(something something how did cat get wedged in box...)

Well. I understood about 3% of the famous cat thought experiment until today - now I'm upping my understanding to about 3.5%.

The article does try to present it in layman's terms, as much as that is possible. Interesting to see the manner of (possible) death changing over time. Poor kitty.

Did not expect references to LeGuin and Paltrow.
posted by davidmsc at 11:22 AM on May 6 [1 favorite]

Both paths are equally well-travelled.
posted by prefpara at 11:43 AM on May 6 [2 favorites]

Surely I can't be the first person to point out that in this thought experiment, the cat is an observer. The cat collapses the wave function before we open the lid.
posted by adamrice at 12:24 PM on May 6 [1 favorite]

Surely I can't be the first person to point out that in this thought experiment, the cat is an observer. The cat collapses the wave function before we open the lid.

Now you're getting into the territory of Wigner's Friend.
posted by BlueDuke at 12:32 PM on May 6 [1 favorite]

The cat collapses the wave function before we open the lid.

That's just the sort of thing a cat would do.
posted by Greg_Ace at 1:02 PM on May 6 [2 favorites]

The thing about certainty and uncertainty is that it's only relevant to us, the limited, temporal being trying to figure out the universe. The point isn't that the cat is alive and dead, it's that it is effectively alive and dead as far as our understanding is concerned and only becomes definitely alive or dead when we look. The thing that changes is the certainty, not the cat.

And we cannot know something without observing it.
posted by Reyturner at 1:04 PM on May 6 [2 favorites]

It's a parallel-universe thing. The superposition means that there's a universe in which the cat lives, and a universe in which the cat dies. Nothing magically resolves when you make the observation--you just find out which universe you're in. That was the implication of a recent Neil DeGrasse Tyson podcast of which I watched five minutes, anyway. I know nothing about it and have no qualifications in the field; I just skipped to the part where they revealed the true nature of the universe.
posted by Sing Or Swim at 1:04 PM on May 6 [7 favorites]

Any interaction which could potentially be observed must collapse the wave function or QM devolves into solipsism — and even that might not be enough.
posted by jamjam at 1:05 PM on May 6 [2 favorites]

Surely I can't be the first person to point out...

So much has been written on the role of the observer in quantum mechanics - the so-called "measurement problem" - that you could probably sink a small ship under the weight of all the books and articles on the topic.

If obserervation affects the outcome of an experiment, what exactly is an "observation"? And what qualifies as an "observer"? Of course there's no settled answer to this. But I remember this question coming up at a lecture I was at years ago - someone asked "If the cat is not an observer in this scenario, what qualifications does one require to be an observer who can force the wave function to collapse?" And the lecturer (I wish I could remember who!) replied, without missing a beat, "A PhD in physics, of course."
posted by crazy_yeti at 1:08 PM on May 6 [6 favorites]

I think the cat problem comes from treating the wavefunction as something it's not. The wavefunction tells us the probability of various measurable events— that's it. A particle has a wavefunction. but you can't say it is its wavefunction. The confusion comes because non-physicists are deeply bothered by a theory that refuses to predict things exactly, giving only probabilities. (Physicists just shrug and go on with their scribbling, i.e. their calculations or grant applications.)

An analogy might be rolling a 3d6. This has a bell-shaped probability curve— there's a nice picture in my copy of the Dungeon Master's Guide. Is that curve real? It's real in the sense that it's a correct representation of the possibilities; it is not real like the dice themselves, or a particular roll of the dice.

On a macro level, like dice, we may expect that probabilities arise from deterministic events. That doesn't work in QM. Feynman's QED is good for exploring how a nondeterministic theory really works.
posted by zompist at 1:29 PM on May 6 [1 favorite]

Since his cat demonstrates how deeply QM depends on consciousness, it must have been tempting to conclude that the universe itself should be conscious under QM.

Here's what Wikipedia says about his beliefs:

His mother was of half Austrian and half English descent; his father was Catholic and his mother was Lutheran. He himself was an atheist.[13] However, he had strong interests in Eastern religions and pantheism, and he used religious symbolism in his works.[14] He also believed his scientific work was an approach to divinity in an intellectual sense.[15]

posted by jamjam at 1:36 PM on May 6 [2 favorites]

Surely I can't be the first person to point out that in this thought experiment, the cat is an observer.

A very reasonable point. The question then becomes "what determines when a collection of atoms switches from 'part of the system' to 'an observer of the system'".

Small numbers of microscopic particle obey the quantum rules, bigger things that people (or cats) can see obey classical rules. But people and cats are just collections of microscopic particles. What is the mechanism by which systems realize they are big enough to behave classically?

Here is my favorite explanation of the problem, by Sean Carroll where he tries to explain why it's a problem.
posted by justkevin at 1:40 PM on May 6 [1 favorite]

pantheism can I get a wooot wooot
posted by St. Peepsburg at 1:51 PM on May 6 [3 favorites]

> think the cat problem comes from treating the wavefunction as something it's not. The wavefunction tells us the probability of various measurable events— that's it. A particle has a wavefunction. but you can't say it is its wavefunction

For it to be "just a particle with a wavefunction", you need to throw away locally real physics. And those are things we do not want to do - non-real/non-local physics doesn't look like "physics", and more like philosophical "what can we possibly know? nothing".

https://www.preposterousuniverse.com/blog/2019/09/21/the-notorious-delayed-choice-quantum-eraser/


You end up with insanity - retrocausality being a tame problem! - if you insist of "collapse" being anything but a perspective. On the other hand, if you treat "collapse" as a matter of perspective, then the universe gets uncomfortably large.
posted by NotAYakk at 2:03 PM on May 6 [2 favorites]

So much has been written on the role of the observer in quantum mechanics - the so-called "measurement problem" - that you could probably sink a small ship under the weight of all the books and articles on the topic.

