Sciency Words: Schrödinger’s Cat

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


Quantum physics has a mascot: a cat.  Specifically, it’s a cat that is somehow, almost magically, both dead and alive at the same time.  Does that sound weird?  Confusing?  It should.  This simultaneously living and dead cat has come to represent everything that makes quantum physics such a weird and confusing subject.

I’m not going to go into the details of how quantum mechanics works because A) I don’t have the math skills to do that properly and B) even if I did, it’s way too big a topic to cover in one blog post.  For the purposes of a Sciency Words post, it’s enough for you to know this: based on a strict interpretation of quantum mechanics, you would be forced to conclude that nothing is real unless it is being observed.

If you find that hard to accept, you’re not alone.  Many of the scientists who came up with quantum mechanics couldn’t accept it.  In 1935, German physicist Erwin Schrödinger—a man who’d received a Nobel Prize for his contributions to quantum theory—had had enough, and he published this article titled “The Present Situation in Quantum Mechanics.”

Don’t let that stolid title fool you.  Schrödinger was mad.  I’d characterize his article as an angry rant about everything wrong with quantum mechanics, or at least everything that was wrong with the strict interpretation of quantum mechanics.  That strict interpretation was becoming increasingly popular among Schrödinger’s colleagues, and it remains very popular among physicists today.

It was in the middle of this angry rant that Schrödinger first presented his now famous cat-in-a-box paradox.  Schrödinger first describes a killing contraption worthy of a James Bond villain.  A radioactive isotope is placed in a box.  A Geiger counter is rigged to trigger a hammer, which will smash a flask of hydrocyanic acid if the Geiger counter detects radioactive decay.  Lastly, a cat is placed in the box.  The box is sealed up so that no one can observe what’s happening inside, and it’s left undisturbed for one hour.

There’s a fifty-fifty chance that that radioactive isotope will decay before the hour is up.  Therefore, there’s a fifty-fifty chance that the cat will die.  So until we open the box and make an observation, a strict interpretation of quantum mechanics would have us believe the isotope simultaneously has and has not decayed.  The Geiger counter simultaneously has and has not gone off, and the cat simultaneously is and is not dead.

Schrödinger’s cat was meant to demonstrate that a strict interpretation of quantum mechanics leads to nonsensical conclusions. “The rejection of realism has logical consequences,” Schrödinger warns us.

No one has ever tried this experiment with an actual cat (I hope), but according to this article from Quanta Magazine, the Schrödinger’s cat phenomenon can and does happen in real life.  Quantum mechanics is weird.  It’s confusing.  It defies common sense.  But as author John Gribbin writes in his cleverly titled book In Search of Schrödinger’s Cat:

Common sense has already been tested as a guide to quantum reality and been found wanting.  The one sure thing we know about the quantum world is not to trust our common sense and only believe things we can see directly or detect unambiguously with our instruments.