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.

https://www.livescience.com/32660-how-does-an-atomic-clock-work.html
https://www.timeanddate.com/time/how-do-atomic-clocks-work.html
https://www.gps.gov/applications/timing/


https://science.howstuffworks.com/question40.htm
https://www.fda.gov/radiation-emitting-products/resources-you-radiation-emitting-products/microwave-oven-radiation
https://science.howstuffworks.com/atomic-clock3.htm
https://www.britannica.com/technology/atomic-clock

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Radio waves

It’s late at night. Maybe you’re staying in a cabin out in the woods. You’re awake. No cell service, no tv, no computer. Only an old battery-powered radio—the kind that has a dial to tune in stations. How about listening to some music? You turn on the radio, get a lot of static, play with the dial until— “Hey! there’s a song I really like!” —but it’s faint, you lose the signal, you slo-o-o-o-owly turn the dial back and forth…there it is! You love this song! It resonates! You start humming along with the music.

Radio waves are electromagnetic energy sent from a broadcasting antenna. The energy is literally sent out in a wavy line. The number of waves sent out per second (their frequency) is expressed in a unit called hertz (Hz). One thousand hertz is a kilohertz (KHz), 1 million hertz is a megahertz (MHz), and 1 billion hertz is a gigahertz (GHz). Hertz is the number you see on a radio’s band.

When you get up between 1 billion and 3 billion hertz (1 GHz – 3Ghz) we’re talking about microwaves.

Microwaves agitate the water molecules in a cold slice of pizza to heat it up.

Inside an atomic clock, a broadcast antenna sends out microwaves inside the vacuum tube where the cesium atoms hang out. The frequency increases and decreases slightly (like tuning in a radio station) until it hits the exact same frequency as the atoms’ oscillations (9,192,631,770 times every second). The atoms resonate with the microwaves and change into a different energy state. They’re in such a fantastic mood they bounce off a detector at the other end of the tube.

https://www.youtube.com/watch?v=sRX2EY5Ubto start at 2:45
https://www.explainthatstuff.com/antennas.html
https://www.nasa.gov/directorates/heo/scan/communications/outreach/funfacts/txt_radio_spectrum.html
https://www.livescience.com/50259-microwaves.html
https://www.britannica.com/video/214986/How-radio-works-overview-radio-waves-frequency-amplitude-modulation

I’m glad the atoms are happy but I still don’t get out how this translates into keeping time. Please continue to hold.

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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.

https://science.howstuffworks.com/atomic-clock3.htm
https://education.jlab.org/qa/atom_02.html

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.

 

Wow—that hold music is awful. Back to the beginning of The Western Civ User’s Guide to Time & Space

 

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.

 

cesium

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.

https://www.livescience.com/37578-cesium.html
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.

 

https://www.britannica.com/science/atom

*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:

 

https://www.school-for-champions.com/science/atoms_solar_systems.htm#.Xyk9hB17nzI

 

 

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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!”

Really.

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32,768 oscillations per second

When you hit a tuning fork against something it vibrates, giving a specific musical note.

We learned that a digital clock is regulated by measuring how many times a quartz crystal oscillates per second—32,768 times. How does it count all those vibrations so quickly? Here’s how: the crystal is purposely cut with a laser to exactly the size and shape (the shape of a tuning fork) that will produce 32,768 oscillations in a second, then stop.* The electric circuit zaps the crystal with electricity, which makes the crystal vibrate until it returns to its original shape. When the vibrating stops, exactly one second has passed. The stopped vibrations trigger the circuit to move the second hand and give the crystal another zap.

The same principle applies in animated entertainment for children. The mouse hits the cat, who oscillates for a second, then resumes his former shape.

Here’s how a tuning fork works: https://www.youtube.com/watch?v=hW-igtIn3A8

Basics of LC oscillators and their measurement


https://en.wikipedia.org/wiki/Crystal_oscillator

* “Because 32768Hz can be so conveniently divided to give a 1 second pulse, it is a very popular size for it to be cut to. Manufacturers can bang them out and be sure they will sell.” https://www.quora.com/Why-does-Quartz-vibrate-exactly-32768-2-15-times-per-second

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The liquid crystal display explained!

Three inventions moved clocks and watches away from being mechanical/analogue so they could become digital: The quartz crystal, the circuit board and the liquid crystal display.

Okay, I sort of explained how a battery works. I kind of explained how a quartz crystal works. The circuit board was easy—even a shmo like me can explain printed metallic ink on a plastic card. But—liquid crystal display? I started this post about 17 times and kept getting lost in the weeds with carrot juice and double melting points and twisted nematics and polarization…

Let’s start here: analogue clocks and watches were inaccurate because they have physical, mechanical moving parts. So we replaced the wound-up mainspring with a battery. We replaced the balance wheel with a vibrating quartz crystal. Now we need to replace the moving mechanical gears, hour-hand and minute-hand with a digital (just the numbers) display of the correct time. How do we do that?

A digital wristwatch made by the Japanese company Casio.

Instead of mechanical gears and hands, we’re going to use electricity and light.

We want a watch-face that will light up and show us what time it is. We want most of the face to light up except the numbers, which should be black so we can read ‘em easily. We’ll block the light in the shape of each number so it shows up black. The numbers will change every minute, so we need a way to change the blocked areas every minute.

In order to block the light, we need a filter. The filter lets us control which rays of light pass through and which rays get blocked. A filter could be a wall of liquid filled with crystals that all face the same direction. The lined-up crystals let the light pass through. We’ll sandwich this wall between 2 plates of glass. The crystals still let light pass through—until we zap them with a little electricity. The electricity upsets the crystals so they don’t line up anymore and light can’t pass through.

We’re only going to zap in certain areas. We want those certain areas to be shaped like numbers. For instance, when we zap the glass in the shape of a ‘3,’ those crystals in the 3-shape get upset and don’t line up with the rest of the crystals in the wall. Light can’t pass through the 3-shape, so we see a black ‘3’ on a lighted watch-face.

Just like on a circuit board, we’ll print the numbers onto the glass in ink. This ink is transparent—and it conducts electricity. Each number is designed as a 7-segment figure, so we can zap only the segments that form a ‘3,’ or whichever number we want. Each segment is wired to the battery.

This is the principle behind LCDs. It’s a simplification. I left out a lot of stuff. But you get the idea, right?


https://electronics.howstuffworks.com/lcd.htm
https://electronics.howstuffworks.com/gadgets/clocks-watches/difference-between-quartz-and-liquid-crystal2.htm
http://www.madehow.com/Volume-1/Liquid-Crystal-Display-LCD.html

Many thanks to a couple of the Western Civ Irregulars, Diana (Ms Physics) and engineering-wiz Don M—both pals of mine since childhood. They pointed me in the right direction when I couldn’t find a way to explain this one.

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