Tag Archives: STEM

Feedback loop

A cesium atom oscillates 9,192,631,770 times every second. That never changes.

What does change is the atoms’ energy state. The excited cesium atoms bounce off the detector every time the microwaves hit the same frequency as the atoms’ oscillations. The detector sends a signal to the microwave resonator, so that the microwave frequency is adjusted to sync better with the atoms. This is called a feedback loop. The detector sends a signal, the signal adjusts the microwave frequency, the microwaves excite the atoms, the atoms bounce off the detector, the detector sends a signal, the signal adjusts the microwave frequency, the microwaves excite the atoms…over and over and over. The time between each signal is exactly one second. No gears, no moving parts to oil, nothing mechanical.

That’s it! That’s how the atomic clock works. Thanks for sticking with me for an entire week on this. Finally, we can get on with our lives!

As with my explanation of the liquid crystal display, this is a simplification. I left out a lot of stuff. It’s the idea, the principle, that I was interested in explaining. Luckily for you, here are links to click on if you’d like more exact, in-depth info about atomic clocks.



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Cæsium steam

Cesium, like every element, is made up of only one kind of atom. There are only cesium atoms in cesium.

I just had this information tattooed on my forehead in mirror writing, so I don’t forget.

Here’s how you get cesium atoms to float around: you boil the cesium. Cesium melts at room temperature, like an ice cube melts into water. So all you need to do to get cesium atoms is boil cesium until it turns into cesium steam. Then you funnel the cesium steam down a tube which is a vacuum—nothing else in there, no air, just cesium atoms and that’s it. Then you expose those atoms to radio waves. When the radio waves hit the exact same frequency as the atoms’ own oscillations—9,192,631,770 times per second—the atoms change to a different energy state.


I guess I need to research radio waves now. Great merciful Zeus, I’m never getting to the end of this. Thanks for holding while I go look up radio waves.


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Please continue to hold while I sort this thing out

Remember how mechanical clocks are prone to lose time? It’s because they’re made out of physical machinery—pendulums or mainsprings and gears. We replaced those mechanical parts with a quartz crystal, zapped it with electricity to make it vibrate and got digital clocks. Digital clocks are more reliable, but they still lose 15 seconds every month.

To make the even-more-reliable atomic clock, we replaced the quartz crystal with atoms. Atoms vibrate on their own. We’re building a clock that’s as free of physical, mechanical parts as we can manage in this bad old fallen world.

Here’s what I’m getting from my exhaustive research so far: somehow cesium atoms are funneled down a tube. How do they get the atoms out of the cesium? I don’t know. The atoms are exposed to radiation—radio microwaves like the kind you use to heat up your old cold French fries—which makes them switch back and forth between energy states. The idea is to tune the radio waves to sync up with the atom’s own vibrations at 9,192,631,770 times every second. It’s not easy to get this exactly right—like tuning in a jazz station from 2 counties over on an old radio with dials. There’s a detector at the end of the tube. When the radio waves are at the exact right frequency (the same frequency as the atoms’ vibrations), the atoms change energy states and bounce off the detector—which means one second has passed. Then what? I dunno. How does the detector know when the atoms change from State B back to State A ? I dunno.

Back to my research. Thanks for your patience. Please continue to hold.

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Hail, Cæsium

Here’s something else about atoms: they vibrate, just like a quartz crystal, but you don’t need to zap them with electricity. An atom vibrates on its own at a steady, predictable rate. Incredibly steady, even.



No-see-ums are annoying bugs. Cesium is an element.

Some of the best atoms for vibrating steadily are the ones that make up the element cesium (SEE zee uhm)—that’s Cs on your periodic table. Cesium is kind of rare and its melting point is room temperature. The cesium atom has only one electron circling its nucleus. The cesium atom vibrates 9,192,631,770 times every second.

Yeah, yeah, we pronounce it SEE zee uhm even though it’s properly spelled caesium or cæsium which means it ought to be pronounced KY zee uhm because the a makes it a hard c but we pronounce cæsar SEE zur instead of KY zar so what are you gonna do. Option-apostrophe for you typography nerds https://www.dictionary.com/browse/caesium

I’m still processing all this info, gang. The atomic clock is still a mystery to me. Thanks for holding.

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Atoms. They’re small.

Here’s something: before I can even begin to figure out how atomic clocks work, it may be helpful to understand what an atom is. An atom is the smallest thing that exists. Anything you can touch is made out of atoms. Lots of ‘em. There is literally nothing physical that’s smaller than an atom.

An atom is made out of subatomic particles, but these can’t be separated so they don’t count as being smaller than a whole atom.* These particles have Greek names (like Aristotle hahajustkidding). In the middle of the atom are neutrons and protons stuck together in a clump, called a nucleus. Around the nucleus are electrons, circling like the moon circles Earth.** The electrons don’t fly away from the nucleus because the neutrons and protons exert a magnetism kind of like gravity.

So, no, I still haven’t figured out how the atomic clock works. This is taking longer than I thought. I’ll be back as soon as have more info. Please continue to hold.



*Ms Physics chimes in: “Atomos (Greek) ‘indivisible’ later proved incorrect!” Well, yes, that’s true. I don’t want you kids getting any ideas. Please, if you manage to isolate an atom—DON’T SPLIT IT!



** Another Western Civ Irregular Jeffrey K takes exception to me comparing an atom’s nucleus to a planet and electrons to orbiting moons. He says “Electrons don’t really orbit like planets– more like moths around a flame (without the usual fatalities). Also electrons are magnetic but the rest is held together by nuclear forces.” I said “So far as I know, nobody’s seen an atom because they’re so teensy.” So he sent me this:





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Atomic Clocks

We’re now at the point where we can talk about…atomic clocks, which lose only one second every 100. Million. Years. Yay!

What is an atomic clock and how does it work? That’s an excellent question. Honestly, I have no idea. You would think, as an adult grown-up-type guy, I’d know something like that. I don’t. I avoided science classes in school so I could hang out in the art room.

I don’t know how you get atoms to float around in a tube so you can zap ‘em with radio waves until they change into a different energy state and bounce off a detector that counts the atoms in their new changed state and funnels the whole mess into a feedback loop…

I need to go away for a few days and marinade myself in sciency research until I figure this one out. I’ll be back. In the meantime, please enjoy this hold music—

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Just a second

Before I got side-tracked into digital clocks and watches, we were talking about satellites and the Global Positioning System. I mentioned that the satellites that send us navigational signals need to have an incredibly accurate clock aboard. Even a second’s difference in time between satellites’ clocks would change significantly your GPS data—and give you the wrong location.

So you’re thinking, “Well, Manders, those quartz crystal clocks lose or gain only 15 seconds a month. That seems pretty accurate to me. How you gonna improve on a system that measures 32,768 oscillations per second? How you gonna do that? How?”

To which I reply, with a rueful smile, “My friend, there is another clock yet to come, whose sandals the quartz crystal clock isn’t fit to lace. I speak of a clock that loses only one second every 100 million years!”


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