How to Teach Physics to Your Dog Read online

  HOW TO TEACH

  PHYSICS

  TO YOUR DOG

  CHAD ORZEL

  SCRIBNER

  A Division of Simon & Schuster, Inc.

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  Copyright © 2009 by Chad Orzel, Ph.D.

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  First Scribner hardcover edition December 2009

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  DESIGNED BY ERICH HOBBING

  Manufactured in the United States of America

  1 3 5 7 9 10 8 6 4 2

  Library of Congress Control Number: 2009021073

  ISBN 978-1-4165-7228-2

  ISBN 978-1-4165-7901-4 (ebook)

  To Kate,

  whose laugh

  started the whole thing

  Contents

  Introduction

  Why Talk to Your Dog about Physics? An Introduction to Quantum Physics

  Chapter 1

  Which Way? Both Ways: Particle-Wave Duality

  Chapter 2

  Where’s My Bone? The Heisenberg Uncertainty Principle

  Chapter 3

  Schrödinger’s Dog: The Copenhagen Interpretation

  Chapter 4

  Many Worlds, Many Treats: The Many-Worlds Interpretation

  Chapter 5

  Are We There Yet? The Quantum Zeno Effect

  Chapter 6

  No Digging Required: Quantum Tunneling

  Chapter 7

  Spooky Barking at a Distance: Quantum Entanglement

  Chapter 8

  Beam Me a Bunny: Quantum Teleportation

  Chapter 9

  Bunnies Made of Cheese: Virtual Particles and Quantum Electrodynamics

  Chapter 10

  Beware of Evil Squirrels: Misuses of Quantum Physics

  Acknowledgments

  Further Reading

  Glossary of Important Terms

  HOW TO TEACH PHYSICS TO YOUR DOG

  INTRODUCTION

  Why Talk to Your Dog about Physics?

  An Introduction to Quantum Physics

  The Mohawk-Hudson Humane Society has set up a little path through the woods near their facility outside Troy, so you can take a walk with a dog you’re thinking of adopting. There’s a bench on the side of the path in a small clearing, and I sit down to look at the dog I’ve taken out.

  She sits down next to the bench, and pokes my hand with her nose, so I scratch behind her ears. My wife and I have looked at a bunch of dogs together, but Kate had to work, so I’ve been dispatched to pick out a dog by myself. This one seems like a good fit.

  She’s a year-old mixed-breed dog, German shepherd and something else. She’s got the classic shepherd black and tan coloring, but she’s small for a shepherd, and has floppy ears. The tag on her kennel door gave her name as “Princess,” but that doesn’t seem appropriate.

  “What do you think, girl?” I ask. “What should we call you?”

  “Call me Emmy!” she says.

  “Why’s that?”

  “Because it’s my name, silly.”

  Being called “silly” by a dog is a little surprising, but I guess she has a point. “Okay, I can’t argue with that. So, do you want to come live with us?”

  “Well, that depends,” she says. “What’s the critter situation like?”

  “Beg pardon?”

  “I like to chase things. Will there be critters for me to chase?”

  “Well, yeah. We’ve got a good-sized yard, and there are lots of birds and squirrels, and the occasional rabbit.”

  “Ooooh! I like bunnies!” She wags her tail happily. “How about walks? Will I get walks?”

  “Of course.”

  “And treats? I like treats.”

  “You’ll get treats if you’re a good dog.”

  She looks faintly offended. “I am a very good dog. You will give me treats. What do you do for a living?”

  “What? Who’s evaluating who, here?”

  “I need to know if you deserve a dog as good as me.” The name “Princess” may have been more apt than I thought. “What do you do for a living?”

  “Well, my wife, Kate, is a lawyer, and I’m a professor of physics at Union College, over in Schenectady. I teach and do research in atomic physics and quantum optics.”

  “Quantum what?”

  “Quantum optics. Broadly defined, it’s the study of the interaction between light and atoms in situations where you have to describe one or both of them using quantum physics.”

  “That sounds complicated.”

  “It is, but it’s fascinating stuff. Quantum physics has all sorts of weird and wonderful properties. Particles behave like waves, and waves behave like particles. Particle properties are indeterminate until you measure them. Empty space is full of ‘virtual particles’ popping in and out of existence. It’s really cool.”

  “Hmmm.” She looks thoughtful, then says, “One last test.”

  “What’s that?”

  “Rub my belly.” She flops over on her back, and I reach down to rub her belly. After a minute of that, she stands up, shakes herself off, and says “Okay, you’re pretty good. Let’s go home.”

  We head back to the kennel to fill out the adoption paperwork. As we’re walking, she says, “Quantum physics, huh? I’ll have to learn something about that.”

  “Well, I’d be happy to explain it to you sometime.”

  Like most dog owners, I spend a lot of time talking to my dog. Most of our conversations are fairly typical—don’t eat that, don’t climb on the furniture, let’s go for a walk. Some of our conversations, though, are about quantum physics.

