How to Teach Relativity to Your Dog

"Yeah, laugh it up, fuzzball. You think this is funny?"

"Sorry," she says, "but at the end there, it sounded like you said—." She barks twice, sounding a little like a cough. "That's really funny in Dog."

"Yeah? What's it mean?"

"Well, it's . . . umm . . . You know, if you can't sniff your own butt, you won't get the joke."

"I'll try to contain my disappointment, then." I turn back to my grading.

"I'll think about it, and see what I can come up with, but translating humor is really hard."

"Translating anything is hard." I say, not looking up.

"Yeah? What do you know about translation?"

"Well, it's what I do for a living."

"You're a physicist, not a diplomat."

"I'm a physics professor," I say, putting my pen down. "In addition to doing physics, I teach physics to other people."

"And dogs!"

"Yes, and dogs. Teaching physics necessarily involves translation. The natural way to express physics is through math, but most people don't think in mathematical terms. So, a lot of the business of teaching physics is finding ways to translate physical ideas from mathematical equations into concepts drawing on everyday language and experience."

"So, making analogies and stuff like that?"

"That's part of it, yes. I also spend a lot of time dealing with people's preconceptions about how the world works. Sometimes, our intuition about how everyday objects behave leads us astray when we think about physics, and the first step in teaching the subject is to break down those preconceptions. Basically, to start over."

"You wouldn't have that problem if you stuck to teaching dogs," she says, looking pleased with herself and her species.

"No?"

"Nope. I have no clue at all about how things work. I'm a clean slate, when it comes to physics."

"I wouldn't go that far, but you at least have a different set of preconceptions than most humans do. Which means that thinking about physics as it appears to a dog can be a useful thing to do—looking at the problem from a different angle, and with an open mind, can sometimes give you insight that you wouldn't get by going straight at your own misconceptions."

"So, when you think about it, teaching physics to me helps you teach physics to humans."

"Yeah, it does."

"Which means that in a sense, it's part of your job, right?" She trots over to me and sits down, looking hopeful.

"I know where you're going with this, so let me remind you that grading these papers is also part of my job. I need to turn my final grades in tomorrow, so that's the more important part right this minute."

"Oh." She deflates a little.

"But tomorrow is also the start of our break, so I'll have time to spend talking to you about physics, if you want."

"Preferably while taking long walks!"

"Sure, that works. So, let me finish grading these exams, while you think about what areas of physics you'd like to learn about, OK?"

"OK!" She trots off in the direction of the library, and I go back to my grading. As I start on the next paper, I hear her saying "Maybe I can finally find out what this Einstein guy was all about . . ."

Asked why Einstein is a famous scientist, even dogs can come up with the equation E=mc², and possibly the words "theory of relativity." Explaining what those mean, and where they come from, is beyond most humans, though, let alone dogs. This is an unfortunate state of affairs, as Einstein's theory of relativity is one of the cornerstones of modern physics. Along with quantum mechanics, relativity completely revolutionized the way scientists view our universe. It provides insight into problems that classical physics can't handle, and poses new problems that physicists still grapple with a hundred years later.

Unfortunately, the features that make relativity so essential to physics also make it extremely intimidating to non-physicists. Relativity deals with situations that are very foreign to our everyday experience of the universe—objects moving thousands of times faster than the fastest man-made objects, astronomical objects packing enormous masses into tiny spaces—and its predictions defy all our normal expectations. Relativity tells us that quantities that seem fundamental—distances through space, and duration in time—in fact vary from one observer to another. A moving clock ticks at a different rate than a stationary one. A clock near a massive object ticks at a different rate than one farther away. And space itself is stretched by the presence of mass, so the length of a path between two points depends on what you pass along the way.

These are all surprising predictions made by the theory of relativity. They are also unequivocally true, confirmed by countless experiments in the century since Einstein first introduced relativity in 1905. The universe we live in is a far stranger place than our everyday intuition leads us to expect. To fully understand it, we have to expand our conception of the universe to include the counterintuitive predictions of the theory of relativity.

This can seem a daunting task, but one way to make it more approachable is to think like a dog. As any pet owner knows, dogs look at the world in a very different way—not entirely without preconceptions, but at least with different preconceptions than their humans, often in ways that make physics easier to understand. To a dog, any time should be dinner time, so the idea of clocks running at different rates for moving observers, or observers in different places, is easier to accept.

If you can learn to think like a dog, to approach the world as an endless source of surprise and wonder, modern physics is much less intimidating. Looking at physics from a dog's point of view allows us to shake off some of our human expectations about how things ought to work, and lets us appreciate the weird and wonderful world revealed by relativity.

This book reproduces a series of conversations with my dog about aspects of both special and general relativity. Each conversation is followed by a more detailed discussion of the physics involved, aimed at interested human readers. Some of the topics covered have achieved fame, or at least notoriety, in the wider culture, like Einstein's famous equation E=mc² (Chapter 7) or the idea of black holes (Chapter 10); others are less familiar to non-physicists, such as the merging of space and time (Chapter 5) or the effects of gravity on time (Chapter 9), but are just as essential to the modern understanding of physics. We'll also talk about some of the innumerable experiments and observations confirming that the universe is a weird and wonderful place.

We won't be able to cover everything that's interesting about relativity—generations of scientists have dedicated their careers to the subject without managing to exhaust its wonders—but we hope this will provide an introduction to the subject giving human and canine readers some sense of what it's about, and why it's important. And the next time some pesky cat asks you to explain why Einstein is famous, you'll have a good answer for them.

"I'm not forgetting you. How am I forgetting you?"

"Oh, sure, you mention the conversations at the start of the chapters. But you didn't tell them that I'll be keeping an eye on you in the middles, too. If you try to leave anything out, or sneak something past without explanation, I'll make sure you get it right."

"You mean, like you're doing now?"

"Yeah. Oh, wait—is this bit going in the book?"

"Yes."

"Oh. Well, then, I guess they're informed. But they should know that I'll be keeping an eye on you, and I'm an excellent watchdog."

"I think you've made that point."

"Also, I'm really cute, and I like long walks, and belly rubs, and chasing bunnies, and bacon. I really like bacon. Also, cheese. And peanut butter."

"I fail to see how this is relevant."

"Well, you know, in case they want to mail me presents. You know, because I'm an excellent physics dog, and all. You are going to put our address in the book, right? So they can send me stuff."

"I think that's just about enough out of you."

"Oh, all right. You're no fun, though."

"Can we please get started with the physics discussions?"

"Sure, absolutely. Lay some physics on me—I'm ready for anything!"