What if serious scientific answers to absurd hypothetical questions summary

what if? Serious Scientific Answers to Absurd Hypothetical Questions

RANDALL MUNROE

HOUGHTON MIFFLIN HARCOURT 2014 • BOSTON • NEW YORK

Copyright © 2014 by xkcd Inc.

ALL RIGHTS RESERVED

For information about permission to reproduce selections from this book, write to Permissions, Houghton Mifflin Harcourt Publishing Company, 215 Park Avenue South, New York, New York 10003.

www.hmhco.com

The Library of Congress has cataloged the print edition as follows: Munroe, Randall, author. What if? : serious scientific answers to absurd hypothetical questions / Randall Munroe. pages cm ISBN 978-0-544-27299-6 (hardback) ISBN 978-0-544-45686-0 (international pbk.) 1. Science—Miscellanea. I. Title. Q173.M965 2014 500—dc23 2014016311

Book design by Christina Gleason Lyrics from “If I Didn’t Have You” © 2011 by Tim Minchin. Reprinted by permission of Tim Minchin.

eISBN 978-0-544-27264-4 V1.0914

http://www.hmhco.com/popular-reading
QUESTIONS

Disclaimer [>]

Introduction [>]

Global Windstorm [>]

Relativistic Baseball [>]

Spent Fuel Pool [>]

Weird (and Worrying) Questions from the What If? Inbox, #1 [>]

New York–Style Time Machine [>]

Soul Mates [>]

Laser Pointer [>]

Periodic Wall of the Elements [>]

Everybody Jump [>]

A Mole of Moles [>]

Hair Dryer [>]

Weird (and Worrying) Questions from the What If? Inbox, #2 [>]

The Last Human Light [>]

Machine-Gun Jetpack [>]

Rising Steadily [>]

Weird (and Worrying) Questions from the What If? Inbox, #3 [>]

Orbital Submarine [>]

Short-Answer Section [>]

Lightning [>]

Weird (and Worrying) Questions from the What If? Inbox, #4 [>]

Human Computer [>]

Little Planet [>]

Steak Drop [>]

Hockey Puck [>]

Common Cold [>]

Glass Half Empty [>]

Weird (and Worrying) Questions from the What If? Inbox, #5 [>]

Alien Astronomers [>]

No More DNA [>]

Interplanetary Cessna [>]

Weird (and Worrying) Questions from the What If? Inbox, #6 [>]

Yoda [>]

Flyover States [>]

Falling with Helium [>]

Everybody Out [>]

Weird (and Worrying) Questions from the What If? Inbox, #7 [>]

Self-Fertilization [>]

High Throw [>]

Lethal Neutrinos [>]

Weird (and Worrying) Questions from the What If? Inbox, #8 [>]

Speed Bump [>]

Lost Immortals [>]

Orbital Speed [>]

FedEx Bandwidth [>]

Free Fall [>]

Weird (and Worrying) Questions from the What If? Inbox, #9 [>]

Sparta [>]

Drain the Oceans [>]

Drain the Oceans: Part II [>]

Twitter [>]

Lego Bridge [>]

Longest Sunset [>]

Random Sneeze Call [>]

Weird (and Worrying) Questions from the What If? Inbox, #10 [>]

Expanding Earth [>]

Weightless Arrow [>]

Sunless Earth [>]

Updating a Printed Wikipedia [>]

Facebook of the Dead [>]

Sunset on the British Empire [>]

Stirring Tea [>]

All the Lightning [>]

Loneliest Human [>]

Weird (and Worrying) Questions from the What If? Inbox, #11 [>]

Raindrop [>]

SAT Guessing [>]

Neutron Bullet [>]

Weird (and Worrying) Questions from the What If? Inbox, #12 [>]

Richter 15 [>]

Acknowledgments [>]

References [>]

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

THIS BOOK IS A collection of answers to hypothetical questions. These questions were submitted to me through my website, where—in addition to serving as a sort

of Dear Abby for mad scientists—I draw xkcd, a stick-figure webcomic. I didn’t start out making comics. I went to school for physics, and after graduating, I worked on

robotics at NASA. I eventually left NASA to draw comics full-time, but my interest in science and math didn’t fade. Eventually, it found a new outlet: answering the Internet’s weird—and sometimes worrying—questions. This book contains a selection of my favorite answers from my website, plus a bunch of new questions answered here for the first time.

