I’m sitting at the computer typing, when the dog bumps up against my legs. I look down, and she’s sniffing the floor around my feet intently.
“What are you doing down there?”
“I’m looking for steak!” she says, wagging her tail hopefully.
“I’m pretty certain that there’s no steak down there,” I say. “I’ve never eaten steak at the computer, and I’ve certainly never dropped any on the floor.”
“You did in some universe,” she says, still sniffing.
I sigh. “I’m going to move the quantum physics books to a higher shelf, so you can’t reach them.”
“It won’t matter. I’ve got Wikipedia.”
“All right, what ridiculous theory has your silly little doggy brain come up with?”
“Well, it’s possible that you would eat steak at the computer, yes?”
“I do eat steak, yes, and I sometimes eat at the computer, so sure.”
“And if you were to eat steak at the computer, you’d probably drop some on the floor.”
“I don’t know about that…”
“Dude, I’ve seen you eat.” Yes, the dog calls me “dude.” There may be obedience classes in her future.
“All right, we’ll allow the possibility.”
“Therefore, it’s possible that you dropped steak on the floor. And according to Everett’s Many Worlds Interpretation of quantum mechanics, that means that you did drop steak on the floor. Which means I just need to find it.”
“Well, technically, what the Many Worlds interpreation says is that there’s some branch of the unitarily evolving wavefunction of the universe in which I dropped steak on the floor.”
“Right, so I just need to find the unitary whatsis.”
“The thing is, though, we can only perceive one branch of the wavefunction.”
“Maybe you can only perceive one branch. I have a very good nose. I can sniff into extra dimensions.”
“That would explain some of your mystery barking fits, but extra dimensions are a completely different thing. In this case, once there has been sufficient decoherence between the branches of the wavefunction that there’s no possibility of interference between the different parts, they’re effectively separate and inaccessible universes.”
“What do you mean, decoherence?”
“Well, say I did have a piece of steak here– stop wagging your tail, it’s a hypothetical– quantum mechanics says that if I dropped it on the floor, then picked it back up, there could be an interference between the wavefunction describing the bit of steak that fell and the wavefunction describing the bit of steak that didn’t fall. Because, of course, there’s only a probability that I’d drop it, so you need both bits.”
“What would that mean?”
“Well, the steak would probably produce some sort of interference pattern. I’m not really sure what that would look like. The point is, though, it doesn’t really matter. The steak is constantly interacting with its environment– the air, the desk, the floor–“
“Whatever. Those interactions are essentially random, and unmeasured. These interactions lead to shifts in the wavefunctions of the different bits of steak, and those shifts make it so the wavefunctions don’t interfere cleanly any more. That process is called ‘decoherence,’ and it happens very fast.”
“How fast?” she asks, looking hopeful.
“It depends on the exact situation, but as a rough guess, I’d say about the same time as the lifetime of a bunny made of cheese. 10-30 seconds or less.”
“Oh.” She deflates a little. “That’s fast.” She still hasn’t caught a bunny made of cheese.
“Yeah. And once that decoherence has happened, the different branches of the wavefunction can’t really interact with each other any more. Which means, essentially, that the different branches become separate universes that are completely inaccessible to one another. Things that happen in these other ‘universes’ have absolutely no effect on what happens in our universe.”
“Why do we only see one branch of the whatchamacallit?”
“Ah, now that’s the big question. Nobody knows. A lot of people think this means that quantum mechanics is fundamentally incomplete, and there’s a whole community of scientists doing research into the fundamentals of quantum theory, and the various interpretations. Matt Leifer has a whole blog talking about this stuff.”
“We don’t like him. He said mean things about me.”
“It wasn’t so much mean, as dismissive. But that’s not the point. The point is, there’s no way you’re going to find steak under my desk, so please get out of there.”
“Oh. OK.” She mopes out from under the desk, head down and tail drooping.
“Hey, look on the bright side,” I say. “In the universe where a version of me dropped a piece of steak on the floor, there’s also a version of you.”
“Yeah?” Her head picks up.
“Yeah. And you’re a mighty hunter, so you probbaly got to the steak before I could pick it up.”
“Yeah?” Her tail starts wagging.
“Yeah. So, in the universe where I dropped steak, you got to eat steak.”
“Oooh!” The tail wags furiously. “I like steak!”
“I know you do.” I save what I was working on. “Tell you what, how about we go for a walk?”
“Ooooh! Good plan!” and she’s off, clattering down the stairs for the back door and the leash.
She’s really a very silly dog.