(Intro)

Hank Green: Hello and welcome to "I Don't Think That Means What You Think It Means", where we look at bits of scientific theory that have wiggled their way into popular culture and have taken on a life of their own.

Today, we're talking about Schrödinger's Cat, a famous thought experiment devised by the Austrian physicist Erwin Schrödinger, who helped piece physics back together after Einstein and his crew blew a giant fricking hole in it back in the early 20th century. It's hard to overstate how much of a giant crap circus the 1920s were for physicists. Until then, everything had pretty much just been good old-fashioned Newtonian physics where you could observe objects moving and predict how they react to various forces. But then along comes all this new research into subatomic particles that shows that they don't act predictably at all. In fact, sometimes stuff seems to be two different things at once, like an electron in a beam might act like a particle sometimes and like a wave at other times, and to make things even more... "gngnngh!" the more you try to observe and measure these particles, the less naturally they behave. Safe to say "what" now?

My friends, I shall now introduce you to one of the biggest mind-flogs of quantum mechanics. It's called superposition, the idea that a particle can exist in all of its theoretically possible states at the same time. So Schrödinger came up with this thought experiment to help everybody understand it.

Say you have a cat, and you put the cat into a steel chamber for an hour with a vial of deadly gas, a Geiger counter, a hammer, and a tiny bit of something radioactive. Bear with me here.

Now say that there's a 50/50 chance that one of the radioactive atoms is going to decay and release some radiation within that hour. If one of the atoms decays, the Geiger counter is going to trigger the hammer, shattering the vial of poisonous gas. Really, Schrödinger? This is not the best way to get people behind the idea of fun in the sciences. So, there's a 50% chance that at the end of the hour, that vial has been broken and the cat is dead, and an equally good chance that the vial hasn't been broken, and the cat's just kicking it, wondering what's for supper.

But, what's actually happening in the box?

According to quantum mechanics, any one of those radioactive atoms would be in a superposition of being both decayed and not decayed at the same time, 'cause that's how quantum objects act. So then, that decayed atom will have both killed and not killed the cat, right?

Well, that's the logical conclusion, but a cat, a cat isn't a quantum object. A cat is a big, normal thing that obeys nice, old-fashioned Newtonian laws, so it, just like every other cat in history, is either alive or dead.

Schrödinger's point, at least one of them, is that the two objects are subject to two separate sets of laws that can't be reconciled. In order to know whether the atom has decayed or not, and whether the cat is dead or not, you have to open the box and see, but in quantum mechanics, the state of superposition can't be observed, so when the evil mad scientist finally opens the chamber to observe, the superposition collapses and the outcome is ensured.

Today, Schrödinger's Cat is often talked about as some kind of undead zombie cat, or discussed as if the cat is actually both alive and dead in the box, but Schrödinger's point wasn't to prove that you can make a cat be both alive and dead, but instead to prove that the quantum world doesn't mesh together well with the, you know, like, normal world. And alternatively, the point is that the universe is pretty fricking weird.

There are other interpretations of quantum mechanics that might resolve the paradox, but none of them are particularly easy to test. My favorite, of course, is the many worlds interpretation, that states that at the end of the experiment and at the end of every superposition, new alternate universes are created, in this case, one in which the cat is alive and one in which the cat is dead. And to be clear, I don't like this interpretation because I think it's the most likely one, I like it because I think it's such an excellent plot device for science fiction novels.

If you have any questions, and I'm sure you do, ask them in the comments below and I'll, you know, do my best. Also, if you have an idea for I Don't Think That Means What You Think It Means, please let us know about that too.