I’ve been using math to try to answer weird questions for as long as I can remember. When I was five years old, my mother had a conversation with me that she wrote down and saved in a photo album. When she heard I was writing this book, she found the transcript and sent it to me. Here it is, reproduced verbatim from her 25-year-old sheet of paper:

Randall: Are there more soft things or hard things in our house? Julie: I don’t know. Randall: How about in the world? Julie: I don’t know. Randall: Well, each house has three or four pillows, right? Julie: Right. Randall: And each house has about 15 magnets, right? Julie: I guess. Randall: So 15 plus 3 or 4, let’s say 4, is 19, right? Julie: Right. Randall: So there are probably about 3 billion soft things, and . . . 5 billion hard things.

Well, which one wins? Julie: I guess hard things.

To this day I have no idea where I got “3 billion” and “5 billion” from. Clearly, I didn’t really get how numbers worked.

My math has gotten a little better over the years, but my reason for doing math is the same as it was when I was five: I want to answer questions.

They say there are no stupid questions. That’s obviously wrong; I think my question about hard and soft things, for example, is pretty stupid. But it turns out that trying to thoroughly answer a stupid question can take you to some pretty interesting places.

I still don’t know whether there are more hard or soft things in the world, but I’ve learned a lot of other stuff along the way. What follows are my favorite parts of that journey.

RANDALL MUNROE

what if?

A.

GLOBAL WINDSTORM

Q. What would happen if the Earth and all terrestrial objects suddenly stopped spinning, but

the atmosphere retained its velocity? —Andrew Brown

NEARLY EVERYONE WOULD DIE. Then things would get inter​esting. At the equator, the Earth’s surface is moving at about 470 meters per second—a little over a

thousand miles per hour—relative to its axis. If the Earth stopped and the air didn’t, the result would be a sudden thousand-mile-per-hour wind.

The wind would be highest at the equator, but everyone and everything living between 42 degrees north and 42 degrees south—which includes about 85 percent of the world’s population—would suddenly experience supersonic winds.

The highest winds would last for only a few minutes near the surface; friction with the ground would slow them down. However, those few minutes would be long enough to reduce virtually all human structures to ruins.

My home in Boston is far enough north to be just barely outside the supersonic wind zone, but the winds there would still be twice as strong as those in the most powerful tornadoes. Buildings, from sheds to skyscrapers, would be smashed flat, torn from their foundations, and sent tumbling across the landscape.

Winds would be lower near the poles, but no human cities are far enough from the equator to escape devastation. Longyearbyen, on the island of Svalbard in Norway—the highest-latitude city on the planet—would be devastated by winds equal to those in the planet’s strongest tropical cyclones.

If you’re going to wait it out, one of the best places to do it might be Helsinki, Finland. While its high latitude—above 60°N—wouldn’t be enough to keep it from being scoured clean by the winds, the bedrock below Helsinki contains a sophisticated network of tunnels, along with a subterranean shopping mall, hockey rink, swimming complex, and more.

No buildings would be safe; even structures strong enough to survive the winds would be in trouble. As comedian Ron White said about hurricanes, “It’s not that the wind is blowing, it’s what the wind is blowing.”

Say you’re in a massive bunker made out of some material that can withstand thousand-mile-per- hour winds.

That’s good, and you’d be fine . . . if you were the only one with a bunker. Unfortunately, you probably have neighbors, and if the neighbor upwind of you has a less-well-anchored bunker, your bunker will have to withstand a thousand-mile-per-hour impact by their bunker.

The human race wouldn’t go extinct.1 In general, very few people above the surface would survive; the flying debris would pulverize anything that wasn’t nuclear-hardened. However, a lot of people below the surface of the ground would survive just fine. If you were in a deep basement (or, better yet, a subway tunnel) when it happened, you would stand a good chance of surviving.

There would be other lucky survivors. The dozens of scientists and staff at the Amundsen–Scott research station at the South Pole would be safe from the winds. For them, the first sign of trouble would be that the outside world had suddenly gone silent.

