What Would Happen if All Matter on the Earth Was Replaced by Blueberries?

A Q&A with the one scientist brave enough to figure it out.

A blueberry in place of planet Earth.
Photo illustration by Slate. Photos by Thinkstock.

The question is so intuitive it’s frankly unbelievable it’s never been asked before: What would happen to Earth if its entire shape were instantaneously replaced by billions of blueberries? That’s the subject of an entertaining paper titled “Blueberry Earth” by University of Oxford researcher Anders Sandberg, which he posted online this week. (I found it thanks to a since-deleted tweet by the Atlantic’s Sarah Zhang.) The result: an Earth that would be, if possible, even more imperiled than the Earth we know, with the planet collapsing to a fraction of its former size and “a steam atmosphere covering an ocean of jam on top of warm blueberry granita.” I spoke to the cheerful Sandberg over Skype to ask him about jam geysers, runaway moons, and why this kind of hypothetical isn’t as frivolous as it may seem.

Dan Kois: OK, let’s start with the simplest question: How long would a human being survive on Blueberry Earth?

Anders Sandberg: I think it wouldn’t be very long. So if you start from the uncompressed state, of course, you would just get this free fall as the planet is imploding. I would expect that within half an hour you would be very uncomfortable.

You describe it as feeling like the worst earthquake you’ve ever felt.

Of course the entire surface layer is literally falling for 700 kilometers. Now gravity is low, but that is still a long free fall.

Anders Sandberg.
Anders Sandberg.
Oxford Future of Humanity Institute

Your paper posits a kind of blueberry hell planet, with geysers of boiling jam spurting up everywhere. Would the jam at least be delicious?

That depends a bit on your taste. I’ve been getting in arguments with friends about whether it’s even properly a jam, because there’s no added sugar. I tend to be a low-sugar person, so I would probably find it delicious if we ignore the boiling part. A blueberry soup, after all, is very good. So I think it would be pretty delicious, at least at first. I think after a while of boiling and mixing with a lot of air, it might start oxidizing and going a bit off.

Am I reading this correctly that the moon would just fly away?!

Yeah. The basic thing is, blueberries are so much lighter than rock, so a lot of the transition is just that all the rock of a blueberry Earth would suddenly become much lighter, about the same mass as the moon. The moon of course is already moving at a fairly brisk pace, and it would no longer be bound to the Earth’s gravity. So it would go off on its merry way.

What was your impetus for doing these calculations and writing this paper?

It began on physics Stack Exchange, which is this question-and-answering site. I’m fairly addicted—I love answering questions. I started going there because I had some weird physics questions of my own, but it’s addictive to try and give good answers to questions.

And then I got this lovely what if question from somebody, I don’t know who it is, about the blueberry Earth. I started writing—I was on a bus trip from a meeting back to my office—and then I found to my great dismay that some moderator had closed the question because it was silly, off topic, not the kind of question one should be dealing with. And I got really annoyed by that! I’d done a bit of work on the bus already. So I decided, let’s write a paper. Anything worth doing well is worth overdoing.

In your research, is this the work you specialize in, the physics of exoplanets?

My specialty is very different. I’m trained as a computational neuroscientist. Which means I do computer models of parts of the brain. But in general I’m interested in mathematical modeling of very different systems. In my normal work here at Oxford I work on global catastrophe research, dealing with radical uncertainty about the Earth’s future. So it’s very nice to get something so concrete and specific. The blueberry planet is not what I usually research but it does feed into those issues. Questions like, what about those exoplanets? What can we infer about them from watching from our telescopes? What about Earth? How will its habitability change as the sun’s heating changes over time? You could say this is kind of a stretching exercise.

Your paper is a kind of answer to the act of deleting it from physics Stack Exchange for being “silly.” It’s an argument that it’s not frivolous.

Yes! If you think about physics, many of the best results have come from seemingly frivolous questions. Like, well if I was traveling at the speed of light and watching a light wave, what would I see? That led Einstein to realize, uh-oh, there is something weird going on here. Newton had all these thought experiments of cannons throwing cannonballs further and further distances. In many cases these thought experiments are leading us in interesting places. Or at the very least they allow us to test how well our theory works.

Now to me these calculations look like they would take me a hundred years, but I also have this maybe fanciful sense that actual scientists knock this crap out in like half an hour. How long did it actually take you to figure out, say, that it would take 42 minutes for the planet to collapse from Earth size to Blueberry Earth size?

The first time I calculated that in physics class it probably took me half an hour. Now I can do it quite quickly; I have a good idea what the answer is right away. I still have to recheck it—I’m a very sloppy researcher, so I always recheck my calculations. All in all this paper probably took me half a day. I was on my way to a conference in Germany. Most of the research I had to do was about blueberry physics.

You had to do some testing yourself to figure out the material strength of blueberries.

Yeah. Well, if you put a sugar cube on a blueberry, it doesn’t collapse. But a milk packet, yep it does. So its strength falls in between those two points. That’s already good enough for the order of magnitude we’re working in. But I still wanted to find a work to cite! So I had great fun searching through research on agricultural technology looking for mentions of blueberries.

Seems like a good avenue of research, to dig in deep to blueberry tensile strength.

When you think about it, if you are building a machine to process blueberries, you don’t want it to be filled with blueberry jam. So that research is useful. I found research about mechanical blueberries that are built, placed on bushes, picked by the machines, and then they test the mechanical blueberry for bruising.

I was really taken by the last line of your paper: “Given how exotic exoplanets have turned out, the physics of blueberry earth is actually fairly normal compared to much that is out there.” Please tell me about an exoplanet with crazy physics!

Well, we can expect some planets to have rain that we might never expect; it might rain materials like quartz or sand, things we tend to think of as solids. Or think about a planet that is primarily carbon. Our planet is so rich in oxygen, so our crust is silicate and oxides. But there you might have an atmosphere of carbon monoxide, oceans of a tarlike compound, and mountains of graphite with the occasional diamond. When I grew up we had much more boring ideas of what planets look like, based mostly on Mars or Venus.

Do you think the answer to this question changes substantially if it’s, like, bananas?

Ha ha! Bananas are much more sturdy. The skin will hold together deeper into the crust. The density will be similar to Blueberry Earth. To some degree it’s going to look the same.

Now the tricky part is that bananas are very rich in potassium. They’re actually so rich in the isotope potassium 40 that they can trip radioactivity detectors in some harbors. I think the banana world would get enough potassium to probably heat up extra because of the radioactivity. I don’t have the numbers, but I think it would be a much hotter world.

And then of course there would be the issue of the taste of the ocean. The banana jam. Because you’d have all the peels and stuff, it would probably be a bit yucky.