Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: JP on 24/03/2010 04:48:00
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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.)
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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.
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As Farsight said in that thread...
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.
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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.?)
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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.
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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.
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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!
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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!
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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:
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fpersonal.tcu.edu%2F%7Emfanelli%2Fimastro%2Fimastro_starstuff_files%2Fhydrogen_21cm1.jpg&hash=7421475958d8dd585342c8eddccc2a74)
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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?
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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.
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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 .......
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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.
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Farsight,
Can you explain your last post for me please.
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.
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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?
Please note:
"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."
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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.
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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.
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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.
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Farsight,
Can you explain your last post for me please.
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.
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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.
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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.
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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.
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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. (http://www.unitconversion.org/time/seconds-to-planck-times-conversion.html) 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.
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Well, as I understands it :)
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Yoron, It's about time you showed up [:D] 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.
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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
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The Atmos is a good example of what I'm on about. (Actually, it's not really a good example because it does not keep very good time.) Anyway, its pendulum has a full one minute period. It rotates around a torsion spring - 30 secs in one direction, then 30 secs in the other direction.
So it goes tick.............................................tock................................................tick.......
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Incidentally, the NIST clock doesn't define time. No single clock does; they use an average of a group of clocks round the world.
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I read that one should send it in for cleaning every twentieth year, The clock I mean, not times arrow. How old is it?
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The Atmos? (Not the NIST I suppose [:D]) It's been going strong for almost forty years. Never had to wind it up once!
Maybe a cleaning would help, but it's never really kept very good time. Mind you, some of that might just be perception. Because I never have to wind it up I hardly ever adjust it, so it's probably going for many months between adjustments.
BTW - for anyone that's wondering what we are yakking on about - http://www.atmosadam.com/
Probably more a piece of kinetic art that a great timepiece, but possibly a bit more interesting than a quartz digital clock!
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The ATMOS as manufactured by Jaeger LeCoultre, the makers of world famous Reverso wrist watches, is a time piece that for generations has represented the wonders of science, technology and remarkable Swiss craftsmanship. Possessing one signifies belonging to an exclusive group of world leaders, famous celebrities, business professionals, and, put more simply, people with exquisite taste.
so geezer to which category do you most identify with?
on a serious note on the fractions of a solar day something that soul surfer touched on the leap seconds that are used are not all in one direction [I will exspand on later but will have to go in new theorys]
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so geezer to which category do you most identify with?
Gosh! It's hard to say really. Can I only select one?
(It's one of the rather basic ones from the early 70's. Wedding present from my Mum and Dad.)
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On behalf of your wife i think it must have been a person with exquisite taste.
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It's not etirely clear Mrs G would agree with that statement [;D]
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Ah your mum and dad found a excellent present for you both my man. I read that you can expect it to work for hundred of years :) Up to a thousand if lucky, and cleaning it, nota bene. I was looking for one here in Sweden (curiosity:) But there were none for sale. People that have them seems to want to keep them too.
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One of the characteristics of clocks seems to be the higher the frequency of the basic oscillator the more accurate they are.
The 'Atmos' that Geezer uses has a torsion oscillator working at .033 Hz and is incredibly inaccurate where as most clocks today use 32768 Hz crystals and are pretty accurate while really stable clocks use microwave Cesium transitions.
The only exceptions that I can think of are gravitational ones based on the rotation of planets etc that use periods of about .000078 Hz and quite accurate.
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The only exceptions that I can think of are gravitational ones based on the rotation of planets etc that use periods of about .000078 Hz and quite accurate.
Heh! - I hadn't thought of that. Hmm... Jupiter, in its orbit, would make a pretty massive oscillator, and if I've done my maths right, it would have a frequency of 2.672026246839270841e-09 Hz. Moving even further out, to use Neptune, would result in a frequency of 1.922923089229784694e-10 Hz [;D]
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Good points! Funny thing about the frequency. I wonder if there is some intrinsic law at work there, or is it more a case of the technology?
Of course, it gets even more complicated, because, the second is really defined by a rather large and very slow pendulum called the Earth [;D]. The SI second is a very good thing for science and engineering of course, but we always end up having to finagle things to line up with our one year pendulum.
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Syhprum you sure have a way with thinking :)
That was a very cool idea.
Now we just wait for LeeE to finish it. I'm putting myself first in the queue for buying one, when you finished your work Lee :)
A stellar watch :)
A very sweet idea.
(or is it a planetary?)
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I'm pretty sure I could rig up something that regularly adjusted the Atmos to keep it reasonably in sync with the RF time signal from Boulder CO, but it might tend to screw up the aesthetics, don't you think?
Synchronizing it with Neptune might be a wee bit trickier.
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Why stop at Neptune?
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I know what your trying to do. You're trying to get me to say that Pluto is not a planet so we have to debate that for the next ten years!
Ho no! I'm not falling for that one. [;D]
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Actually, a solar orbit clock wouldn't be much good for measuring durations less than the orbital period because the orbits are all slightly eccentric.
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Before the development of the marine chronometer by Harrison et al the chief source of timekeeping for navigation was the passage of the Moon thru the star field.
As Lee points out the eccentricity of the orbit causes a lot of problems and prediction tables had to be prepared so that adjustments could be made.
When I suggested that large bodies made good time keepers I had in mind their rotational periods not of course their orbital time about other bodies.
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Before the development of the marine chronometer by Harrison et al the chief source of timekeeping for navigation was the passage of the Moon thru the star field.
As Lee points out the eccentricity of the orbit causes a lot of problems and prediction tables had to be prepared so that adjustments could be made.
When I suggested that large bodies made good time keepers I had in mind their rotational periods not of course their orbital time about other bodies.
So here's the thing. Does the second define the period of the "Earth pendulum", or does the period of the "Earth pendulum" define the second?
(I think it's really the latter, but I suspect I'll get some heat for saying that!)
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It's the former: as I thought we'd already agreed, the second is defined by the "transition between the two hyperfine levels of the ground state of the caesium 133 atom" and most definitely not by the Earth's rotational and/or orbital periods (which are subject to stuff like earthquakes etc.).
The Earth's rotational and orbital periods are defined in terms of the second.
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It's the former: as I thought we'd already agreed, the second is defined by the "transition between the two hyperfine levels of the ground state of the caesium 133 atom" and most definitely not by the Earth's rotational and/or orbital periods (which are subject to stuff like earthquakes etc.).
The Earth's rotational and orbital periods are defined in terms of the second.
As I said, I'll probably get some heat [;D]
As you say, today, the second is defined in terms of atomic activity. However, originally the second was a subdivision of the period of Earth's orbit. We still have to make adjustments to reconcile the two.
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Geezer
No the second was never defined with reference to the Earths orbital time it was defined with reference to the Earths rotational period relative to the distant stars.
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Geezer
No the second was never defined with reference to the Earths orbital time it was defined with reference to the Earths rotational period relative to the distant stars.
Thanks Syhprum. If I understand correctly, that means the second is a fraction of the time it takes for the Earth to rotate 360 degrees on its axis.
Who came up with the concept of the second first?