  Why do I talk to my dog about quantum physics? Well, it’s what I do for a living: I’m a college physics professor. As a result, I spend a lot of time thinking about quantum physics.

  What is quantum physics? Quantum physics is one part of “modern physics,” meaning physics based on laws discovered after about 1900. Laws and principles of physics that were developed before about 1900 are considered “classical” physics.

  Classical physics is the physics of everyday objects—tennis balls and squeaky toys, stoves and ice cubes, magnets and electrical wiring. Classical laws of motion govern the motion of anything large enough to see with the naked eye. Classical thermodynamics explains the physics of heating and cooling objects, and the operation of engines and refrigerators. Classical electromagnetism explains the behavior of lightbulbs, radios, and magnets.

  Modern physics describes the stranger world that we see when we go beyond the everyday. This world was first revealed in experiments done in the late 1800s and early 1900s, which cannot be explained with classical laws of physics. New fields with different rules needed to be developed.

  Modern physics is divided into two parts, each representing a radical departure from classical rules. One part, relativity, deals with objects that move very fast, or are in the presence of strong gravitational forces. Albert Einstein introduced relativity in 1905, and
it’s a fascinating subject in its own right, but beyond the scope of this book.

  The other part of modern physics is what I talk to my dog about. Quantum physics or quantum mechanics* is the name given to the part of modern physics dealing with light and things that are very small—molecules, single atoms, subatomic particles. Max Planck coined the word “quantum” in 1900, and Einstein won the Nobel Prize for presenting the first quantum theory of light.† The full theory of quantum mechanics was developed over the next thirty years or so.

  The people who made the theory, from early pioneers like Planck and Niels Bohr, who made the first quantum model of the hydrogen atom, to later visionaries like Richard Feynman and Julian Schwinger, who each independently worked out what we now call “quantum electrodynamics” (QED), are rightly regarded as titans of physics. Some elements of quantum theory have even escaped the realm of physics and captured the popular imagination, like Werner Heisenberg’s uncertainty principle, Erwin Schrödinger’s cat paradox, and the parallel universes of Hugh Everett’s many-worlds interpretation.

  Modern life would be impossible without quantum mechanics. Without an understanding of the quantum nature of the electron, it would be impossible to make the semiconductor chips that run our computers. Without an understanding of the quantum nature of light and atoms, it would be impossible to make the lasers we use to send messages over fiber-optic communication lines.

  Quantum theory’s effect on science goes beyond the merely practical—it forces physicists to grapple with issues of philosophy. Quantum physics places limits on what we can know about the universe and the properties of objects in it. Quantum mechanics even changes our understanding of what it means to make a measurement. It requires a complete rethinking of the nature of reality at the most fundamental level.

  Quantum mechanics describes an utterly bizarre world, where nothing is certain and objects don’t have definite properties until you measure them. It’s a world where distant objects are connected in strange ways, where there are entire universes with different histories right next to our own, and where “virtual particles” pop in and out of existence in otherwise empty space.

  Quantum physics may sound like the stuff of fantasy fiction, but it’s science. The world described in quantum theory is our world, at a microscopic scale.* The strange effects predicted by quantum physics are real, with real consequences and applications. Quantum theory has been tested to an incredible level of precision, making it the most accurately tested theory in the history of scientific theories. Even its strangest predictions have been verified experimentally (as we’ll see in chapters 7, 8, and 9).

  So, quantum physics is neat stuff. But what does it have to do with dogs?

  Dogs come to quantum physics in a better position than most humans. They approach the world with fewer preconceptions than humans, and always expect the unexpected. A dog can walk down the same street every day for a year, and it will be a new experience every day. Every rock, every bush, every tree will be sniffed as if it had never been sniffed before.

  If dog treats appeared out of empty space in the middle of a kitchen, a human would freak out, but a dog would take it in stride. Indeed, for most dogs, the spontaneous generation of treats would be vindication—they always expect treats to appear at any moment, for no obvious reason.

  Quantum mechanics seems baffling and troubling to humans because it confounds our commonsense expectations about how the world works. Dogs are a much more receptive audience. The everyday world is a strange and marvelous place to a dog, and the predictions of quantum theory are no stranger or more marvelous than, say, the operation of a doorknob.*

  Discussing quantum physics with my dog is useful because it helps me see how to discuss quantum mechanics with humans. Part of learning quantum mechanics is learning to think like a dog. If you can look at the world the way a dog does, as an endless source of surprise and wonder, then quantum mechanics will seem a lot more approachable.

  This book reproduces a series of conversations with my dog about quantum physics. Each conversation is followed by a detailed discussion of the physics involved, aimed at interested human readers. The topics range from ideas many people have heard of, like particle-wave duality (chapter 1) and the uncertainty principle (chapter 2), to the more advanced ideas of virtual particles and QED (chapter 9). These explanations include discussion of both the weird predictions of the theory (both practical and philosophical), and the experiments that demonstrate these predictions. They’re selected for what dogs find most interesting and also illustrate the parts that humans find surprising.