The mysterious silence would probably distract them for a while, but eventually someone would notice something even stranger:

The air As the surface winds died down, things would get weirder.

The wind blast would translate to a heat blast. Normally, the kinetic energy of rushing wind is small enough to be negligible, but this would not be normal wind. As it tumbled to a turbulent stop, the air would heat up.

Over land, this would lead to scorching temperature increases and—in areas where the air is moist—global thunderstorms.

At the same time, wind sweeping over the oceans would churn up and atomize the surface layer of the water. For a while, the ocean would cease to have a surface at all; it would be impossible to tell

where the spray ended and the sea began. Oceans are cold. Below the thin surface layer, they’re a fairly uniform 4°C. The tempest would

churn up cold water from the depths. The influx of cold spray into superheated air would create a type of weather never before seen on Earth—a roiling mix of wind, spray, fog, and rapid temperature changes.

This upwelling would lead to blooms of life, as fresh nutrients flooded the upper layers. At the same time, it would lead to huge die-offs of fish, crabs, sea turtles, and animals unable to cope with the influx of low-oxygen water from the depths. Any animal that needs to breathe—such as whales and dolphins—would be hard-pressed to survive in the turbulent sea-air interface.

The waves would sweep around the globe, east to west, and every east-facing shore would encounter the largest storm surge in world history. A blinding cloud of sea spray would sweep inland, and behind it, a turbulent, roiling wall of water would advance like a tsunami. In some places, the waves would reach many miles inland.

The windstorms would inject huge amounts of dust and debris into the atmosphere. At the same time, a dense blanket of fog would form over the cold ocean surfaces. Normally, this would cause global temperatures to plummet. And they would.

At least, on one side of the Earth. If the Earth stopped spinning, the normal cycle of day and night would end. The Sun wouldn’t

completely stop moving across the sky, but instead of rising and setting once a day, it would rise and set once a year.

Day and night would each be six months long, even at the equator. On the day side, the surface would bake under the constant sunlight, while on the night side the temperature would plummet.

Convection on the day side would lead to massive storms in the area directly beneath the Sun.2

In some ways, this Earth would resemble one of the tidally locked exoplanets commonly found in a red dwarf star’s habitable zone, but a better comparison might be a very early Venus. Due to its rotation, Venus—like our stopped Earth—keeps the same face pointed toward the Sun for months at a time. However, its thick atmosphere circulates quite quickly, which results in the day and the night side having about the same temperature.

Although the length of the day would change, the length of the month would not! The Moon hasn’t stopped rotating around the Earth. However, without the Earth’s rotation feeding it tidal energy, the Moon would stop drifting away from the Earth (as it is doing currently) and would start to slowly drift back toward us.

In fact, the Moon—our faithful companion—would act to undo the damage Andrew’s scenario caused. Right now, the Earth spins faster than the Moon, and our tides slow down the Earth’s rotation

while pushing the Moon away from us.3 If we stopped rotating, the Moon would stop drifting away from us. Instead of slowing us down, its tides would accelerate our spin. Quietly, gently, the Moon’s gravity would tug on our planet . . .

. . . and Earth would start turning again.

1 I mean, not right away.

2 Although without the Coriolis force, it’s anyone’s guess which way they would spin.

3 See “Leap Seconds,” http://what-if.xkcd.com/26, for an explanation of why this happens.

http://what-if.xkcd.com/26
A.

RELATIVISTIC BASEBALL

Q. What would happen if you tried to hit a baseball pitched at 90 percent the speed of light?

—Ellen McManis

Let’s set aside the question of how we got the baseball moving that fast. We’ll suppose it’s a normal pitch, except in the instant the pitcher releases the ball, it magically accelerates to 0.9c. From that point onward, everything proceeds according to normal physics.

THE ANSWER TURNS OUT to be “a lot of things,” and they all happen very quickly, and it doesn’t end well for the batter (or the pitcher). I sat down with some physics books, a Nolan Ryan

action figure, and a bunch of videotapes of nuclear tests and tried to sort it all out. What follows is my best guess at a nanosecond-by-nanosecond portrait.

