What If You Tried to Swallow a Whole Cloud?

What does a star smell like? What’s the tensile strength of snow? How much actual dinosaur does a toy dinosaur contain? If you drove a car to the edge of the universe, how much gas would you use? What’s the market value of a shoebox full of LSD? Are all the churches in the world big enough to hold all the bananas? 

“I like ridiculous questions, because nobody is expected to know the answer,” Randall Munroe writes in the introduction to What If? 2, his second book of serious answers to hypothetical questions sent in by fans. You may know Munroe as the creator of the web comic xkcd, which offers wry commentary (usually involving stick figures wearing various hats) on science, technology, history, politics, and the daily micro- and macro-­follies of human life. There’s a sincerity about his projects, a nerdy excitement that reminds you of talking to a 10-year-old—assuming you know a 10-year-old who can explain what a naked singularity is and why theoretical physicists hate them. 

Nerdy-kid excitement is what WIRED is all about. The following pages contain Munroe’s answers to two questions: What if some of outer space turned to soup? And what if you tried to swallow a cloud?

If the solar system were full of soup out to Jupiter, things might be okay for some people for a few minutes. Then, for the next half hour, things would definitely not be okay for anyone. After that, time as we know it would end.

Filling the solar system would take about 2 × 10³⁹ liters of soup. If the soup is tomato, that works out to about 10⁴² calories’ worth, more energy than the sun has put out over its entire lifetime.

The soup would be so heavy that nothing would be able to escape its enormous gravitational pull; it would be a black hole. The event horizon of the black hole, the region where the pull is too strong for light to escape, would extend to the orbit of Uranus. Pluto would be outside the event horizon at first, but that doesn’t mean it would escape. It would just have a chance to broadcast out a radio message before being vacuumed up.

What would the soup look like from inside?

You wouldn’t want to stand on the surface of Earth. Even if we assume the soup is rotating in sync with the planets in the solar system, with little whirlpools surrounding each planet so the soup is stationary where it touches their surfaces, the pressure due to Earth’s gravity would crush anyone on the planet within seconds. Earth’s gravity may not be as strong as a black hole’s, but it’s more than enough to pull an ocean of soup down hard enough to squish you. After all, the pressure of our regular water oceans under Earth's gravity can do that, and Amelia's soup is a lot deeper than the ocean.

If you were floating between the planets, away from Earth’s gravity, you’d actually be okay for a little while, which is kind of weird. Even if the soup didn’t kill you, you’d still be inside a black hole. Shouldn’t you die instantly from … something?

Strangely enough, no! Normally, when you get close to a black hole, tidal forces tear you apart. But tidal forces are weaker for larger black holes, and the Jupiter Soup black hole would be about 1/500 the mass of the Milky Way. That’s a monster even by astronomical standards—it would be comparable in size to some of the largest known black holes. Amelia’s souper-massive black hole would be large enough that the different parts of your body would experience about the same pull, so you wouldn’t be able to feel any tidal forces at first.

Even though you wouldn’t be able to feel the soup’s gravitational pull, it would still accelerate you, and you would immediately begin to plunge toward the center. After a second had passed, you’d have fallen 20 kilometers and you’d be traveling at 40 kilometers per second, faster than most spacecraft. But since the soup would be falling along with you, you’d feel like nothing was wrong.

As the soup collapsed inward toward the center of the solar system, its molecules would be squeezed closer together and the pressure would rise. It would take a few minutes for this pressure to build up to levels that would crush you. If you were in some kind of a soup bathyscaphe, the pressure vessels that people use to visit deep ocean trenches, you could conceivably last a little longer.

There would be nothing you could do to escape the soup. Everything inside it would flow inward toward the singularity. In the regular universe, we’re all dragged forward through time with no way to stop or back up. Inside a black hole’s event horizon, in a sense, time stops flowing forward and starts flowing inward. All time lines converge toward the center.

From the point of view of an unlucky observer inside our black hole, it would take about half an hour for the soup and everything in it to fall to the center. After that, our definition of time—and our understanding of physics in general—breaks down.

Outside the soup, time would continue passing and problems would keep happening. The black hole of soup would start slurping up the rest of the solar system, starting with Pluto almost immediately, and the Kuiper belt shortly thereafter. Over the course of the next few million years, the black hole would cut a large swath through the Milky Way, gobbling up stars and scattering more in all directions.

This leaves us with one more question: What kind of soup is this, anyway?

If Amelia fills the solar system with broth, and there are planets floating in it, is it planet soup? If there are already noodles in the soup, does it become planet-and-noodle soup, or are the planets more like croutons? If you make a noodle soup, then someone sprinkles some rocks and dirt in it, is it really noodle-and-dirt soup, or is it just noodle soup that got dirty? Does the presence of the sun make this star soup?

The internet loves arguing about soup categorization. Luckily, physics can settle the debate in this particular case. It’s believed that black holes don’t retain the characteristics of the matter that goes into them. Physicists call this the “no-hair theorem,” because it says that black holes don’t have any distinguishing traits or defining characteristics. Other than a handful of simple variables like mass, spin, and electric charge, all black holes are identical.

In other words, it doesn’t matter what kind of ingredients you put into a black hole soup. The recipe always turns out the same in the end.

Clouds are made of water, which is edible. Or drinkable, I guess. Potable? I’ve never been sure where the line between eating and drinking is.

Clouds also contain air. We don’t usually count air as part of food, since it escapes from your mouth as you chew or—in some cases—soon after you swallow.

You can certainly put a piece of a cloud in your mouth and swallow the water it contains. The problem is that you’ll need to let the air escape—but air that’s been inside your body will have absorbed a lot of moisture. When it leaves your mouth, it will carry that moisture with it, and once it encounters the cool, cloudy air, it will condense. In other words, if you try to eat a cloud, you’ll just burp out more cloud faster than you can eat it.

But if you can collect the droplets together—perhaps by passing the cloud through a fine mesh and squeezing it out, or ionizing the droplets and collecting them on charged wires—you could absolutely eat a small cloud.

A fluffy cumulus cloud the size of a house could contain about a liter of liquid water, or two or three large glasses, which is about the volume a human stomach can comfortably hold at one time. You couldn’t eat a huge cloud, but you could absolutely eat one of those small house-size ones that briefly block the sun for a second or two when they pass overhead.

A cloud is just about the largest thing you could eat in one sitting. There aren’t a lot of things puffier and lower-density. Whipped cream seems pretty fluffy, but I’m told it’s 15 percent as dense as water,* so a gallon of whipped cream would weigh about a pound. Even accounting for all the air that would escape, you couldn’t eat more than a small bucket of it. Cotton candy, one of the most cloudlike foods, has a very low density—about 5 percent that of water—which means that you could in theory eat about a cubic foot of it in one sitting. That wouldn’t necessarily be healthy, but it would be possible. But even if you spent your entire life eating cotton candy, you'd probably die before you ate enough to fill a house.

Other extremely lightweight edible substances include snow, meringues, and bags of potato chips, but you probably couldn’t eat a cubic foot of any of them in a single sitting.

So if you want to eat a cloud, you’ll need to do some work, but if you succeed, you’ll have the satisfaction of knowing that you’ve eaten the largest thing you can possibly eat.

Just remember to store your cloud in a reusable bottle. There’s no need to waste all that plastic!

* Citation: Tracy V. Wilson, host of the podcast Stuff You Missed in History Class, who happened to have a cooking scale and a can of whipped cream on hand when I got this question.

From What If? 2, by Randall Munroe. Published by arrangement with Riverhead. Copyright © 2022 by xkcd inc.

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