  “I don’t know. I think it needs . . . more.”

  “More what?”

  “More me. You don’t talk about the fact that I’m an exceptionally smart dog.”

  “Well, okay—”

  “And exceptionally cute, too.”

  “Sure, but—”

  “And don’t forget good. I’m way better than those other dogs.”

  “What other dogs?”

  “Dogs who aren’t me.”

  “Look, this is really a book about physics, not a book about you.”

  “Well, it ought to be more about me, that’s all I’m saying.”

  “It’s not, and you’ll just have to live with that.”

  “Okay, fine. You need my help with the physics stuff, though.”

  “What do you mean?”

  “Well, sometimes you leave some stuff out, and don’t answer all of my questions. You shouldn’t do that.”

  “Like what? Give me an example.”

  “Ummm . . . I can’t think of one now. If you read it to me, though, I’ll point them out, and help fix them.”

  “Okay, that sounds fair. Here’s what we’ll do. We’ll go over the book together, and if there are places where you think I’ve left stuff out, we can talk about them, and I’ll put your comments in the book.”

  “Talk about them like we’re doing now?”

  “Yeah, like we’re doing now.”

  “And you’ll put the conversation in the book?”

  “Yes, I will.”

  “In that case, we should talk about how I’m the very best, and I’m cute, and I should get more treats, and—”

  “Okay, that’s about enough of that.”

  “For now.”

  * The terms “quantum physics,” “quantum theory,” and “quantum mechanics” are more or less interchangeable.

  † Inventing relativity didn’t exactly hurt, but the official reason for Einstein’s Nobel was his quantum theory of the photoelectric effect (page 22).

  * “Microscopic” for a physicist means anything too small to be seen with the naked eye. This covers a range from bacteria to atoms to electrons. It’s a wide range of sizes, but physicists think it would be confusing to have more than one word for small things.

  * Which unquestionably follows classical rules, but does, alas, require opposable thumbs to operate.

  CHAPTER 1

  Which Way? Both Ways: Particle-Wave Duality

  We’re out for a walk, when the dog spots a squirrel up ahead and takes off in pursuit. The squirrel flees into a yard and dodges around a small ornamental maple. Emmy doesn’t alter her course in the slightest, and just before she slams into the tree, I pull her up short.

  “What’d you do that for?” she asks, indignantly.

  “What do you mean? You were about to run into a tree, and I stopped you.”

  “No I wasn’t.” She looks off after the squirrel, now safely up a bigger tree on the other side of the yard. “Because of quantum.”

  We start walking again. “Okay, you’re going to have to explain that,” I say.

  “Well, I have this plan,” she says. “You know how when I chase the bunnies in the backyard, when I run to the right of the pond, they go left, and get away?”

  “Yes.”

  “And when I run to the left of the pond, they go right, and get away?”

  “Yes.”

  �
��Well, I’ve thought of a new way to run, so they can’t escape.”

  “What, through the middle of the pond?” It’s only about eight inches deep and a couple of feet across.

  “No, silly. I’m going to go both ways. I’ll trap the bunnies between me.”

  “Uh-huh. That’s an . . . interesting theory.”

  “It’s not a theory, it’s quantum physics. Material particles have wave nature and can diffract around objects. If you send a beam of electrons at a barrier, they’ll go around it to the left and to the right, at the same time.” She’s really getting into this, and she doesn’t even notice the cat sunning itself in the yard across the street. “So, I’ll just make use of my wave nature, and go around both sides of the pond.”

  “And where does running headfirst into a tree come in?”

  “Oh, well.” She looks a little sheepish. “I thought I would try it out on something smaller first. I got a good running start, and I was just about to go around when you stopped me.”

  “Ah. Like I said, an interesting theory. It won’t work, you know.”

  “You’re not going to try to claim I don’t have wave nature, are you? Because I do. It’s in your physics books.”

  “No, no, you’ve got wave nature, all right. You’ve also got Buddha nature—”

  “I’m an enlightened dog!”

  “—which will do you about as much good. You see, a tree is big, and your wavelength is small. At walking speed, a twenty-kilogram dog like you has a wavelength of about 10-35 meters. You need your wavelength to be comparable to the size of the tree—maybe ten centimeters—in order to diffract around it, and you’re thirty-four orders of magnitude off.”

  “I’ll just change my wavelength by changing my momentum. I can run very fast.”

  “Nice try, but the wavelength gets shorter as you go faster. To get your wavelength up to the millimeter or so you’d need to diffract around a tree, you’d have to be moving at 10-30 meters per second, and that’s impossibly slow. It would take a billion years to cross the nucleus of an atom at that speed, which is way too slow to catch a bunny.”