Schroedinger's Cat

This is a part of the Eureka! Stories web site. These are stories for children about famous historical scientists. They are under constant development, so check back for additions.

Cartoon by Ken Otter

A long time ago somebody asked the question "If a tree falls in a forest, and nobody is there, is there still a sound?" The problem is, do things happen the same when nobody can watch them, and can we ever find out? And what if its not even possible to watch them? Scientists and philosophers have argued about this for a long long long time.

In the last hundred years a lot of scientists have been very disturbed by something called Quantum Mechanics. They all agree about how it works, and that it does work, but they can't all agree about what it means. Here's an example: if you have very small balls going through a wall that has two very small holes that are close together, you get a strange pattern for how the balls come out on the other side. You might think that you could measure which hole each of the balls went through. Well, the funny thing is that if you do anything that lets you measure which hole they go through, you get a different pattern for the balls coming out on the other side. If everything is very small, you change everything too much just by trying to watch.

Scientists can agree on this much, but can't agree on what it means. If you can't measure which hole a ball goes through, does it still go through just one of them, or does it do something unexpected? Does it even make sense to talk about which hole it went through when you could never find out?

Two very famous scientists who couldn't agree on the answer were Einstein and Schroedinger. One day Schroedinger explained his opinion to Einstein by talking about his cat. He suggested that you put a cat in a box with a bottle of poison, and the bottle will open and kill the cat at some time that you don't know. Then at some later time is the cat dead or alive? Einstein says that it is one or the other, but we can't know which until we open the box. Shroedinger says that it is neither - the cat is a mixture of dead and alive, and only becomes one or the other when we open the box.

Now in case you are worried about Schroedinger and Einstein's cats, you should know that they were not only famous for being great scientists, Einstein especially was also famous for being very kind. Besides that, there were a kind of scientist called a theoretician. This means that mostly they came up with ideas about how the world worked and made predictions about what should happen if you did different things. Actually doing the experiments to find out which theoreticians are right is usually the job of experimentalists. The problem with trying to do an real experiment with real cats to find out the answer is that Schroedinger made up the question in a way that makes it impossible to do it. The question is what is happening in the box when you aren't making any measurements, so if we do make measurements we've broken the rules of the puzzle.