other times after 9192640000 cycles (which is pretty good accuracy for most practical purposes) didn't suddenly become obsolete when we adopted the cesium standard.

12. Apr 5, 2015

### sophiecentaur

Or, of course, if we happened to be on a distant planet who's rotation period we happened not to know and needed to start from scratch, with no basic equipment - just some ol' books.

13. Apr 5, 2015

### Buckleymanor

That's all very good but why should you trust the 9192631770 cycles of a cesium atom surely you have only arrived at that standard from the breakdown of a day or a wristwatch that only ticks after 9192620000 cycles.So if you were starting from scratch you would have to check it.

14. Apr 5, 2015

### sophiecentaur

It's not a matter of "trusting", surely. The only trust involving is to rely on your friend, the other end of the line, having the same conditions as you do for his reference atomic oscillator. That will give you both the same standard 'second'. The accuracy that you can work to is potentially far greater than just the ticks of your one second clock mechanism. You can measure tiny fractions of the tick interval and get the phase slope and, hence the frequency accuracy can be as good as your measurement time will allow. To get the best reliability, you would need to do the usual thing of averaging over a long period of time

15. Apr 5, 2015

### jbriggs444

Check it? The second is 9192631770 cycles of the radiation corresponding to the hyperfine transition of the ground state of the cesium atom by definition. You don't check the second against the other (non-existent or broken by hypothesis) clocks. You check those other non-standard clocks against the standard definition.

If you were starting from scratch you could define a new unit of time, call it the "fuzzletynobbit" and say "to hell with the second". But that was not the challenge posed in post #1 above.

16. Apr 5, 2015

### dextercioby

The question in the original post is not trivial, but rather interesting if reformulated: if one were to suddenly 'lose' all clocks in the universe, thus the ability to measure time at microseconds to years scale, how would one measure the energy of the photon released when the cesium atom undergoes a hyperfine transition for its chemical-bond electron?

17. Apr 5, 2015

### sophiecentaur

That's not how it's done- i.e. you don't just look at individual photons. You use a cell containing the gas and build an oscillator that interacts with a vast number of atoms in the cell at the appropriate transition frequency. It's effectively a resonance you are using (as with an atomic absorption line). By keeping the cell under the right conditions of temperature, pressure etc. the error in finding the frequency of this resonance can be reduced to the level that's acceptable for that standard.
I don't know the details of building a frequency standard or why particular elements have been used over the years but you could google something like 'frequency standard technology' and find some article to suit your level of understanding. Needless to say, you will find some articles that will totally flummox you (and me!) but there will be some that make sense to you.

18. Apr 6, 2015

### Buckleymanor

As Sophie points out you have to average out the time period over a long period of time.The longer the more accurate the definition of the second untill other influences make the reading less accurate.All this averageing though is tied indirectly to your broken hypothetical clocks.If it was not then your fuzzletynobbit might well end up having 1234567890 cycles and why not! Because your clock alarm would be waking you to go to work in the middle of the night when you are not a shift worker.Time and the second have been intricately linked to the rotation of the planet for good reasons.Inventing a new clock whatever it timeing mechanism has to improve and keep those links intact.

19. Apr 6, 2015

### Buckleymanor

Trust is probably the wrong word.Reliant would be better.
No doubt about the accuracy of the cesium clock it's the reliability of it to wake me when I wantYou can't take these things for granted.

20. Apr 6, 2015

### jbriggs444

What are you talking about? An atomic clock is plenty accurate enough not to set off a shift worker's alarm in the middle of the night.