How to Teach Quantum Physics to Your Dog
By Chad Orzel
4.5/5
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About this ebook
When physics professor Chad Orzel went to the pound to adopt a dog, he never imagined Emmy. She wasn’t just a friendly mutt who needed a home. Soon she was trying to use the strange ideas of quantum mechanics for the really important things in her life: chasing critters, getting treats, and going for walks. She peppered Chad with questions: Could she use quantum tunneling to get through the neighbor’s fence and chase bunnies? What about quantum teleportation to catch squirrels before they climb out of reach? Where are all the universes in which Chad drops steak on the floor?
With great humor and clarity, Chad Orzel explains to Emmy, and to human readers, just what quantum mechanics is and how it works—and why, although you can’t use it to catch squirrels or eat steak, it’s still bizarre, amazing, and important to every dog and human.
Chad Orzel
Chad Orzel was born and raised in central New York, and received a degree in physics from Williams College, and his Ph.D. in chemical physics from the University of Maryland. He is now a professor in the Department of Physics and Astronomy at Union College in Schenectady, New York. He lives near campus with his wife Kate, their daughter, and, of course, Emmy, the Queen of Niskayuna.
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Reviews for How to Teach Quantum Physics to Your Dog
41 ratings11 reviews
- Rating: 4 out of 5 stars4/5excellent book, easy to read and understand the Quantum physics
- Rating: 4 out of 5 stars4/5Quantum physics may still remain an unsolved mystery to me, for the most part, but I feel I understand some of its quirky aspects better now. I've read a few books for laymen on this subject, and the sad fact is that it may be simply so counter-intuitive that it my brain won't accept it. My 'that don't make sense' filter seems to kick in. When I read such books, I keep stopping to question the findings and ask, "How could that be?" (Unfortunately, my math skills are inadequate to help me overcome this.) Still, somehow I feel there must be some underlying order in the universe, but, then again, perhaps not.
In any event, this book is a (supposedly) imagined conversation between a physics professor and his dog. I found it quite entertaining. Things I especially liked were the one Terry Pratchett reference (and footnotes)and the last chapter, which is about the charlatans and other profiteers who misuse the language of quantum physics to promote bogus things like free energy and magic healing. If you're fascinated by quantum and what it means, pick up this book. I can't say you'll understand quantum electrodynamics when you're done reading it, but you will be entertained and perhaps more knowledgeably confused, and, most importantly, less likely to be taken in by hucksters misusing the terms to sell quantum snake oil.2 people found this helpful
- Rating: 4 out of 5 stars4/5Well, it certainly has a unique selling point.A quixotic quest, you would think: rendering the world of quantum physics understandable for the layman - approached quixotically: by fabricating a dialogue between the author, a physics professor, and his rabbit-chasing, treat-loving Alsatian dog.There is something oddly Socratic about Orzal's interlocutions with Emmy the Alsatian. Only in the Dialogues, no-one says "that sounds ridiculous, Socrates". Emmy has no such qualms.As a conceptual device it works as well as it can be expected to, though at times both Chad and Emmy expect too much of their readers. That is one smart dog.Over the years I've read plenty of popular science treatments of quantum physics (more than your average bear, I dare say, but of course that's not to say I necessarily understood them), and I still found my eyes glazing over at some of the depth to which Orzal was obliged (or at any rate inclined) to descend in expounding quantum theory. Emmy stays with him throughout, and eggs him on.I have always harboured suspicions about the scientific inviolability claimed of this sort of physics. Real, falsifiable empirical evidence seems in short supply (often being suspiciously forbidden by the very terms of the theory, or at least buried under many sedimentary layers of mathematical assumption) and quantum effects have a habit of conveniently being unobservable in any dimension meaningful to everyday life. Or vanishing (er, I mean, collapsing the wave function) when you try to measure them.Which, to this old sceptic, gives them a religious sort of disposition - true by definition; true because smart men learned in arcane lore say so. (I should say I'm not alone in this view: properly credentialised physicists like Peter Woit and Lee Smolin have also expressed it).That said, Orzal is no (ahem) dogmatist (indeed, trying to de-mystify the scriptures as he does makes him more like a sort of Lutheran reformer), and I think is prepared to admit of some missing links in the overall theory (I couldn't work out whether quantum entanglement, which is "non local", falsifies relativity or not).Then again, the only practical upshot I could derive of all this colossally brain-contorting discipline is the possibility of "quantum computing" - apparently faster and cheaper than boring old silicon.Orzal's main objective, finally arrived at in the closing chapter, is to debunk phoney new age baloney which purports to trade on quantum underpinnings - quantum healing, and that kind of thing. This is done effectively, but at some cost: by underlining the singular uselessness of quantum theory in every day volumes, velocities and quantities. Whenever it would come in handy (often, when chasing squirrels, as Emmy astutely observes), by its own theory, quantum effects would be unobservably minuscule.Which makes this old goat wonder why we bother digging up the Swiss countryside and dropping trillions of dollars of supercooled electromagnets into it just to find another unobservable subatomic particle. Surely we can figure out whether quantum computing works by trying to building a quantum computer?In the final analysis, and as other reviewers have said, I put this book down having a better general understanding of the gist of a number of really quite difficult concepts - enough to keep my end of the conversation up if sat next to a physicist at dinner - even if the details and implications below that remain entirely murky - and so in that regard, it is a tough job imaginatively and successfully done.And, now matter how cute the device seems, you can't help but like the irrepressible Emmy, even if she does understand Schrodinger's indeterminacy better than I do.
