# The Naked Scientists Forum

### Author Topic: How do we define the second?  (Read 13437 times)

#### JP

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##### How do we define the second?
« on: 24/03/2010 04:48:00 »
How is the second defined?  What physical process(es) are we measuring?

(This was brought up in the thread here: http://www.thenakedscientists.com/forum/index.php?topic=29238.0;topicseen
I wasn't quite following the discussion in that other thread, so I thought to make the point clearer we could discuss the point here.)

#### Soul Surfer

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##### How do we define the second?
« Reply #1 on: 24/03/2010 09:45:50 »
The historical definition of the second came as a specific fraction of the 24 hour mean solar day.  (day lengths do actually vary throughout the year by a small amount because the earth's orbit is elliptical and the earth's rotation is very nearly constant).  This was reinterpreted to be a particular pendulum but as measurements became more precise it was reinterpreted as a specific number of cycles of a particularly stable spectrum line.

Wikipedia gives this information on the Second   among a lot of more detailed descriptions.

"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 caesium 133 atom."

but there is a more stable frequency source that is being considered as an alternative

Nowadays this is more precise and stable than the rotation of the earth and so normal clock time is adjusted by small amounts to keep it in step  it used to be "leap seconds" every few years but now it is smaller and more frequent.

« Last Edit: 24/03/2010 09:53:56 by Soul Surfer »

#### LeeE

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##### How do we define the second?
« Reply #2 on: 24/03/2010 09:49:55 »
As Farsight said in that thread...

Quote
Since 1967, the second has been defined to be 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 caesium 133 atom.

That atomic clock employs microwave radiation, which is essentially light. You count 9,192,631,770 microwave peaks going past and call it a second.
« Last Edit: 26/03/2010 02:02:35 by LeeE »

#### JP

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##### How do we define the second?
« Reply #3 on: 24/03/2010 10:09:56 »
Ok.  That makes sense.  So is there a difference in the result you get between counting the number of waves going past and the number of transitions in the cesium atom?  (There would be, I suppose, if you let the atom move at relativistic speeds with respect to you, but isn't that all controlled for in the definition of measuring the second?  The atom is at rest, 0 K, etc.?)

#### PhysBang

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##### How do we define the second?
« Reply #4 on: 24/03/2010 12:14:14 »
Technically, the definition rests on the processes of the electron, not on the motion of the light. The light is how one measures the transitions, but the timing of the transitions is determined by the transition of the electron. The transition from one state to another is not a transition of motion but of energy.

The reliance on light is a practical restriction. The definition of a second as the transition is a practical definition, too. It is something that we can use as a standard of accuracy.

#### Geezer

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##### How do we define the second?
« Reply #5 on: 24/03/2010 17:43:43 »
Technically, the definition rests on the processes of the electron, not on the motion of the light. The light is how one measures the transitions, but the timing of the transitions is determined by the transition of the electron. The transition from one state to another is not a transition of motion but of energy.

The reliance on light is a practical restriction. The definition of a second as the transition is a practical definition, too. It is something that we can use as a standard of accuracy.

Well said Physbang. Ultimately we are using the cesium atom as an oscillator, as we do with all timing devices. The transitions in a cesium atom happen to provide a very stable timing reference. EM Radiation (it's a long way from being visible light) is merely the means used to detect those transitions.

Atomic clocks are rather sophisticated clocks that happen to use atoms as oscillators, but they are still clocks nonetheless.

#### syhprum

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##### How do we define the second?
« Reply #6 on: 26/03/2010 04:49:43 »
There seem to be two basic forms of oscillator that are used in 'clocks' those that depend on gravitational forces i.e the rotation of bodies, pendulums etc and those that depend on inter atomic electromagnetic forces i.e springs, crystals etc.
could atomic clocks be said to form a third class ?.
"(day lengths do actually vary throughout the year by a small amount because the earth's orbit is elliptical and the earth's rotation is very nearly constant)"
My sundial was 20 minutes out at Xmas!
« Last Edit: 26/03/2010 19:15:47 by syhprum »

#### Geezer

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##### How do we define the second?
« Reply #7 on: 26/03/2010 04:56:45 »

My sundial was 20 minutes out at Xmas!

That's nothing. I have an Atmos. I never have to wind it up, but it's 20 minutes out every month!

#### Farsight

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##### How do we define the second?
« Reply #8 on: 26/03/2010 10:48:39 »
It's important to remember that you are not counting the caesium hyperfine transitions, you're counting the passing microwave peaks that result from it. The hyperfine transition is a spin flip, like the hydrogen depiction below, and you aren't counting spin flips:

#### JP

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##### How do we define the second?
« Reply #9 on: 26/03/2010 11:00:00 »
Ok, Farsight, but as I understand it light is just the tool we use to extract information about the atomic transitions, and we make all sorts of requirements to keep the light from influencing the measurement.  Is there any practical difference in this case between the number of atomic transitions and the number of peaks of the light that pass by?

