dead and alive at the same time. In the box.”
“Schrödinger’s cat?”
“Yeah! That’s the one!” She wags her tail excitedly. “I like that experiment. You should do that.”
“For one thing, it’s just a thought experiment to show the absurdity of quantum predictions. Nobody ever did it for real.For another, I doubt that people would appreciate it if we started killing cats.”
“I don’t care about the killing. I just like the idea of putting cats in boxes. Cats belong in boxes.”
“I’ll pass that on to the scientific community. But what does this have to do with your treat?”
“Well, the treat could be in your left hand, and it could be in your right hand. I don’t know which it’s in, and you won’t let me sniff to see where it is, so that means that the treat is in a superposition state of both left and right hands. Until I measure which hand it’s in, the answer is that it’s in both hands at the same time.”
“That’s an interesting argument. It doesn’t apply here, though.”
“Yes it does. It’s basic quantum mechanics.”
“Well, yeah, it’s true that unmeasured objects exist in superposition states as a general matter,” I say, “but those superposition states are extremely fragile. Any disturbance at all—absorbing or emitting even a single photon—will cause them to collapse into classical states with a definite value.”
“People have seen them, though.”
“Sure, there have been lots of ‘cat state’ experiments done, but the largest superposition anybody has managed to make involved something like a billion electrons. * That’s nowhere near the size of a dog treat, which would contain something like 10 22 atoms.”
“Oh.”
“And on top of that, even in the most extreme variant of the Copenhagen interpretation, the wavefunction is collapsed by the act of observation by a conscious observer. Now, you can argue about who counts as an observer—”
“Not a cat, that’s for sure. Cats are dumb.”
“—but by any reasonable standard, I count as an observer. I know which hand the treat is in. So you’re dealing with a classical probability distribution, in which the treat is in either one hand or the other, not a quantum superposition in which the treat is in both hands at the same time.”
“Oh. Okay.” She looks disappointed.
“So, guess which hand the treat is in.”
“Ummm . . . I still say both.”
“Why is that?”
“Because I am an
excellent
dog, and I deserve
two
treats!”
“Well, yeah. Also, I’m a sap.” I give her both of the treats.
“Ooooo! Treats!” she says, crunching happily.
One of the most vexing things about studying quantum mechanics is how stubbornly classical the world is. Quantum physics features all sorts of marvelous things—particles behaving like waves, objects being in two places at the same time, cats that are both alive and dead—and yet, we don’t see any of those things in the world around us. When we look at an everyday object, we see it in a definite classical state—with some particular position, velocity, energy, and so on—and not in any of the strange combinations of states allowed by quantum mechanics. Particles and waves look completely different, dogs can only pass on one side or the other of an obstacle, and cats are stubbornly, irritatingly alive and not happy about being sniffed by strange dogs.
We directly observe the stranger features of quantum mechanics only with a great deal of work, in carefully controlled conditions. Quantum states turn out to be remarkably fragile and easily destroyed, and the reason for this fragility is not immediately obvious. In fact, determining why quantum rules don’t seem to apply in the macroscopic world of everyday dogs and cats is a surprisingly difficult problem. Exactly what happens in the transition from the microscopic to the macroscopic hastroubled many of the best physicists of the last hundred years, and there’s still no clear answer.
In
“Schrödinger’s cat?”
“Yeah! That’s the one!” She wags her tail excitedly. “I like that experiment. You should do that.”
“For one thing, it’s just a thought experiment to show the absurdity of quantum predictions. Nobody ever did it for real.For another, I doubt that people would appreciate it if we started killing cats.”
“I don’t care about the killing. I just like the idea of putting cats in boxes. Cats belong in boxes.”
“I’ll pass that on to the scientific community. But what does this have to do with your treat?”
“Well, the treat could be in your left hand, and it could be in your right hand. I don’t know which it’s in, and you won’t let me sniff to see where it is, so that means that the treat is in a superposition state of both left and right hands. Until I measure which hand it’s in, the answer is that it’s in both hands at the same time.”
“That’s an interesting argument. It doesn’t apply here, though.”
“Yes it does. It’s basic quantum mechanics.”
“Well, yeah, it’s true that unmeasured objects exist in superposition states as a general matter,” I say, “but those superposition states are extremely fragile. Any disturbance at all—absorbing or emitting even a single photon—will cause them to collapse into classical states with a definite value.”
“People have seen them, though.”
“Sure, there have been lots of ‘cat state’ experiments done, but the largest superposition anybody has managed to make involved something like a billion electrons. * That’s nowhere near the size of a dog treat, which would contain something like 10 22 atoms.”
“Oh.”
“And on top of that, even in the most extreme variant of the Copenhagen interpretation, the wavefunction is collapsed by the act of observation by a conscious observer. Now, you can argue about who counts as an observer—”
“Not a cat, that’s for sure. Cats are dumb.”
“—but by any reasonable standard, I count as an observer. I know which hand the treat is in. So you’re dealing with a classical probability distribution, in which the treat is in either one hand or the other, not a quantum superposition in which the treat is in both hands at the same time.”
“Oh. Okay.” She looks disappointed.
“So, guess which hand the treat is in.”
“Ummm . . . I still say both.”
“Why is that?”
“Because I am an
excellent
dog, and I deserve
two
treats!”
“Well, yeah. Also, I’m a sap.” I give her both of the treats.
“Ooooo! Treats!” she says, crunching happily.
One of the most vexing things about studying quantum mechanics is how stubbornly classical the world is. Quantum physics features all sorts of marvelous things—particles behaving like waves, objects being in two places at the same time, cats that are both alive and dead—and yet, we don’t see any of those things in the world around us. When we look at an everyday object, we see it in a definite classical state—with some particular position, velocity, energy, and so on—and not in any of the strange combinations of states allowed by quantum mechanics. Particles and waves look completely different, dogs can only pass on one side or the other of an obstacle, and cats are stubbornly, irritatingly alive and not happy about being sniffed by strange dogs.
We directly observe the stranger features of quantum mechanics only with a great deal of work, in carefully controlled conditions. Quantum states turn out to be remarkably fragile and easily destroyed, and the reason for this fragility is not immediately obvious. In fact, determining why quantum rules don’t seem to apply in the macroscopic world of everyday dogs and cats is a surprisingly difficult problem. Exactly what happens in the transition from the microscopic to the macroscopic hastroubled many of the best physicists of the last hundred years, and there’s still no clear answer.
In
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