2 people found this helpful
- Rating: 4 out of 5 stars4/5The best explanation I've found of quantum physics' basic principles for a lay lay lay person. You can infer from the title that this book does not belong on a doctoral candidate's Works Cited page. If, however, you've found yourself unable to recall the major foundations of quantum physics no matter how many cute animated YouTube videos you've watched, you'll have no trouble after reading this. A humorous midpoint between a tome and a For Dummies, and a quick read.
1 person found this helpful
- Rating: 4 out of 5 stars4/5A very readable introduction to the current knowledge about quantum physics. The interludes with the dog both gave examples of the concepts at a simplified level and provided good in-book breaks from the more dense discussion (and let's face it; QM is complicated enough that even a popularized version takes some brainpower to read).
1 person found this helpful
- Rating: 4 out of 5 stars4/5This had more physics and less dog that I could have asked for, but it was totally readable and fascinating. I don't have the background to comment on how accurate it is, but I would totally recommend it as a quick overview of the basic concepts in quantum physics.
1 person found this helpful
- Rating: 4 out of 5 stars4/5Generally good clear descriptions of quantum physics in an accessible fashion. However in some respects the attempt to do this by teaching it to a dog was less than perfect. At times the links to the dog were effectively abandoned, at others the links and analogies were perhaps rather forced.
1 person found this helpful
- Rating: 4 out of 5 stars4/5I was initially slightly uneasy about this book. Throughout the first pages, topics I had previously encountered in formal education were presented: I wasn't sure that I'd understand them if I'd been encountering them for the first time. The books was presented in a carefully thought out manner, with ten clearly defined chapters. As the book developed my initial doubts went away. The subject matter is not easy, but I finished the book feeling satisfied I had learnt something and that I was able to (at least partially) understand all of the material.Before reading the book I was well aware that I am not a dog lover! But that didn't matter, the dog element didn't get in the way, and the two "characters" helped lighten the tone and make for a more enjoyable read.
1 person found this helpful
- Rating: 4 out of 5 stars4/5If 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 [... dogs] always expect treats to appear at any moment, for no obvious reason. [...] 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.Chad Orzel enlists his German shepherd, Emmy, to provide that perspective in How to Teach Physics to Your Dog. Each chapter opens with Emmy voicing some problem (usually related to improving her chances of nabbing a squirrel or bunnie or treat) that gives Chad a jumping-off point into physics concepts like particle-wave duality, uncertainty, measurement, teleportation. It’s a cute device but it’s also effective -- alternating between theory and man-dog conversation both lightens the content and applies it to the everyday.I endured a year of classical physics in college but came to this book after having developed an adulthood interest in quantum physics. I came away much more aware of the concepts involved, but only slightly more in command of them. (Maybe that’s the way with physics, little by little?) But to be clear: this is a physics book and Orzel writes for a pop-sci physics audience: “Most people wouldn’t know a wavefunction if they tripped over one, but almost everyone has heard of the uncertainty principle…” If that’s not you, or if you’re not interested in photons, probability, and polarization, this is not the book for you. If that is you, enjoy! -- Orzel’s approach is positive and curious, and Emmy is just plain fun.
