There are hundreds of atomic clocks in orbit right now, perched on satellites all over Earth. We depend on them for GPS location, Internet timing, stock trading ... and space navigation?Today on the show, hosts Emily Kwong and Regina G. Barber learn how to build a better clock. In order to do that, they ask: How do atomic clocks really work, anyway? What makes a clock precise? And how could that process be improved for even greater accuracy?For more about Holly's Optical Atomic Strontium Ion Clock, check out the OASIC project on NASA's website.For more about the Longitude Problem, check out Dava Sobel's book, Longitude. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.Have questions or story ideas? Let us know by emailing [email protected]!Learn more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: What historical problem does the episode address?
GPS, internet timing protocol, stock trading, all of these things rely on more accurate systems.
But the atomic clock system, as it stands right now, is error prone. Yeah. GPS clocks are estimated to drift by about 10 nanoseconds a day, which I know doesn't sound like a lot. But an error of even a microsecond in space can translate to an error of 300 meters on the ground. Right.
So to correct for clock drift, GPS clocks will send a signal a few times a day down to Earth and ask, you know, hey, am I on time?
And then the Earth says, OK, your clock is this has accumulated this much error. It's this much seconds off or time off. And then they send another signal back.
But this process is kind of a pain, you know, this constant like phoning home. So for years now, NASA has been searching for a clock that is capable of autonomous navigation, able to operate as its own unit with minimal updates and be even more precise. All of this reminds me of what the Board of Longitude was trying to do all those centuries ago. Holly, she calls her clock OASIC.
Optical Atomic Astronium Ion Clock.
OASIC. It's a science OASIS cover band.
I'm going to explain why OASIC holds such promise, what all those different words in that sentence mean. But I need to call upon the spirit of my grandfather, Bob, who was a clock repairman, and first explain how an atomic clock works. As a physicist, I still like struggle with this. So let's do it. It's like the Mr. Potato Head of science. You have to smash so much tech together to make it go.
So all you need to know about a clock, this is true of all clocks, is they are feedback loops. And there's generally three elements that talk internally to each other within the clock to keep it steadily ticking. Mm hmm. The first part is an oscillator.
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Chapter 2: How did clocks impact navigation in the past?
What's the third thing that makes it a clock? Your reference. So the reference ensures that the oscillator vibrates at the right frequency and doesn't cause the clock to drift. And that's where the atoms come in. An atomic clock is called that because it uses part of an atom as its reference.
Atoms have this really special quality, and I'm going to turn it over to you now, Gina, to explain how atoms go from a grounded state to an excited state.
Yeah, so most atomic clocks use an atom of cesium or rubidium, but in general, I think it's, like, easiest to explain this process with, like, the element hydrogen because it just has one proton at its center and one electron orbiting it. And like orbit is a bit of a simplification for now, but let's just say orbit. Electrons, they have these different orbits.
Each of them are associated with like a different energy. And if an atom absorbs energy, let's say through like a little chunk of light or a photon, the electron will change its orbit. It'll go to this higher energy state. It'll go to a higher orbit. And then when the electron eventually goes down, energy is released from that atom as another photon.
Okay, so that. In the 1950s, scientists... hacked this particular ability of an atom and forced this energy transition in the atom at a regular interval and designed a clock that would count every time energy is released as the electron goes back down. And that is the frequency of the atomic clock. Okay, and they did this with light, right? Right.
So traditional atomic clocks, the ones used for GPS, use microwaves, which is a form of light. How the clock works is it bombards an atom with microwaves, and that forces the atom from its grounded state to its excited state, and that transition happens at a steady pulse by which the whole clock is referenced.
But those clocks are accurate to the 10 to the minus—the best ones are 10 to the minus 16—
Which is not good enough for Holly as an atomic physicist. It's so precise, though. I know, but microwave is not precise enough for her. She and other atomic physicists work with optical light. Optical light has a shorter wavelength, so it's a better light source by which to control an atom.
Instead of going from shining microwave light on the atoms, we can go shine optical light or use lasers on the atoms. We can get to 10 to the minus 17, 10 to the minus 18, and even 10 to the minus 19. So these are... you know, up to three orders of magnitude improved over current microwave clocks.
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