#### LeeE

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##### How do we define the second?
« Reply #10 on: 26/03/2010 14:38:04 »
I agree with JP here; although it is the light that is detected and counted, it is the transitions that create the light that act as the timing mechanism.  The emitted light just acts as a 'messenger' to bring us the information.

#### Geezer

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##### How do we define the second?
« Reply #11 on: 26/03/2010 18:23:51 »
From Wiki - Atomic Clocks.

"Since 1967, the International System of Units (SI) has defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom."

Even the definition says it's an atomic standard.

If we used light to measure the speed of light, I'm pretty sure we would soon disappear up our own .......

#### Farsight

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##### How do we define the second?
« Reply #12 on: 26/03/2010 18:48:22 »
JP: yes.

Lee: yes, the transitions create the light that acts as a timing mechanism, and light does act as a messenger. But the duration between hyperfine transitions isn't defining the second. The hyperfine transition is something like the pluck of a guitar string. The nature of the guitar string determines the emitted sound, just as the nature of the atom determines the emitted microwaves.

#### Geezer

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##### How do we define the second?
« Reply #13 on: 26/03/2010 19:05:16 »
Farsight,

Are you saying that atomic clocks keep time based on properties of atoms, or not?

And if you are saying that they don't keep time based on properties of atoms, please explain what they are using to keep time.

#### Geezer

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##### How do we define the second?
« Reply #14 on: 26/03/2010 19:46:23 »
Cesium fountain atomic clock.

http://tf.nist.gov/cesium/fountain.htm

As it employs a fountain, would that mean we are really measuring time with water?

"Eventually, a microwave frequency is found that alters the states of most of the cesium atoms and maximizes their fluorescence. This frequency is the natural resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency used to define the second."
« Last Edit: 26/03/2010 20:22:17 by Geezer »

#### lightarrow

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##### How do we define the second?
« Reply #15 on: 26/03/2010 21:56:43 »
What is measured is a property of an em radiation (the frequency). The fact that radiation is emitted by an atom is not relevant here, but the practical fact this radiation is particularly stable, when emitted by that atom with that electronic transition.

#### lightarrow

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##### How do we define the second?
« Reply #16 on: 26/03/2010 22:04:37 »
Ok, Farsight, but as I understand it light is just the tool we use to extract information about the atomic transitions, and we make all sorts of requirements to keep the light from influencing the measurement.  Is there any practical difference in this case between the number of atomic transitions and the number of peaks of the light that pass by?
Farsight has already answered, but I want to remark that there is absolutely no relation at all between the two things; in theory you could get that radiation with a completely different mechanism, not involving atoms or electrons at all.

#### Geezer

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##### How do we define the second?
« Reply #17 on: 26/03/2010 22:26:42 »
Farsight has already answered, but I want to remark that there is absolutely no relation at all between the two things; in theory you could get that radiation with a completely different mechanism, not involving atoms or electrons at all.

Indeed you could. There are many ways to produce radiation, but they would not produce a very accurate clock. The point you might be missing though is that the Cesium atoms in an atomic clock are controlling the frequency of the radiation. There is a rather complicated feedback mechanism involved to achieve that, but it is there, nonetheless.

I think the NIST page makes that rather clear. It's a bit more difficult to identify it on the Wiki page on atomic clocks, but it does mention it.

#### Farsight

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##### How do we define the second?
« Reply #18 on: 27/03/2010 10:51:58 »
Farsight,

Are you saying that atomic clocks keep time based on properties of atoms, or not?

And if you are saying that they don't keep time based on properties of atoms, please explain what they are using to keep time.
Yes, of course atomic clocks keep time based on the properties of atoms, just as a guitar note depends on the properties of the guitar string. An atom undergoes a hyperfine transition and emits a photon, then can absorb a photon and undergo the hyperfine transition again. The frequency of that photon depends on the atom you use, the particular hyperfine transition, and other factors such as temperature and gravitational time dilation. But note that frequency is measured in hertz, which is defined as cycles per second, so we have to count cycles to define the second.

#### Geezer

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##### How do we define the second?
« Reply #19 on: 27/03/2010 15:42:25 »

But note that frequency is measured in hertz, which is defined as cycles per second, so we have to count cycles to define the second.

The clock is not measuring frequency which is what you seem to be implying. It's simply counting events. The time between those events happens to be  1/9,192,631,770 seconds. It's really no different from any other clock that uses a pendulum, spring, or whatever.

We don't count cycles to "define" the second. We count cycles to measure a second, or any other interval of time we choose.

You continually imply that there is something circular and unreliable about this process. There isn't. It's a clock.

