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:
SCHRÖDINGER’S CAT
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.
Planet Pailly 
Wigner took this a step further with his “Wigner’s friend” thought experiment. No cats are harmed by Wigner, although still no conclusive answer can be derived. Discussions of quantum mechanical paradoxes rapidly become very technical, and even experts do not agree how to interpret these ideas. This is, of course, what makes QM so fascinating!
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But what I think is now definitely accepted by the majority of working physicists is that consciousness plays no role in the transition from quantum to classical behaviour. Roughly speaking, large objects (like cats, or friends) very rapidly lose their quantum superposition through the natural process of interference with their environment (quantum decoherence). The cat is either dead or alive long before the scientist opens the box, just as common sense would have it, and as Wigner’s thought experiment suggests. The main outstanding question (I believe) is whether the wavefunction collapses (as scientists like Bohr, Einstein and Schrodinger believed) and thus there is a single reality, or whether all quantum states are preserved (as in Everett’s many-world theory) and there are many realities, one for every possible quantum outcome. From a practical point of view, I don’t think it actually matters, and no one has ever suggested an experiment that could tell the difference between the two. Those other universes would not interact with our own and would not be observable to us.
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That’s my understanding as well. I’ve long wanted to do a Sciency Words post on what the word observation means to quantum physicists. Consciousness is absolutely not required, as I understand it. But still, at this point I don’t feel like I understand this topic well enough to write that post. Maybe someday.
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John Gribbin writes about the Wigner’s friend thought experiment too, and he starts turning it into a case of infinite regression. Schrodinger’s cat has to be observed by Wigner’s friend, who then has to be observed by Wigner, who then has to be observed by news reporters, who then have to be observed by a curious general public. It’s superposition all the way down!
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Schrodinger’s Cat is one of those things we should remember whenever anybody uses a reductio ad absurdum argument. Reality is absurd. Absurdity, in and of itself, is no argument against a proposition, only whether or not there is evidence for it.
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Not to brag, but I did that very thing a few years back. I won an argument about time travel by citing Schrodinger’s cat and then saying nature is not obligated to obey our human perception of common sense. Obviously that doesn’t totally settle the issue of time travel, but it did shut down the ad absurdum argument.
For context, this happened at a critique group session when I was presenting a scene from Tomorrow News Network.
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That kind of critique for a a time travel story seems out of place. Lots of people don’t buy time travel, but if they’re going to read a time travel story, not suspending disbelief seems lacking in goodwill.
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Not everyone who joins a critique group knows how to give useful critiques. But I was proud of myself at the time for having an answer ready.
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I don’t have the math to understand quantum physics. I can’t appreciate Persian poetry either because I don’t speak that language. But did you know that Schrodinger’s cat is not the only famous physics cat?
When I learned about F.D.C. Willard I had to post about him, and I hope you won’t mind too much if I add a link here in your comments: https://katerauner.wordpress.com/2018/02/14/famous-physics-cat-second-only-to-schrodingers-physics-science-cats-research-quote/
Or look him up in Wikipedia.
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I remember that. I’m pretty sure your blog post was the first time I’d heard about F.D.C. Willard. It’s a really neat and fun story!
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The observation is made right after the box is opened. But the conclusion is regarding the state of affairs a fraction of a second before the box is opened. You are attempting to observe something that no longer exists.
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This is something I wonder about whenever I read about Schrödinger’s cat-style experiments. Supposedly we have observed quantum superpositions in action, but wouldn’t the observation break the superposition? I don’t get how that works. Or maybe I just don’t really get what “observation” means to quantum physicists.
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This is the crux of the matter. We never observe superpositions. Cats are alive or dead. Atoms are spin up or spin down. The photon goes through one slit or another. Superpositions only happen when no one is looking 🙂 That’s what makes QM so infuriating.
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That is the problem as I understand it. But occasionally I read things that say something like “scientists observed water molecules in a superposition,” and I’m left thinking “What does that mean?” It leaves a lot of uncertainty (pardon the pun) in my understanding of quantum mechanics.
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You’re right. That is confusing (sloppy?) use of language. We cannot observe the superposition state, merely deduce its existence. An example is the 2-slit experiment. We can only observe photons arriving at a definite end point. But we deduce that they passed through both slits to reach it, being in a superposition of 2 states.
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Time progresses along a continuum that consists of an infinite number of dots. Any observation or measurement always takes place after the event.
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Coy of Alien Resort, you are describing an intuitive classical way of thinking about the universe. QM isn’t like that. The observation *is* the event.
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Maybe it isn’t possible for an observation and event to occupy the same point in time just as two objects can’t occupy the same space. This is probably classical too–I don’t have science training; I’m just using my imagination.
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So, two questions. 1) What is an observation? 2) What is an event?
In QM, an event is something like a subatomic particle bouncing off another particle, or perhaps a particle and its antiparticle colliding and annihilating each other.
An observation of a subatomic particle would involve the scientist bouncing a second particle off it, or interacting with the system in some way similar to this.
So you can see that the observation and event don’t simply occur at the same time, they are in fact the same thing. The observation is the event; the event is the observation. In classical physics, we are not used to thinking this way.
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They shouldn’t use the word “observation” if the scientist is an active participant.
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You’re absolutely right: they shouldn’t use the word observation. It creates a lot of confusion. It confuses the heck out of me all the time.
Unfortunately, quantum mechanics has been using that word for a century now, and we’re kind of stuck with it. This seems to happen a lot in science. Scientists pick a term for something before they fully understand what that something is. By the time they do understand it, the word has become too deeply embedded in the scientific lexicon for anybody to change it.
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The problem is that in quantum physics, the only way to observe something, to learn about its position, charge, momentum, etc, is to interact with it, such as bouncing at least one photon off it. On macroscopic scales, there’s enough electromagnetic radiation bouncing around, that we can observe something like a rock without the observation itself affecting it.
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But the scientist is *always* an active participant.
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True. As it has been explained to me many times over, in classical physics the scientist has such a negligible effect that you can safely ignore it. But on the quantum scale, you can’t disentangled what you’re measuring from the instruments you’re using to measuring it.
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I want to thank everyone who’s participated in this comment thread. I don’t know about you, but I feel like this has been a very productive conversation, and it’s helped clarify some of my own thinking on all this quantum stuff.
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Although, as Richard Feynman said, “If you think you understand quantum mechanics, you don’t understand quantum mechanics”.
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So this means the cat is simultaneously hungry and not hungry. My cat does not approve of this experiment. She is always hungry.
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One of my favorite takes on Schrodinger’s Cat was in Neil Gaimon’s book American Gods. One of the characters says something like “I can believe in a cat that’s dead and alive at the same time, but someone better be feeding her.”
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