1 person found this helpful
- Rating: 4 out of 5 stars4/5Since reading The Dancing Wu Li Masters an Overview of the New Physics by Gary Zukav many years ago, I have been fascinated by Quantum Physics. I do not claim to understand it, but I am intrigued. It makes perfect sense to me that there is not only order in the universe, thus in everyday life, but chaos as well. I have read several books on the subject, and always find myself nodding and saying to myself, yes! of course!Orzel's book caught my attention immediately because of the dog. There is a dog on the cover, and a dog who is part of the book. This time quantum physics is presented as an explanation to the dog. I love dogs. I am intrigued by the subject, so whats not to like?I found this book to be entertaining in it approach, and I fell immediately in love with Emmy the cutest smartest do in the world,as she carefully explains. This is not a book of humor. It is a book that approaches a serious and fascinating subject with humor and whimsy. Perfect! Read it. Really.. you won't be sorry/
1 person found this helpful
- Rating: 4 out of 5 stars4/5o
Book preview
How to Teach Quantum Physics to Your Dog - Chad Orzel
HOW TO TEACH
QUANTUM PHYSICS
TO YOUR DOG
CHAD ORZEL
SCRIBNER
A Division of Simon & Schuster, Inc.
1230 Avenue of the Americas
New York, NY 10020
www.SimonandSchuster.com
Copyright © 2009 by Chad Orzel, Ph.D.
All rights reserved, including the right to reproduce this book or portions thereof in any form whatsoever. For information, address Scribner Subsidiary Rights Department, 1230 Avenue of the Americas, New York, NY 10020.
First Scribner hardcover edition December 2009
SCRIBNER and design are registered trademarks of The Gale Group, Inc., used under license by Simon & Schuster, Inc., the publisher of this work.
For information about special discounts for bulk purchases, please contact Simon & Schuster Special Sales at 1-866-506-1949 or [email protected].
The Simon & Schuster Speakers Bureau can bring authors to your live event. For more information or to book an event, contact the Simon & Schuster Speakers Bureau at 1-866-248-3049 or visit our website at www.simonspeakers.com.
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."
So, you’re saying I need a new plan?
You need a new plan.
Her tail droops, and we walk in silence for a few seconds. Hey,
she says, can you help me with my new plan?
I can try.
How do I use my Buddha nature to go around both sides of the pond at the same time?
I really can’t think of anything to say to that, but a flash of gray fur saves me. Look! A squirrel!
I say.
Oooooh!
And we’re off in pursuit.
Quantum physics has many strange and fascinating aspects, but the discovery that launched the theory was particle-wave duality, or the fact that both light and matter have particle-like and wavelike properties at the same time. A beam of light, which is generally thought of as a wave, turns out to behave like a stream of particles in some experiments. At the same time, a beam of electrons, which is generally thought of as a stream of particles, turns out to behave like a wave in some experiments. Particle and wave properties seem to be contradictory, and yet everything in the universe somehow manages to be both a particle and a wave.
The discovery in the early 1900s that light behaves like a particle is the launching point for all of quantum mechanics. In this chapter, we’ll describe the history of how physicists discovered this strange duality. In order to appreciate just what a strange development this is, though, we need to talk about the particles and waves that we see in everyday life.
PARTICLES AND WAVES AROUND YOU: CLASSICAL PHYSICS
Everybody is familiar with the behavior of material particles. Pretty much all the objects you see around you—bones, balls, squeaky toys—behave like particles in the classical sense, with their motion determined by classical physics. They have different shapes, but you can predict their essential motion by imagining each as a small, featureless ball with some mass—a particle—and applying Newton’s laws of motion.* A tennis ball and a long bone tumbling end over end look very different in flight, but if they’re thrown in the same direction with the same speed, they’ll land in the same place, and you can predict that place using classical physics.
A particle-like object has a definite position (you know right where it is), a definite velocity (you know how fast it’s moving, and in what direction), and a definite mass (you know how big it is). You can multiply the mass and velocity together, to find the momentum. A great big Labrador retriever has more momentum than a little French poodle when they’re both moving at the same speed, and a fast-moving border collie has more