We'll see.
posted by ChurchHatesTucker at 2:56 PM on May 6 [1 favorite]

> And what qualifies as an "observer"?

In the first half of the 20th century, when the Copenhagen interpretation was coming into vogue, it was proposed that while a human being could be considered an observer--and therefore the agent behind the collapse of the wavefunction--a mouse, with its limited intelligence, could not.

And thus ended respect for the Copenhagen interpretation among many physicists.
posted by Gordion Knott at 3:03 PM on May 6

my grandmother liked Robert Frost and attended parlor, believe it was the spring of 1927. My grandmother doesn't like cats so she'd comment on the fact that there was many cats around the house. she would comment about cats and anybody's house.
when Henry Ford moved The Ann Arbor House, house that Frost lived in, to Greenfield Village my grandmother went to visit and she saw a cat.

I stayed the weekend at Greenfield Village on a school field trip and what did I see in front of Frosts house, nothing but out back was a cat.

the cat's there, the cats always been there.
posted by clavdivs at 3:10 PM on May 6

I'm taking the fork in the road where our theories are incomplete and this metaphor strained. The big thing that annoys me is just the word "observe" because when it comes to the atoms themselves, ain't nobody just looking at the wave and particle, they're actively fucking with it, lacking a better means of direct observation. We are primitive beings with crude technology, Schrödinger's cat isn't much more sophisticated than thought experiments about ether and the four humors. If humanity continues on for longer than our tiny blip, they will be able to look back at this whole notion and laugh like we do at phrenologists.
posted by GoblinHoney at 3:18 PM on May 6

Never buy a pig in a poke lest the cat be out of the bag.
posted by y2karl at 4:01 PM on May 6

I'd like to talk briefly on the quantum nature of a cat owner's brain.

Not 20 min ago we caught our cat Schroedinger Bonnie, on the kitchen counter (where she's never allowed), eating some freshly-grated cheddar intended for our tacos. We yelled and chased her off.

Later we will be in front of the TV, eating said tacos (with cat-licked cheese) and watching Jeopardy, with the cat on someone's lap.

Gotta be a paper in there somewhere.
posted by Artful Codger at 4:12 PM on May 6

I don't understand quantum mechanics at all as a concept. But I get the question posed for the poor cat. Things might have happened, ending with either a live or dead cat. And those things are based on semi-random behavior of particles and their decay. If we had sensors inside the box, we would know if the cat were alive or dead before we opened the box, wouldn't we?

But that takes all the fun out of the concept and the question.
posted by Windopaene at 4:25 PM on May 6

> isn't that the cat is alive and dead, it's that it is effectively alive and dead as far as our understanding is concerned

> The wavefunction tells us the probability of various measurable events— that's it. A particle has a wavefunction. but you can't say it is its wavefunction

These are not actually true. It'd be nice if they were! But the double-slit experiment shows that that's not really how it works.

In the double slit experiment, we have a barrier. Behind the barrier is a device to count photons as they arrive at different x positions, and we fire photons at the barrier. If there's one slit in the barrier to allow photons through, we get a nice bell curve on the detector. If there's two slits, then we get an interference pattern, not just the overlay of two bell curves. Next, we fire a single photon at a time, and expect that to be detected it must go through one or the other slit, but that's not the result. Instead, even when we fire a single photon at a time, we still get the interference pattern on the detector once enough photons have been fired through the apparatus.

The interpretation is that the wave function of each photon passed through both slits at once, and *interferes with itself* as it does so. It is empirically not true that the photon passed through either the left slit or the right slit, we just don't know which one. The results only make sense if it passed through both with a certain probability wave function.
posted by vibratory manner of working at 4:40 PM on May 6 [1 favorite]

Reminds me of a story I heard once about Schrödinger: while a professor in Austria during the '30s, he was pulled over while out for a drive. Schrödinger had made it known he was in opposition to Nazism, so there was a bit of anxiety for the physicist while the officer gave the vehicle a once-over.

The officer, finishing the inspection, approached the driver's window. "Herr Schrödinger, do you know you have a dead cat in your trunk?"

Schrödinger: "I do now."
posted by Theophrastus Johnson at 5:18 PM on May 6 [1 favorite]

even when we fire a single photon at a time, we still get the interference pattern on the detector once enough photons have been fired through the apparatus.

No, when we fire a single photon, we do not get an interference pattern. We get a single detection at one particular location. We never observe a photon "smeared out".

The patterns come when we accumulate a whole bunch of observations. We can't predict where a photon will be detected, but we can predict the probability of where it'll be detected. The wavefunction gives you that probability; taking it as something else is philosophy, not physics. And if you don't reify the wavefunction, you don't have to invent a mechanism where it "collapses" because of an "observer."

Better minds than ours have worried about how this can be the case, and the best answer so far if we don't know, and positing "hidden variables" (a deeper deterministic layer) doesn't help.
posted by zompist at 6:05 PM on May 6

please leave the cat alone in the box while it is interfering with itself
posted by Sperry Topsider at 6:05 PM on May 6

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