The ball would be going so fast that everything else would be practically stationary. Even the molecules in the air would stand still. Air molecules would vibrate back and forth at a few hundred miles per hour, but the ball would be moving through them at 600 million miles per hour. This means that as far as the ball is concerned, they would just be hanging there, frozen.

The ideas of aerodynamics wouldn’t apply here. Normally, air would flow around anything moving through it. But the air molecules in front of this ball wouldn’t have time to be jostled out of the way. The ball would smack into them so hard that the atoms in the air molecules would actually fuse with the atoms in the ball’s surface. Each collision would release a burst of gamma rays and scattered

particles.1

These gamma rays and debris would expand outward in a bubble centered on the pitcher’s mound. They would start to tear apart the molecules in the air, ripping the electrons from the nuclei and turning the air in the stadium into an expanding bubble of incandescent plasma. The wall of this bubble would approach the batter at about the speed of light—only slightly ahead of the ball itself.

The constant fusion at the front of the ball would push back on it, slowing it down, as if the ball were a rocket flying tail-first while firing its engines. Unfortunately, the ball would be going so fast that even the tremendous force from this ongoing thermonuclear explosion would barely slow it down at all. It would, however, start to eat away at the surface, blasting tiny fragments of the ball in all directions. These fragments would be going so fast that when they hit air molecules, they would trigger two or three more rounds of fusion.

After about 70 nanoseconds the ball would arrive at home plate. The batter wouldn’t even have seen the pitcher let go of the ball, since the light carrying that information would arrive at about the same time the ball would. Collisions with the air would have eaten the ball away almost completely, and it would now be a bullet-shaped cloud of expanding plasma (mainly carbon, oxygen, hydrogen, and nitrogen) ramming into the air and triggering more fusion as it went. The shell of x-rays would hit

the batter first, and a handful of nanoseconds later the debris cloud would hit.

When it would reach home plate, the center of the cloud would still be moving at an appreciable fraction of the speed of light. It would hit the bat first, but then the batter, plate, and catcher would all be scooped up and carried backward through the backstop as they disintegrated. The shell of x-rays and superheated plasma would expand outward and upward, swallowing the backstop, both teams, the stands, and the surrounding neighborhood—all in the first microsecond.

Suppose you’re watching from a hilltop outside the city. The first thing you would see would be a blinding light, far outshining the sun. This would gradually fade over the course of a few seconds, and a growing fireball would rise into a mushroom cloud. Then, with a great roar, the blast wave would arrive, tearing up trees and shredding houses.

Everything within roughly a mile of the park would be leveled, and a firestorm would engulf the surrounding city. The baseball diamond, now a sizable crater, would be centered a few hundred feet behind the former location of the backstop.

Major League Baseball Rule 6.08(b) suggests that in this situation, the batter would be considered “hit by pitch,” and would be eligible to advance to first base.

1 After I initially published this article, MIT physicist Hans Rinderknecht contacted me to say that he’d simulated this scenario on their lab’s computers. He found that early in the ball’s flight, most of the air molecules were actually moving too quickly to cause fusion, and would pass right through the ball, heating it more slowly and uniformly than my original article described.

A.

SPENT FUEL POOL

Q. What if I took a swim in a typical spent nuclear fuel pool? Would I need to dive to actually

experience a fatal amount of radiation? How long could I stay safely at the surface?

—Jonathan Bastien-Filiatrault

ASSUMING YOU’RE A REASONABLY good swimmer, you could probably survive treading water anywhere from 10 to 40 hours. At that point, you would black out from fatigue and

drown. This is also true for a pool without nuclear fuel in the bottom. Spent fuel from nuclear reactors is highly radioactive. Water is good for both radiation shielding

and cooling, so fuel is stored at the bottom of pools for a couple of decades until it’s inert enough to be moved into dry casks. We haven’t really agreed on where to put those dry casks yet. One of these days we should probably figure that out.

Here’s the geometry of a typical fuel storage pool:

The heat wouldn’t be a big problem. The water temperature in a fuel pool can in theory go as high as 50°C, but in practice it’s generally between 25°C and 35°C—warmer than most pools but cooler than a hot tub.