#### Bored chemist

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##### How do we define the second?
« Reply #20 on: 27/03/2010 17:15:36 »
It jolly well is measuring a frequency; specifically, the frequency of a microwave oscillator. That oscillator is locked to the absorbtion frequency of caesium atoms in a "fountain".
In principle, you could have the events that control the frequency of the oscillator happen as seldom as you like- once a day or whatever. In practice they happen very frequently, but that's just for engineering reasons. They certainly don't happen exactly 9 point something billion times a second.
« Last Edit: 27/03/2010 17:19:29 by Bored chemist »

#### Geezer

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##### How do we define the second?
« Reply #21 on: 27/03/2010 18:37:57 »
It jolly well is measuring a frequency; specifically, the frequency of a microwave oscillator. That oscillator is locked to the absorbtion frequency of caesium atoms in a "fountain".
In principle, you could have the events that control the frequency of the oscillator happen as seldom as you like- once a day or whatever. In practice they happen very frequently, but that's just for engineering reasons. They certainly don't happen exactly 9 point something billion times a second.

I fully agree with your description of how it operates. The atomic activity is used to tune the microwave oscillator to a frequency that corresponds with the resonant frequency of the cesium atom. The microwave resonator might "flywheel" for long intervals between adjustments. There is nothing unusual about that. It's a technique that is used in lots of high stability reference clocks.

Ultimately the clock is comparing the frequency of the microwave oscillator with the resonant frequency of the atoms, although, the process that it uses to do that is rather indirect. It's certainly not like a phase-locked loop or anything that simple. It makes very slight adjustments to the microwave resonator so that it produces events that occur 9 whatever billion times a second.

However, I don't fully agree with you when you say it is "measuring frequency" unless you are referring to the comparison process between the oscillator and the atomic resonator. I would agree that it is producing a frequency based on a comparison. It's a subtle difference that may be of little consequence.

My objection to Farsights comment was that it might imply that the clock, or clocks in general, somehow count events in a given amount of time, which is what it would do if it was "measuring frequency". It's really the other way around. It's producing events with (hopefully)a constant amount of time between each event, then counting off those events.

Hope that makes more sense.

#### yor_on

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##### How do we define the second?
« Reply #22 on: 28/03/2010 05:34:48 »
As with all time measuring we want to split times arrow as closely as possible, I assume? And for one second that would be 1.855094832e+43 'splits' according to Plank time conversion. If we assume that to be the ultimate definition of times arrow we will have the ultimate Plank frequency here it seems to me.

If we now had something oscillating that quick, and we also had something able to measure those oscillations and transfer that to a a dial f.ex we would have a extremely smooth dial movement :) So how do we fit those oscillations, 1.855094832e+43 times per second to the speed of light,

How far will light 'travel' for one second? Light travels at a speed of 299,792,458 meters roughly per second in a 'perfect' vacuum. So what will we get as a distance then? Splitting it against the oscillations/events I mean? Well as I understand it we will get a Plank length. "1 Planck length per Planck time is the speed of light in a vacuum."

So if we had something that exact I think we could say that we actually was measuring the 'speed of light in a vacuum'. But as it is we don't do that, the events we use for measuring is nowhere that exact as I understands it? That the photons radiated are in themselves traveling at 'C' don't make the clock 'work' at 'C' as I see it.

We would still be unable to measure 'times arrow' in 'real time' even if we had such an exact 'time splitter' though, as we still would measure something happening before we could observe it. So to do it in 'real time' seems quite impossible. But we would be close (Ah, this was a joke:).

In reality we are, all of us, working at 'real' Plank time :) even though we have no way of measuring it. And that is true as long as we all observe and interact via 'photons'. But to measure is another thing. that isn't about that ethereal 'now' forever disappearing, never to be 'caught' as all processes measuring 'now' will have to take some time, from thoughts to ....

So do this mean that times arrow actually is 'events', as we have a natural limit in the Planck time measured? Not really, it just make a statement about what we see as 'meaningful' for transitions/events observed. Everything faster than that will be unable for us to measure, ever, inside our SpaceTime as I understands it.
==

Well, as I understands it :)

« Last Edit: 28/03/2010 05:49:22 by yor_on »

#### Geezer

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##### How do we define the second?
« Reply #23 on: 28/03/2010 06:36:13 »
Yoron, It's about time you showed up for the debate.

I kind of think I understand what your saying.

Just wanted to point out something about the clock and "events".

The actual frequency that the clock oscillates at is not really all that important. If we knew of some substance that had a resonant characteristic that turned out to be more stable than cesium, even though it had a period of multiple seconds, somebody would probably figure out a way to use it to make a more accurate clock. In fact, as BC pointed out, the NIST clock isn't being continuously adjusted. There can be quite long intervals between adjustments.

Once we have a really stable reference, it's possible to synthesize any frequency that electronics will allow, and it will be almost as stable as the "master" clock, and certainly quite accurate over long periods.

So, to measure the speed of light accurately, we "just" need a very accurate clock. It does not necessarily have to operate with a very fast timebase, although it likely will.

Hope this makes sense.

#### yor_on

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##### How do we define the second?
« Reply #24 on: 28/03/2010 16:30:51 »
That's a way too Geezer. Still, the clock on my cell phone is sufficient for me. Nowadays I feel it goes to fast too? I liked that Atmos you wrote about

#### The Naked Scientists Forum

##### How do we define the second?
« Reply #24 on: 28/03/2010 16:30:51 »