Skip to main content
No. 347:
Schrödinger's Cat

Today, Schrödinger's Cat changes our view of science. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Philosopher Abner Shimony makes a puzzling remark. He says:

Physical systems cannot be said to have definite properties independent of our observations.

Does he mean we give our world existence by looking at it? That sounds like a paranoid delusion, but Shimony is quite sane. He's explaining Shrödinger's Cat, a creature born in the strange new thinking of quantum mechanics.

The riddle of the cat begins with Heisenberg's Uncertainty idea: the most precise measurement we could ever make would be to shoot one photon of light at a moving object. But even so delicate a peek will change the position and motion we're trying to measure. At best, you always measure with some uncertainty.

That's easy enough to understand. But an awesome subtlety turns it into a new tenet of scientific faith. It makes precise measurement unthinkable. And that means we no longer have reason for thinking the world has any ultimate precision to measure.

So we take the last terrible step. We admit the world is indeterminate. We admit that electrons have fuzzy edges. When one collides, it may bounce one way. It may bounce the other.

Schrödinger said that if that's the case, let's seal a cat, a geiger counter, a fragment of radioactive material, and a bottle of poison gas into a box for one hour. There's a 50-50 chance that radioactive decay will trigger the geiger counter, activate a mechanism that breaks the bottle, and poison the cat. He asks if we'll find a live cat or a dead one when we open the box.

That sounds like the "Lady or the Tiger," but it's much worse. The man who has to open either of two doors knows a lady is behind one and a killer tiger behind the other. He doesn't know which door leads to the tiger, but the answer is knowable. Radioactive decay occurs on the level of indeterminancy. No knowledge of the system inside the box will ever let you predict the fate of Schrödinger's Cat. Whether it lives or dies is absolutely unknowable -- until you open the box.

Physicists agonize while that Cheshire cat sits and smiles. They try to write wave functions for cats and gamma radiation. They conclude goofy things: maybe the cat in the unopened box is both alive and dead at the same time. Steven Hawking, the physicist who writes about black holes from his wheelchair, throws up his hands and cries: "When I hear of Schrödinger's cat, I reach for my gun."

But in the end we have to look inside the box to learn whether the cat is alive or dead. So it is that the observer determines the the truth. This makes an odd commentary on objective science. We're left to wonder if scientists aren't far more deeply interwoven with the world they observe than they would like to be.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Barrow, J.D. and Tipler, F.J., Quantum Mechanics and the Anthropic Principle. The Anthropic Cosmological Principle. New York: Oxford University Press, 1988, Chapter 7.