The most highly radioactive fuel rods are those recently removed from a reactor. For the kinds of radiation coming off spent nuclear fuel, every 7 centimeters of water cuts the amount of radiation in half. Based on the activity levels provided by Ontario Hydro in this report, this would be the region of danger for fresh fuel rods:

Swimming to the bottom, touching your elbows to a fresh fuel canister, and immediately swimming back up would probably be enough to kill you.

Yet outside the outer boundary, you could swim around as long as you wanted —the dose from the core would be less than the normal background dose you get walking around. In fact, as long as you were underwater, you would be shielded from most of that normal background dose. You may actually receive a lower dose of radiation treading water in a spent fuel pool than walking around on the street.

Remember: I am a cartoonist. If you follow my advice on safety around nuclear materials, you probably deserve whatever happens to you.

That’s if everything goes as planned. If there’s corrosion in the spent fuel rod casings, there may be some fission products in the water. They do a pretty good job of keeping the water clean, and it wouldn’t hurt you to swim in it, but it’s radioactive enough that it wouldn’t be legal to sell it as

bottled water.1

We know spent fuel pools can be safe to swim in because they’re routinely serviced by human divers.

However, these divers have to be careful. On August 31, 2010, a diver was servicing the spent fuel pool at the Leibstadt nuclear reactor in

Switzerland. He spotted an unidentified length of tubing on the bottom of the pool and radioed his supervisor to ask what to do. He was told to put it in his tool basket, which he did. Due to bubble noise in the pool, he didn’t hear his radiation alarm.

When the tool basket was lifted from the water, the room’s radiation alarms went off. The basket was dropped back in the water and the diver left the pool. The diver’s dosimeter badges showed that he’d received a higher-than-normal whole-body dose, and the dose in his right hand was extremely high.

The object turned out to be protective tubing from a radiation monitor in the reactor core, made highly radioactive by neutron flux. It had been accidentally sheared off while a capsule was being

closed in 2006. It sank to a remote corner of the pool, where it sat unnoticed for four years. The tubing was so radioactive that if he’d tucked it into a tool belt or shoulder bag, where it sat

close to his body, he could’ve been killed. As it was, the water protected him, and only his hand—a body part more resistant to radiation than the delicate internal organs—received a heavy dose.

So, as far as swimming safety goes, the bottom line is that you’d probably be OK, as long as you didn’t dive to the bottom or pick up anything strange.

But just to be sure, I got in touch with a friend of mine who works at a research reactor, and asked him what he thought would happen to someone who tried to swim in their radiation containment pool.

“In our reactor?” He thought about it for a moment. “You’d die pretty quickly, before reaching the water, from gunshot wounds.”

1 Which is too bad — it’d make a hell of an energy drink.

WEIRD (AND WORRYING) QUESTIONS FROM THE WHAT IF? INBOX, #1

Q. Would it be possible to get your teeth to such a cold temperature that they would shatter upon drinking a hot cup of coffee?

—Shelby Hebert

Q. How many houses are burned down in the United States every year? What would be the easiest way to increase that number by a significant

amount (say, at least 15%)? —Anonymous

NEW YORK–STYLE TIME MACHINE

Q. I assume when you travel back in time you end up at the same spot on the Earth’s surface. At least, that’s how it worked in the Back to the

Future movies. If so, what would it be like if you traveled back in time, starting in Times Square, New York, 1000 years? 10,000 years? 100,000 years? 1,000,000 years? 1,000,000,000 years? What about forward in time 1,000,000 years?

—Mark Dettling

1000 years back Manhattan has been continuously inhabited for the past 3000 years, and was first settled by humans perhaps 9000 years ago.

In the 1600s, when Europeans arrived, the area was inhabited by the Lenape people.1 The Lenape were a loose confederation of tribes who lived in what is now Connecticut, New York, New Jersey, and Delaware.

A thousand years ago, the area was probably inhabited by a similar collection of tribes, but those inhabitants lived half a millennium before European contact. They were as far removed from the Lenape of the 1600s as the Lenape of the 1600s are from the modern day.

To see what Times Square looked like before a city was there, we turn to a remarkable project called Welikia, which grew out of a smaller project called Mannahatta. The Welikia project has produced a detailed ecological map of the landscape in New York City at the time of the arrival of Europeans.

