How to Teach Physics to Your Dog (2 page)

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.

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.

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 momentum than a waddling basset hound of the same mass. Momentum determines what will happen when two particles collide. When a moving object hits a stationary one, the moving object will slow down, losing momentum, while the stationary object will speed up, gaining momentum.

The other notable feature of particles is something that seems almost too obvious to mention: particles can be counted. When you have some collection of objects, you can look at them and determine exactly how many of them you have—one bone, two squeaky toys, three squirrels under a tree in the backyard.

Waves, on the other hand, are slipperier. A wave is a moving disturbance in something, like the patterns of crests and troughs formed by water splashing in a backyard pond. Waves are spread out over some region of space by their nature, forming a pattern that changes and moves over time. No physical objects move anywhere—the water stays in the pond—but the pattern of the disturbance changes, and we see that as the motion of a wave.

If you want to understand a wave, there are two ways of looking at it that provide useful information. One is to imagine taking a snapshot of the whole wave, and looking at the pattern of the disturbance in space. For a single simple wave, you see a pattern of regular peaks and valleys, like this:

As you move along the pattern, you see the medium moving up and down by an amount called the “amplitude” of the wave. If you measure the distance between two neighboring crests of the wave (or two troughs), you’ve measured the “wavelength,” which is one of the numbers used to describe a wave.

The other thing you can do is to look at one little piece of the
wave pattern, and watch it for a long time—imagine watching a duck bobbing up and down on a lake, say. If you watch carefully, you’ll see that the disturbance gets bigger and smaller in a very regular way—sometimes the duck is higher up, sometimes lower down—and makes a pattern in time very much like the pattern in space. You can measure how often the wave repeats itself in a given amount of time—how many times the duck reaches its maximum height in a minute, say—and that gives you the “frequency” of the wave, which is another critical number used to describe the wave. Wavelength and frequency are related to each other—longer wavelengths mean lower frequency, and vice versa.