What If? 2 Read online

  Riverhead Books

  An imprint of Penguin Random House LLC

  penguinrandomhouse.com

  Copyright © 2022 by xkcd inc.

  Penguin Random House supports copyright. Copyright fuels creativity, encourages diverse voices, promotes free speech, and creates a vibrant culture. Thank you for buying an authorized edition of this book and for complying with copyright laws by not reproducing, scanning, or distributing any part of it in any form without permission. You are supporting writers and allowing Penguin Random House to continue to publish books for every reader.

  Riverhead and the R colophon are registered trademarks of Penguin Random House LLC.

  Some of the questions and answers previously appeared on the author’s blog, what-if.xkcd.com, sometimes in different form.

  ISBN 9780525537113 (hardcover)

  ISBN 9780525537120 (ebook)

  ISBN 9780593542903 (international)

  Cover illustrations: Randall Munroe

  Book design by Christina Gleason, adapted for ebook by Maggie Hunt

  pid_prh_6.0_140849174_c0_r0

  Questions

  INTRODUCTION

  1. Soupiter

  2. Helicopter Ride

  3. Dangerously Cold

  4. Ironic Vaporization

  5. Cosmic Road Trip

  6. Pigeon Chair

  S SHORT ANSWERS #1

  7. T. Rex Calories

  8. Geyser

  9. Pew, Pew, Pew

  10. Reading Every Book

  W WEIRD & WORRYING #1

  11. Banana Church

  12. Catch!

  13. Lose Weight the Slow and Incredibly Difficult Way

  14. Paint the Earth

  15. Jupiter Comes to Town

  16. Star Sand

  17. Swing Set

  18. Airliner Catapult

  S SHORT ANSWERS #2

  19. Slow Dinosaur Apocalypse

  20. Elemental Worlds

  21. One-Second Day

  22. Billion-Story Building

  23. $2 Undecillion Lawsuit

  24. Star Ownership

  25. Tire Rubber

  26. Plastic Dinosaurs

  S SHORT ANSWERS #3

  27. Suction Aquarium

  28. Earth Eye

  29. Build Rome in a Day

  30. Mariana Trench Tube

  31. Expensive Shoebox

  32. MRI Compass

  33. Ancestor Fraction

  34. Bird Car

  35. No-Rules NASCAR

  W WEIRD & WORRYING #2

  36. Vacuum Tube Smartphone

  37. Laser Umbrella

  38. Eat a Cloud

  39. Tall Sunsets

  40. Lava Lamp

  41. Sisyphean Refrigerators

  42. Blood Alcohol

  43. Basketball Earth

  44. Spiders vs. the Sun

  45. Inhale a Person

  46. Candy Crush Lightning

  S SHORT ANSWERS #4

  47. Toasty Warm

  48. Proton Earth, Electron Moon

  49. Eyeball

  50. Japan Runs an Errand

  51. Fire from Moonlight

  52. Read All the Laws

  W WEIRD & WORRYING #3

  53. Saliva Pool

  54. Snowball

  55. Niagara Straw

  56. Walking Backward in Time

  57. Ammonia Tube

  58. Earth-Moon Fire Pole

  S SHORT ANSWERS #5

  59. Global Snow

  60. Dog Overload

  61. Into the Sun

  62. Sunscreen

  63. Walking on the Sun

  64. Lemon Drops and Gumdrops

  ACKNOWLEDGMENTS

  REFERENCES

  INDEX

  Disclaimer

  Do not try any of this at home.

  The author of this book is an internet cartoonist, not a health or safety expert. He likes it when things catch fire or explode, which means he does not have your best interests in mind. The publisher and the author disclaim responsibility for any adverse effects resulting, directly or indirectly, from information contained in this book.

  Introduction

  I like ridiculous questions because nobody is expected to know the answer, which means it’s okay to be confused.

  I studied physics in college, so there’s a lot of stuff I feel like I’m supposed to know—like the mass of an electron or why your hair sticks up when you rub a balloon against it. If you ask me how much an electron weighs, I feel a little rush of anxiety, like it’s a pop quiz and I’m going to be in trouble if I don’t know the answer without looking it up.

  But if you ask me how much all the electrons in a bottlenose dolphin weigh, that’s a different situation. No one knows that number off the top of their head—unless they have an extremely cool job—which means it’s okay to feel confused and a little silly and take some time to look stuff up. (The answer, in case anyone ever asks you, is about half a pound.)

  Sometimes simple questions turn out to be unexpectedly hard. Why does your hair stand on end when you rub a balloon on it, anyway? The usual answer from science class is that electrons are transferred from your hair to the balloon, leaving your hair positively charged. The charged hairs repel each other and stick out.

  Except . . . why do electrons get transferred from the hair to the balloon? Why don’t they go the other way?

  That’s a great question, and the answer is that no one knows. Physicists don’t have a good general theory for why some materials shed electrons from their surfaces on contact while other materials pick them up. This phenomenon, called triboelectric charging, is an area of cutting-edge research.

  The same kind of science is used to answer serious questions and silly ones. Triboelectric charging is important to understanding how lightning forms in storms. Counting the number of subatomic particles in an organism is something physicists do when modeling radiation hazards. Trying to answer silly questions can take you through some serious science.

  And even if the answers aren’t useful for anything, knowing them is fun. The book you’re holding weighs about as much as the electrons in two dolphins. That information probably isn’t useful for anything, but I hope you enjoy it, anyway.

