No, the Earth’s core has NOT stopped spinning

Debunkening

You can’t debunk something unless it’s bunk to start with

A little gendankenexperiment for you.

Imagine you’re in a car on a highway. Another car is in the other lane right next to you, going the same speed. You see the landscape going past you at, say, 100 kilometers per hour, but the other car appears motionless, keeping pace with you.

Now imagine you hit the accelerator, just a little. Your car speeds up to 101 kph. When that happens, what does the other car appear to do?

It looks like it’s moving backwards relative to you as it falls behind you at 1 kph. It’s still moving forward, of course — the driver of that car still sees the landscape going past them, but they see you moving ahead slowly.

To be painfully clear: Both cars are moving forward the whole time, but because one of them is slightly faster, it moves ahead, and sees the other car lag behind. This is relative motion.

Simple, right? This same concept, though, seems to have confused a lot of people — a lot — due to a recent science story that made many headlines.

But I can understand their confusion! For one thing, the concept is a little more complicated in the story, dealing with rotation, not linear motion. But worse — oh, so much worse — was that the actual scientist authors of the paper the story is based on wrote it in a confusing way.

The story is about the rotation of the Earth’s core. Our planet has many layers in it with distinct properties, broadly divided into four sections: The thin crust, the much thicker mantle, the outer core, and the inner core. Both the outer and inner cores are loaded with heavy elements like iron and nickel, but the inner core is solid and the outer core liquid.

Perhaps surprisingly, the different layers don’t all rotate at the same speed! The layers aren’t bolted together, though, so there’s no rule that says they have to spin at the same rate. I actually wrote about this in 2022 on The Old Site, about a paper showing that not only does the inner core not spin at the same rate as the mantle and crust, but that its rotational speed can change!

The method used to figure this out is super cool — nuclear bomb tests generating seismic waves which moved through the Earth, allowing scientists to measure the motions inside the planet — but the conclusion is that the rotational speed of the inner core changes by a small amount, sometimes going faster than the mantle high above it (called superrotation) and sometimes slower (subrotation). Around the year 2000, for example, the core rotated faster than the mantle by about 1 degree per year.

That news was from last year. More recently, a paper came out on the same topic, showing that the change in the core’s rotation rate has slowed, and that at the moment it’s spinning at the same rate as the Earth’s upper layers. That’s interesting, especially since we don’t have a firm grasp on what’s making these changes, and what effects this might have on the surface as well as Earth’s magnetic field (which is generated in the liquid outer core).

Here’s where things get dicey. A lot of science articles reporting on this story came out saying that the Earth’s core has stopped spinning. Here’s one from MSN (“Uh, Earth’s Inner Core Just Stopped Spinning”), and here’s another from CNN (“Earth’s inner core may have stopped turning and could go into reverse, study suggests”).

Not to put too fine a point on it, but this is impossible.

The Earth’s inner core is roughly the size of Pluto (2,400 km wide), and the amount of energy it would take to stop its rotation is huge. VAST. That energy depends on the mass, size, and spin rate of the core, and using the equations for rotational kinetic energy I get it would take about 4 x 10^33 ergs to stop it; about the same as the energy the entire Sun emits over one second. Or, if you prefer, the same amount of energy released when you detonate 100 billion one-megaton nuclear weapons.

So yeah, a lot. If the Earth’s core suddenly stopped it would emit that energy due to friction, and that would be bad.

But it didn’t. It only slowed or sped up enough such that it’s now spinning at the same rate as the rest of the planet. The change was very small.

When I saw all the articles about this talking about the core having stopped, I shook my head ruefully. Since I wrote about this last year, and explained in that article what it meant, I knew where the article writers had gone wrong.

But this is where things go completely off the rails: In the journal paper, the scientist authors didn’t choose their words carefully enough. They wrote that the core had indeed stopped spinning, implying it was relative motion, not absolute, without saying it outright. A geoscientist (or someone familiar with the topic) would know what they meant, but it was extremely easy to read their abstract and come to the very wrong conclusion.

They quickly issued a correction to the paper (no doubt as people widely misunderstood the point and word got back to them), saying, “Some language in this article has been updated to more precisely describe the inner-core rotation discussed in this study as differential inner-core rotation, the difference in rotation between the inner core and the mantle. In the fourth sentence of the Abstract, now reading ‘This globally consistent pattern suggests that differential inner-core rotation has recently paused,’ the clarifying text ‘differential’ has now been included.”

So at least it got fixed, but I’ll note many articles (including the two I linked above) never issued corrections. I’ll also note that quotations by the scientists themselves in the articles used the confusing terminology, so it’s no wonder this story got so screwed up so widely.

I want to be careful here, too: As far as the scientists themselves were concerned, what they said was fine. And given that they in general talk to other scientists, that’s correct! They used terms other scientists would understand. But when things get out to the public the terms used by scientists can be easily misconstrued, especially when scientists use those terms differently than the public does. Here’s a great article about that, with a list of confusing terms. I’ve written on this myself, about the words theory versus hypothesis.

Back to the issue of Earth’s rotation, my colleague Corey Powell did a tweet thread on it, too:

I first heard about all this when my friend Jon Rogers tweeted about it (he wrote the screenplay for “The Core”, and we have a fun history) and I got other questions about it as well.

So there you go. It’s common for science stories to get things wrong due to lots of reasons — I’m guilty of this as well — but it’s rare for it to be due to reading the words of scientists literally*. The specific and different ways words are used is pretty ingrained in both the public and in scientists, but in my opinion it’s up to scientists to work on this; it’s much easier for us as a group to change the way we talk and write — or at least do so more clearly when talking to the public — than it is for billions of laypeople. I’m not saying it’s easy, or even how we’d go about changing it, but if we want to fix this problem I think the onus is pretty clearly on us.

* Well, to be fair, anti-science boneheads like creationists and climate science deniers do this all the time, but that’s different: It’s malicious, usually on purpose to bamboozle others.

Et alia

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