Physicists disagree


Getting back to our satellites—traveling around Earth at 17,000 miles per hour, a satellite’s time slows down, according to Einstein. Also according to Einstein, since gravity is weaker up where the satellites are, time moves a little faster. If that’s the case, wouldn’t a satellite’s GPS signal be inaccurate?

As I researched this question online, I got 2 answers to it: yes and no.

Yes. The satellites’ times slow down, but those amazing scientists who put the satellites up there in the first place cleverly adjusted their signals to compensate for it. Each satellite carries an atomic clock and sends a time & location signal at the speed of light, slightly adjusted (through programmed computer chips) for Einstein’s Special Theory. The clock takes into consideration both the slowing down because of speed and the speeding up because of weak gravity.

No. The satellites’ times slow down, but since they’re all zooming along at 17,000 miles per hour, all their times will be slowed down by the same ratio. So, it doesn’t matter. If all the satellites’ times agree with each other, the GPS system will be accurate. Or, the slowing down is cancelled out by the speeding up caused by weak gravity, so it’s what they call a ‘wash.’ Or, Einstein’s theories are a bunch of baloney so we shouldn’t pay any attention to them.
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Um. I don’t know what to say here, gang. As you must be aware, I’m just some shmo who is learning as I write this. If you put me on the spot for which is the right answer, I’m going with the bloggers who have better proofreading/grammar skills and citations. If any of my loyal readers want to chime in, please do!

UPDATE: Regarding a satellite’s time difference because of relativity, our indefatigable physics consultant, Ms Physics, says:

Infinitesimal difference, yet a difference. You need to approach the speed of light 3.0 x 10^8 m/s (670,616,629 mph) to have a significant difference. Any way John you remember the precision of the Cesium clock, these differences would really become significant in GPSA calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

A microsecond is one-millionth of a second.

This is why I stick to drawing pictures and let others do the heavy brain-work.

I borrowed parts of this composition for my sketch. I removed the horses and put everybody in lab coats.

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The speed of light!

The speed of light stays the same—186,282 miles per second in a vacuum. It doesn’t change relative to other objects. If you switch on a light in your rocketship while cruising along at 12 parsecs, the light doesn’t travel at 186,282 miles per second plus 12 parsecs. It stays at 186,282 miles per second. This is because light is an electromagnetic wave and has no mass. Signals from a radio station are electromagnetic waves, too. They wouldn’t speed up or slow down if the radio station were moving.

I say ‘in a vacuum’ because outside of a vacuum light’s going to be slowed down by atmosphere: dust particles, car exhaust, hairspray from actors in 1970s sci-fi movies, bird poop, cigar smoke, &c, &c. Beyond Earth’s atmosphere, in outer space, it’s a vacuum.

My vacuum seems to have a lot of dust particles and dog fur in it, so that slows the light down somewhat.

A lightyear is the distance light can travel in a vacuum—at 186,282 miles per second—in one year. Earth-year, that is!

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Einstein’s Special Theory of Relativity

As you move faster, time slows down for you. Even though that’s true when you’re riding in a car, the slowdown is so teeny-tiny that it’s not worth measuring. But, if you were to travel to another galaxy at almost the speed of light, time would slow down—for you—to the point where you would age more slowly than your pals back on Earth.

When you got back from your trip, your friends would be old and wrinkly but you’d be ready to graduate from high school.

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Classical Relativity

Believe it or not, we’re coming to the end of The Western Civ User’s Guide to Time & Space. I would be cheating you customers if I didn’t spend a little time talking about TimeSpace.

The name Einstein has come to be shorthand for genius. It’s still difficult to imagine the abstract way he trained his brain to think about time and space. Back in 1905, Albert Einstein published his theories about Relativity. He had 2 theories, a Special and a General Theory. But first, let’s talk about Classical Relativity.

Classical Relativity: everything moves relative to everything else.

If you’re riding in a train going 45 miles per hour and throw a ball from the back of the car to the front, the ball seems to you to travel at 10 mph. To a cow standing in a field watching you throw the ball, the ball is traveling at 55 mph: 45 mph for the train plus 10 mph for the ball.

A speedometer can say we’re driving an automobile at 45 miles per hour, but that 45 mph is the car’s speed relative to the surface of good ol’ Planet Earth. Earth is also spinning around and circling the Sun. The Sun also circles around in the Milky Way, taking us along with him. The Milky Way circles around in the Universe. Even the Universe is moving—it’s expanding. How fast something is moving can only truly be measured as it relates to something else. That’s Classical Relativity.

I once tried talking my way out of a speeding ticket using this concept but the trooper wasn’t buying it. Just kidding! Actually, I started sobbing uncontrollably and he walked away in disgust.

UPDATE: My pal Ross sent this interesting article about how a fighter jet ran into its own bullets (the jet landed safely). It seems air-friction slowed the bullets but the jet’s engines allowed it to speed up. At any rate, probably I should redraw the above cartoon with the kid inside a box car, instead of standing on a flat car.

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So atomic clocks saved the day—yay! Satellites are synchronized with each other to nanosecond accuracy. Their signals let our GPSs figure out where we are because even though those satellites are hurtling through space at 17,000 miles per hour, their atomic clocks will always show the correct time, right? Nothing’s gonna interfere with each satellite’s time, nothing! No sir! Not one thing!

Okay, maybe one thing: Einstein.

I can see you loyal-yet-exasperated readers flinging your half-eaten peanut-butter-and-jelly sandwiches across the room and yelling, “Oh, come off it, Manders! Have you finally gone around the bend? Should we have you fitted for a straight-jacket and a drool-cup? What’s Einstein got to do with my global positioning system?”

Listen, and I will tell you all about it.

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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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. start at 2:45

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.

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.



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

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

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