  1. SOUPITER

  What would happen if the Solar System was filled with soup out to Jupiter?

  —Amelia, age 5

  Please make sure everyone is safely out of the Solar System before you fill it with soup.

  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 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 the 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 the 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 1/500th the mass of the Milky Way. That’s a monster even by astronomical standards—it would be comparable in size to 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.

  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 for 10 or 15 minutes.

  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 thousand 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.

  2. HELICOPTER RIDE

  What if you were hanging on a helicopter blade by your hands and then someone turned it on?

  —Corban Blanset

  You may be picturing a cool movie action scene like this:

  If so, you’re going to be disappointed, because what would actually happen would be more like this:

  Helicopter rotors take a little while to get up to speed. Once the rotor starts moving, it might take 10 or 15 seconds for it to make its first full turn, so you’d have an uncomfortable amount of time to make eye contact with the pilot before you rotated out of view.

  Luckily, you probably won’t have to pass in front of the pilot a second time, because you’ll fall off embarrassingly quickly.

  Hanging on to the smooth surface of the blade would be hard enough when it was standing still, but even if you found a comfortable handhold, you’d probably lose your grip before the blade finished a single turn.

  Helicopter blades are pretty big, which makes them look like they’re moving more slowly than they really are. We’re not used to large objects moving around that fast. When a helicopter is sitting on the pad with the rotor revolving slowly, it may look pretty gentle, like a dangling mobile rotating over a baby’s crib. But if you tried to hang on to the end of the rotor, you’d find yourself flung outward surprisingly hard.

  It might take 5 to 10 seconds from the time when the rotor starts moving to when it makes its first half-turn. If you were hanging on, by that point you’d already be swinging noticeably outward and you’d feel an extra 10 or 20 pounds of weight from the centrifugal force. Luckily, most helicopter rotors are close enough to the ground that you’d probably survive the fall with only minor injuries and bruised dignity.

  If you do manage to hang on, things will get worse very fast. By the time the blade makes one full turn, [*] the centrifugal force will be pulling on you even harder than gravity, causing you to swing way outward. The extra force would be the equivalent of the weight of another person clinging to you.

  Even if you had a really good grip, you’d probably struggle to hang on. If you wanted to ride the rotor all the way around, you’d need to arrange some kind of system to keep your hands attached to the blade.

  If the rotor kept accelerating at its normal rate, and you somehow stayed attached, then after another full rotation you’d be swinging almost straight outward, with your hands trying to support many times your own body weight. If you hung on for 20 seconds, the rotor would be making one revolution per second, putting several tons of force on your hands. After 30 seconds, you’d have lost your grip on the helicopter one way or another. If your hands stay attached to the rotor, they won’t stay attached to your body.

  This experience won’t be any more pleasant for the helicopter than for you. The rotor wouldn’t be able to keep accelerating like it would during a normal start-up. After all, if your hands are experiencing this much force, then so is the helicopter. A helicopter blade is designed to handle many tons of tension, but that tension is carefully balanced between the blades. If one blade is exerting more force than the other, it will yank the helicopter back and forth, like an unbalanced washing machine.

  Adding just a few ounces of weight to the base of a blade can cause (or cancel out) uncomfortably strong vibrations. Adding a human-size weight to the end of a blade would cause the helicopter to flip itself over and tear itself apart long before it got up to speed.

  Come to think of it, maybe this would make a good movie action scene. You know the scene where the villain’s helicopter is escaping, and the hero runs and jumps and dangles from the landing skids?

  If the hero really wants to keep the villain from escaping . . .

  . . . they should just grab a little higher.

  3. DANGEROUSLY COLD

  Would there be any danger from standing next to a lar
ge object that was 0 Kelvin?

  —Christopher

  So you’ve decided to install an ultracold cube of iron in your living room.

  First of all, definitely don’t touch it. As long as you resist the urge to touch it, you probably won’t suffer any immediate harm.

  Cold things and hot things are different. [citation needed] Standing near a hot object can kill you very fast—for more on this, flip to basically any other random page of this book—but standing near a cold thing won’t freeze you instantly. Hot objects emit thermal radiation that heats up things around them, but cold objects don’t emit cold radiation. They just sit there.

  Even though it doesn’t give off cold radiation, the lack of heat radiation can make you feel cold. Your body, like all warm objects, is constantly radiating heat. Luckily for you, everything around you—like furniture and walls and trees—is also radiating heat, and that incoming radiation partly balances out the heat you’re losing. We usually measure room temperatures in Fahrenheit or Celsius, but setting our thermostats to Kelvin would make it clearer that most of the stuff in the room has roughly the same absolute heat level—since it’s all 250 or 300 degrees Kelvin—so it all radiates heat.

  When you stand near something much colder than room temperature, the heat you’re losing in that direction isn’t balanced by any incoming heat, so that side of your body gets cold much faster. From your point of view, it feels like the object is radiating cold.

  You can feel this “cold radiation” by looking up at the stars on a summer night. Your face will feel cold since your body heat is pouring away into space. If you hold up an umbrella to block your view of the sky, you’ll feel warmer—almost as if the umbrella is “blocking the cold” from the sky. This “cold sky” effect can cool things down to below the ambient air temperature. If you leave out a tray of water under a clear sky, it can turn to ice overnight even if the air temperature stays well above freezing.