If you’ve spent time around physics types, listened to media appearances by science educators like Michio Kaku and Brian Greene, or even watched episodes of The Big Bang Theory, you’ve almost certainly heard of Schrödinger’s cat.
But what is it, why is it important, and what does it really have to do with cats? Most importantly, if you’re a cat lover, does it involve harming cats?
I promise you, if you stick with me and have a little patience, you’ll not only understand Schrödinger’s cat, but a hugely important element of physics will be demystified for you.
Let’s take a step back. First, we all learned in school that Isaac Newton was the “father of physics,” and Albert Einstein came along about two centuries later, revolutionizing physics by adding to Newton’s work and coming up with his own, more accurate model.
To this day, Newton and Einstein are in a class by themselves among physicists because they single-handedly changed everything we know about the natural world.
We all remember the famous story about Newton watching an apple fall from a tree, wondering why the apple fell down instead of up, and eventually developing his theory of gravity. Newton went on to develop his theories, which describe everything we see in the natural world, from that apple falling to the complexities of orbital mechanics.
Everything seemed to work perfectly, until a physicist named Max Planck came along in 1905 and published a paper introducing quantum physics.
What is quantum physics?
Now the word “quantum” has been incorporated into practically everything these days and has been so utterly abused as a marketing buzzword, a way to add a veneer of science to things that are otherwise nonsense, that it’s essentially a meaningless word to most people. Practically everything is described as quantum, from deodorants to claims of psychic telepathy.
But the gist of it is this: While Newtonian physics does indeed describe everything we see with our own eyes accurately, it does not accurately describe things at the subatomic level.
In other words, there are two sets of rules in our universe. Everything larger than an atom behaves according to one set of rules in our universe, and everything the size of an atom or smaller — which includes subatomic particles — behaves according to a different set of rules.
Not only that, but at the quantum scale, things get really, really weird.
They behave in ways that are completely at odds with everything we intuitively understand about reality, so much so that even Einstein himself was disturbed by what he found. Einstein famously described quantum entanglement — the ability of two different objects to be linked and share properties, regardless of how far apart they are — as “spooky action at a distance.”
So what the hell does this have to do with cats?
Ready to get even weirder?
Thanks to Planck, Einstein, John Stewart Bell and innumerable physicists — who are still studying these concepts, and still winning Nobel prizes for them in 2022 — we know that two particles can be “entangled” and will remain that way no matter how far apart they are.
You could take one particle, transport it 10,000 light years away, and it would still be entangled with the other particle.
But it gets even stranger than that.
Our entangled particles have certain properties, such as their spin, which are unknowable until we measure them. In fact, they exist simultaneously in all possible states until the moment when we observe them, at which time the wave function “collapses.” It’s called quantum superposition.
Not only that, but when we measure one particle in an entangled pair, the second particle’s wave also “collapses” (settles on a certain state) and we know its spin instantaneously, regardless of how far apart the particles are.
If I measure an entangled particle here on Earth and find its spin is up, I know the corresponding particle that’s been moved to, say, Epsilon Eridani, 10.47 light years away, is spin down.
You can see why this would be profoundly disturbing to scientists. It violates the speed of light, and it’s completely counterintuitive. How can the mere act of observation change something in the physical world, and how can it change something else potentially thousands or millions of light years away? Everything we know, every gut instinct we have, screams that this should not be true.
But it is true.
These aren’t just ideas kicked around by scientists smoking the sticky stuff, by the way. They’ve been proven experimentally many times over. No matter how much we might dislike the idea, no matter how weird or spooky it may seem, it’s true.
Enter Erwin Schrödinger, an Austrian physicist. He devised a thought experiment that goes like this:
Imagine you have a sealed box with a cat inside. Inside the box are two buttons: One button feeds the cat a yummy treat, the other button kills the cat. There is an equal (50/50) chance of the cat pressing either button. (Other versions use a more complex system involving radioactive material, or poison, that could kill the cat, again with a 50/50 chance.)
We don’t know if the cat is alive or dead until the moment we open the box. So in this thought experiment, we can think of the cat as both alive and dead until we “measure” or “observe” by opening the box.
That’s what’s happening in the above example of quantum entanglement and the idea that a particle is neither in one position or another until we measure it.
Why is measurement the key here? No one knows. Scientists are still arguing about that. Some believe that there’s some special quality of consciousness that interacts with the universe, so the mere act of observing something can change physical reality.
Others scoff at that idea and insist we’re missing something, that it’s not the act of observation that determines the final state of a particle at all.
Regardless, the important thing here for cat lovers is that Schrödinger’s cat is just a thought experiment.
Schrödinger never had a cat, as far as anyone has been able to ascertain, and no one has used an actual cat in an attempt to reconstruct the thought experiment because 1) You wouldn’t learn anything, since cats are not subatomic particles, and 2) Anyone intelligent enough to be a physicist is presumably intelligent enough to understand how absurd, pointless and cruel it would be to use a living being in an experiment that can’t give you any answers.
For those of us who aren’t geniuses, here’s Sheldon explaining the thought experiment as a child (in Young Sheldon) and as an adult (in The Big Bang Theory):