As I’m driving down the street, a squirrel darts out into the road a block or so ahead of me. From the back seat, the dog says “Gun it!!!! Hit the squirrel, hit the squirrel, hitthesquirrel!”
“Will you sit down and be quiet?” We’re having some work done on the house, and I’m taking her to work with me so she’s not underfoot for the contractors.
The squirrel makes it to the other side of the road, and up a tree to safety. “Awwww,” says the dog. “Dude, you totally could’ve gotten that one. This car is way faster than a stupid squirrel.”
“That may be, but I have a class to teach today. I don’t have time to be careening around like a maniac trying to squash squirrels with the car.”
“No, no– you’d have plenty of time. Time slows down when you go faster.”
I look in the rear-view mirror. She’s standing on the seat, wagging her tail and looking pleased with herself.
“Oh, God,” I say. “Don’t tell me you ran out of quantum physics books, and have started reading about relativity.”
“OK, I won’t tell you.” She’s quiet for a few second, then, “Relativity is pretty cool, though. I can slow time!”
This is not going to go anywhere good, I can tell. We come to a traffic light, and I stop.
“One thing I don’t understand, though…”
I sigh. “OK, what is it you want me to explain.”
“Why do they call it that?”
“Why do they call what, what?”
“Why do they call relativity ‘relativity?’ Why not something cooler, like Superfast Timeslowing Squirrelcatching Dynamics?”
“Well, for starters, that’s a bad name for a physics theory. Physicists don’t care much about squirrels. More importantly, though, the name ‘Relativity’ comes from one of the theory’s most basic elements: the idea that relative motion is the only thing that matters. There is no absolute frame of reference against which we can measure the motion of everything in the universe.” The light changes, and I start driving again.
“Yeah, but that’s silly. Of course there’s a fixed frame of reference.”
“Really? What is it?”
“Well, my house, silly. And the yard, with the big oak tree. And the other tree. And the other other tree. And–“
“OK, ok, I get it.”
“The house is where I keep my stuff!”
“Yeah, ok. But look, the house only looks like it’s a fixed frame of reference. I mean, it’s on the Earth, right? And the Earth is rotating.”
“I guess so…”
“And it also moves around the sun, which is why we have seasons. The thing you’re using as a fixed reference point is really in constant motion, and all you’re doing is measuring your motion relative to it.”
“OK, but I can still tell the difference between when I’m standing still and when I’m moving.”
“Well, when I’m moving, I walk past stuff, and sniff things, and chase bunnies and squirrels. When I’m not, I just sit there.”
“Sure, but how can you tell the difference between a situation where you’re moving and a situation where you’re sitting still, and everything else is moving in the opposite direction?”
“Well, that would be silly.” We come to another red light, and I stop again. “Anyway, I can tell that I’m the one moving, because my legs are moving.”
“OK, fine, but how about when you’re in the car, like we are now?”
“What do you mean?”
“Well, we’re sitting still right now, but when we start moving again…” the light changes, just at the right moment. I accelerate a bit, then cruise at a constant speed. “How can you tell that we’re moving, rather than sitting still and watching the rest of the world move by?”
“Ummm… The engine is going.”
“Yeah, but we could be on a treadmill, with fake scenery moving past us. This whole trip could be a fiendish illusion.”
She looks really worried. “I don’t like fiendish illusions.”
“Calm down, it’s just a hypothetical.” She looks somewhat mollified. “Anyway, the answer is that there’s no physics measurement you can do to distinguish between sitting still and moving at a constant velocity, the way we are now. You can detect acceleration,” I step on the gas, and speed up, “but when we’re moving at a constant speed, all the laws of physics are exactly the same as when you’re standing still.”
“So how do you tell when you’re moving?”
“You can’t. The only thing you can say is that you’re moving relative to some other object. Which is why the theory is called ‘Relativity.'”
I check the mirror, and she’s looking thoughtful. “So,” she says, “The only thing we can measure is relative velocity?”
“Like your velocity relative to that car with the lights?”
“What?” I look behind us, and see a cop car pulling out lights flashing. I look down, and my foot is still on the gas. “Crap! Well, maybe he’s after someone else…”
The cop car pulls in behind me. “I don’t think so,” says the dog cheerfully. “He’s got you nailed.”
I pull over. “This is all your fault, you know,” I say to the dog as I kill the engine.
“Yeah? Good luck explaining that to the cop.” She turns toward the window, and wags her tail cutely, just in case the policeman has dog treats.