The interactive map, available online at welikia.org, is a fantastic snapshot of a different New York. In 1609, the island of Manhattan was part of a landscape of rolling hills, marshes, woodlands, lakes, and rivers.

The Times Square of 1000 years ago may have looked ecologically similar to the Times Square described by Welikia. Superficially, it probably resembled the old-growth forests that are still found

http://www.welikia.org
in a few locations in the northeastern US. However, there would be some notable differences. There would be more large animals 1000 years ago. Today’s disconnected patchwork of

northeastern old-growth forests is nearly free of large predators; we have some bears, few wolves and coyotes, and virtually no mountain lions. (Our deer populations, on the other hand, have exploded, thanks in part to the removal of large predators.)

The forests of New York 1000 years ago would be full of chestnut trees. Before a blight passed through in the early twentieth century, the hardwood forests of eastern North America were about 25 percent chestnut. Now, only their stumps survive.

You can still come across these stumps in New England forests today. They periodically sprout new shoots, only to see them wither as the blight takes hold. Someday, before too long, the last of the stumps will die.

Wolves would be common in the forests, especially as you moved inland. You might also encounter

mountain lions2,3,4,5,6 and passenger pigeons.7

There’s one thing you would not see: earthworms. There were no earthworms in New England when the European colonists arrived. To see the reason for the worms’ absence, let’s take our next step into the past.

10,000 years back The Earth of 10,000 years ago was just emerging from a deep cold period.

The great ice sheets that covered New England had departed. As of 22,000 years ago, the southern

edge of the ice was near Staten Island, but by 18,000 years ago it had retreated north past Yonkers. 8

By the time of our arrival, 10,000 years ago, the ice had largely withdrawn across the present-day Canadian border.

The ice sheets scoured the landscape down to bedrock. Over the next 10,000 years, life crept slowly back northward. Some species moved north faster than others; when Europeans arrived in New England, earthworms had not yet returned.

As the ice sheets withdrew, large chunks of ice broke off and were left behind.

When these chunks melted, they left behind water-filled depressions in the ground called kettlehole ponds. Oakland Lake, near the north end of Springfield Boulevard in Queens, is one of these kettlehole ponds. The ice sheets also dropped boulders they’d picked up on their journey; some of these rocks, called glacial erratics, can be found in Central Park today.

Below the ice, rivers of meltwater flowed at high pressure, depositing sand and gravel as they went. These deposits, which remain as ridges called eskers, crisscross the landscape in the woods outside my home in Boston. They are responsible for a variety of odd landforms, including the world’s only vertical U-shaped riverbeds.

100,000 years back The world of 100,000 years ago might have looked a lot like our own.9 We live in an era of rapid,

pulsating glaciations, but for 10,000 years our climate has been stable10 and warm. A hundred thousand years ago, Earth was near the end of a similar period of climate stability. It

was called the Sangamon interglacial, and it probably supported a developed ecology that would look familiar to us.

The coastal geography would be totally different; Staten Island, Long Island, Nantucket, and Martha’s Vineyard were all berms pushed up by the most recent bulldozer-like advance of the ice. A hundred millennia ago, different islands dotted the coast.

Many of today’s animals would be found in those woods—birds, squirrels, deer, wolves, black bears—but there would be a few dramatic additions. To learn about those, we turn to the mystery of the pronghorn.

The modern pronghorn (American antelope) presents a puzzle. It’s a fast runner—in fact, it’s much faster than it needs to be. It can run at 55 mph, and sustain that speed over long distances. Yet its fastest predators, wolves and coyotes, barely break 35 mph in a sprint. Why did the pronghorn evolve such speed?

The answer is that the world in which the pronghorn evolved was a much more dangerous place than ours. A hundred thousand years ago, North American woods were home to Canis dirus (the dire wolf), Arctodus (the short-faced bear), and Smilodon fatalis (sabre-toothed cat), each of which may have been faster and deadlier than modern predators. All died out in the Quaternary extinction event,

which occured shortly after the first humans colonized the continent.11

If we go back a little further, we will meet another frightening predator.