Schroedinger's Dog in Copenhagen

May 27, 2009

My bloggy buddy Chad Orzel, over at Uncertain Principles, has a new book coming out in December, called How To Teach Physics To Your Dog. The dog in question is Emmy, known to Chad's readers as "The Queen of Niskayuna," who has a tendency to browse Chad's physics books when she's bored. And oh, yes, fame will most certainly go to her head.

Chapter 3 deals with the "Copenhagen Interpretation" of quantum mechanics, specifically, the infamous thought experiment known as "Schroedinger's Cat." Back in 1935, physicist Erwin Schroedinger illustrated how ludicrously counter-intuitive the implications of quantum mechanics could be by suggesting one could place a cat in a closed box with a single uranium atom (a highly unstable element), right next to a Geiger counter. The uranium atom has a 50% chance of decaying and emitting an electron, and that tiny bit of radiation would set off the Geiger counter. To up the stakes, Schroedinger pictured a hammer rigged to smash a small vial containing cyanide, should the Geiger counter detect any radiation, instantly killing the poor kitty. (Emmy, not surprisingly, has no moral qualms about this.)

That's not the mind-bending part of the thought experiment. According to quantum mechanics, we have no way of knowing before we open the box whether the cat is alive or dead. And until we do so, the cat inhabits a bizarre superposition of states -- both alive and dead at the same time -- until we open the box and look for ourselves. This constitutes a "measurement" or observation, and the cat's wavefunction collapses into either an alive or dead state. So in that sense, observation determines reality.

"Preposterous!" you say. So did lots of people, including physicists, when the implications of quantum mechanics were first being debated. Albert Einstein was famously skeptical, once asking Niels Bohr if he truly believed that, say, the moon is not there when we don't happen to be looking at it.

Einstein had a point: quantum mechanics only applies to the subatomic world; on the macroscopic level, we don't see those bizarre effects. Cats are alive or dead, not both at the same time. That's because cats are macroscopic objects, made of billions of subatomic particles. And an "observation" doesn't necessarily imply a human (or canine) observer. Any interaction whatsoever within the system -- a particle in the air interacting with a single particle of the cat -- is sufficient to cause the wave function to collapse and destroy any superposition of states. The moon doesn't actually need us to look at it -- although why wouldn't you, on a nice clear night when it's hanging low in the sky?

You can say all of what I just said above, and watch your listeners' eyes glaze over in bafflement. Or you can take the humorous approach, like Chad, and try explaining it in the simplest possible terms to your dog. In the book, Emmy clearly stands in for the Everyman (or Everydog), asking the kind of basic "why is the sky blue" questions that most of us are reluctant to ask for fear of looking, well, stupid. Dogs don't have this hang-up. Here's Chad reading aloud from Chapter 3, with accompanying photos:

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[Jumper]

But who collapsed Chad?