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We almost never notice it, but the ground under our feet is moving all the time. Earth’s crust is separated into plates, and they’re pushed into and away from each other by currents of magma.

Like, every year, that magma spreads apart the Atlantic Ocean, and New York gets a couple of centimeters farther from London. With scales like that, measuring these movements is easier said than done — but how these plates move also affects everything from earthquakes to the shapes of continents. So to learn more about plate tectonics, researchers have turned to an unlikely helper: quasars in space, millions of light-years away.

Quasars are some of the brightest, most distant objects ever discovered. They’re caused by gas and dust circling and falling into supermassive black holes, and heating up and releasing light as they go. And by tracking these objects, astronomers have been able to map the movements of continents down to a few millimeters.

Their technique is called Very Long Baseline Interferometry, or VLBI, and it’s similar to the way people can tell where a sound is coming from. One of the key ways people localize sound depends on having two ears. Unless the sound is coming from straight ahead, it will hit the closest ear first, then the farther one.

Then, the brain can take the information about that time delay, factor in how far apart someone’s ears are, and pinpoint exactly where the sound came from. VLBI takes this general concept and kind of reverses it. Now, in space research, this technique is a bit different — it’s used to look at distant objects.

But in geodynamics, it’s all about using distant objects to learn something about Earth. In this method, you start by knowing where an object is in space. Then, you see how long it takes for its light to hit two different telescopes.

Based on the time delay, you can then work out exactly how far apart the telescopes are. And here’s the thing about telescopes: They seem immovable, but they’re also on the ground, on moving tectonic plates. So by tracking the telescopes’ motions, you can also track the plates underneath them.

Overall, quasars make great targets for these studies because they’re so bright and far away. They’re like point sources of light that are effectively always in the same spot relative to the Earth, so they can help us make really detailed measurements about how fast plates are moving. And that’s a pretty big deal!

The speed of the plates partly depends on what’s happening beneath the surface, so knowing how the ground is moving lets geophysicists better understand the Earth’s inner layers. And this technique has been used to calculate all sorts of things. Like, it allowed us to measure plate motion for the first time.

And along with historical data, it’s shown us that the process of plate tectonics hasn’t changed much in millions of years. Since these telescopes are fixed to the surface, researchers can also use them to track the Earth’s wobble, the exact speed of its rotation, and how it’s oriented compared to other objects in space. All thanks to some incredibly bright quasars, millions of light-years away.

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You can send it to our patrons-only QQ inbox over at Patreon.com/SciShow. And if you’re not a patron yet but you want to learn more about becoming one, you can head over to that website, as well. [♪ OUTRO].